Over the last ten years, hexacyanometalates have

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New Trends in Coordination, Bioinorganic, and Applied Inorganic Chemistry. Edited by ...... F. Basolo, R. G. Pearson, Mechanisms of inorganic reactions. A study ...
NEW TRENDS IN COORDINATION, BIOINORGANIC, AND APPLIED INORGANIC CHEMISTRY

Monograph Series of the International Conferences on Coordination and Bioinorganic Chemistry held periodically at Smolenice in Slovakia

Volume 10

Editors

Milan Melník, Peter Segľa, and Miroslav Tatarko

Department of Inorganic Chemistry Faculty of Chemical and Food Technology Slovak University of Technology Bratislava, Slovakia

Slovak University of Technology Press Bratislava 2011

New Trends in Coordination, Bioinorganic, and Applied Inorganic Chemistry Edited by M. Melník, P. Segľa, M. Tatarko Slovak University of Technology Press, Bratislava © 2011

ANTIBACTERIAL ACTIVITY AND REDUCTION OF CYTOCHROME C. REDOX-ACTIVE METAL(II) COMPLEXES OF STERICALLY HINDERED O-DIPHENOLIC DERIVATIVES OF THIOGLYCOLIC ACID T. V. Kavalchuk, N. V. Loginova, Y. V. Faletrov, H. T. Hres, H. I. Polozov, and N. P. Osipovich Department of Chemistry, Belarusian State University, Leningradskaya 14, 220030 Minsk, Belarus e-mail

[email protected]

ABSTRACT The synthesis and physico-chemical characterization of Fe(II) and Mn(II) complexes of 2-[4,6-di(tert-butyl)-2,3-dihydroxyphenylsulfanyl]acetic acid (HLI) and 2-[4,6-di(tert-butyl)2,3-dihydroxyphenylsulfinyl]acetic acid (HLII) were carried out. Antibacterial activity of the Fe(II) and Mn(II) complexes was evaluated in comparison with Cu(II), Co(II), Ni(II) and Zn(II) complexes and three standard antibiotics; it was found to follow the order: 1) Сu(LI)2>Mn(LI)2>HLI>Ni(LI)2∼Zn(LI)2≥Fe(LI)2>Co(H2O)2LI 2) Сu(LII)2>Co(LII)2> >Mn(H2O)2(LII)2>Ni(LII)2>Fe(LII)2∼Zn(LII)2≥HLII their

reducing

ability

(determined

electrochemically)

followed

the

same

order.

Spectrophotometric investigation was carried out with the aim to estimate the rate of the reduction of bovine heart сytochrome c with the ligands and their metal(II) complexes. It was shown that the reduction of сytochrome c with these compounds can be related to the facility of their oxidation, ionization and lipophilicity. INTRODUCTION Redox transformations of endogenic and most of exogenic substances, including therapeutically active substances, are catalyzed by oxidoreductases [1]. For some therapeutically active substances the ability of being oxidized or reduced by oxidoreductases is the key factor responsible for their pharmacologic properties [2, 3]. One of the possible types of biological macromolecular targets of redox-active antimicrobial agents may be comprised by cytochromes c which are components of mammalian and bacterial electron transport chains [1]. Cytochromes c of bacteria carry out the transport of electrons to cytochrome c-depending cytochrome oxidases of respiratory chains, nitrite-reductases (NrfH) [4], NO-reductases (cNOR) [5], cytochrome c peroxidases [6], thus participating in the processes of respiration and utilization/rendering harmless of NO, NO2– and H2O2. Due to 198

New Trends in Coordination, Bioinorganic, and Applied Inorganic Chemistry Edited by M. Melník, P. Segľa, M. Tatarko Slovak University of Technology Press, Bratislava © 2011

their subcellular localization (periplasm or plasmatic membrane) they are one of the first oxidoreductases which can participate in realization of mechanisms of action and metabolism of redox-active antimicrobial agents, thus providing sensitivity or resistance of bacteria to these agents [7]. Previously we have found that some mono- and di-substituted derivatives of sterically hindered phenolic ligands as well as their complexes with Cu(II), Co(II), Ni(II) and Zn(II) ions exhibit antimicrobial and antiviral activities, the complexes mostly demonstrating a higher level of activity than the parent ligands [8–13]. Furthermore, using the method of cyclic voltammetry, we have shown these compounds to be also of a pronounced reducing ability correlating with antimicrobial activity in a limited series of these compounds [13, 14]. This suggests that redox conversions of these compounds play a dominant part in realization of their antimicrobial activity. As the values of redox potentials govern the ability of compounds to participate in redox interactions, studying electrochemical properties of pharmacologically active compounds may provide a useful approach to evaluation of their ability to undergo redox transformations in biosystems and, particularly, mechanisms of realization of antimicrobial activity. The coordination peculiarities of compounds have significant impact on their biological activity. All these considerations prompted us to extend our research to other phenolic ligands and their metal(II) complexes. In the present work the reduction of bovine heart cytochrome c (Cyt c), selected as a plausible model of bacterial cytochrome c, with two derivatives of sterically

hindered

sulphur-containing

2,3-dihydroxyphenylsulfanyl]acetic

acid

phenolic I

(HL )

ligands

2-[4,6-di(tert-butyl)-

and

2-[4,6-di(tert-butyl)-

2,3-dihydroxyphenylsulfinyl]acetic acid (HLII):

OH

OH

OH

OH SCH2COOH

SCH2COOH HLI

O

II

HL

Fig. 1 Schematic representation of 2-[4,6-di(tert-butyl)-2,3-dihydroxyphenylsulfanyl]acetic acid (HLI) and 2-[4,6-di(tert-butyl)-2,3-dihydroxyphenylsulfinyl]acetic acid (HLII)

as well as with their redox-active transition metal (copper, cobalt, nickel, zinc, manganese, iron) complexes was investigated spectrophotometrically. We also report here the synthesis and characterization of Fe(II) and Mn(II) complexes with aforesaid ligands in order to 199

New Trends in Coordination, Bioinorganic, and Applied Inorganic Chemistry Edited by M. Melník, P. Segľa, M. Tatarko Slovak University of Technology Press, Bratislava © 2011

compare their coordinative behavior in relation to Fe(II), Mn(II) ions with the results obtained earlier for their Cu(II), Co(II), Ni(II) and Zn(II) complexes [15], and to assess the influence of complexation on their biological activity and redox properties determined electrochemically. The results obtained are discussed in the context of presumed interconnection of the capacity of the compounds under study for reducing Cyt c, their antibacterial activity, redox properties determined electrochemically, and other physico-chemical characteristics. EXPERIMENTAL PART Reagents and solutions Chemicals were purchased from commercial sources and were used without further purification.

2-[4,6-di(tert-butyl)-2,3-dihydroxyphenylsulfanyl]acetic

acid

(HLI)

and

2-[4,6-di(tert-butyl)-2,3-dihydroxyphenylsulfinyl]acetic acid (HLII) were prepared according to the methods reported previously [11, 12]. The purity of compounds was checked by TLC. Apparatus and equipment Elemental analyses were carried out with an instrument Vario EL (CHNS mode). Metals and sulphur were determined using an atomic emission spectrometer with an inductively coupled plasma excitation source (Spectroflame Modula). Infrared spectra of solids were recorded with a Nicolet 380 spectrometer in the wavelength range 4000–400 cm–1 at room temperature, using a «Smart Performer»; spectra in the range 400–50 cm–1 were registered using a «Vertex 70» instrument (Bruker Optik GmbH). Thermal analysis was performed with a Simultaneous Thermal Analyzer STA 449 C. XRD analysis was carried out with a HZG 4A diffractometer (CoKα or CuKα radiation, MnO2-filter). ESR spectra of polycrystalline samples were measured with an ERS-220 X-band spectrometer (9.45 GHz) at room temperature and at 77 K, using 100-kHz field modulation; g factors were quoted relative to the standard marker DPPH. The magnetic moment measurements of the investigated complexes were carried out using a SQUID magnetometer. UV-Vis spectra were recorded with a SPECORD M500 spectrophotometer. The molar conductance of 10–3 mol·l–1 solutions of the metal(II) complexes in acetonitrile was measured at 20 °C using a TESLA BMS91 conductometer (cell constant 1.0). The lipophilicity test was made by determining the n-octanol/water partition coefficient (Pow) [16]. Electrochemical measurements were performed under dry nitrogen in a three-electrode two-compartment electrochemical cell using a glassy-carbon (GC) working electrode, Pt auxiliary electrode and Ag|AgCl|0.1 mol·l–1 (C2H5)4NCl reference electrode. The supporting electrolyte was 0.1 mol·l–1 (C2H5)4NClO4. The Ag|AgCl|0.1 mol·l–1 (C2H5)4NCl

200

New Trends in Coordination, Bioinorganic, and Applied Inorganic Chemistry Edited by M. Melník, P. Segľa, M. Tatarko Slovak University of Technology Press, Bratislava © 2011

reference electrode was calibrated with the ferrocenium | ferrocene redox couple located at E1/2=+0.52 V. Acetonitrile was used as a solvent. Synthesis of the metal(II) complexes with 2-[4,6-di(tert-butyl)-2,3-dihydroxyphenylsulfanyl]acetic acid (HLI) and 2-[4,6-di(tert-butyl)-2,3-dihydroxyphenylsulfinyl]acetic acid (HLII) Based on our previous findings [8, 9], the preparation of metal(II) complexes followed a common procedure. A solution of 0.050 mmol M(CH3COO)2 (M=Fe(II) and Mn(II)) in 10 ml of water was added dropwise to a colorless solution of 0.100 mmol of a ligand dissolved in 10 ml of ethanol (molar ratio M(II):L=1:2). As these ligands can be oxidized by oxygen, argon was bubbled through the solutions (pH≤6) during the synthesis to ensure the absence of oxygen. Colored precipitates of Fe(II) and Mn(II) complexes formed instantaneously. After 1.5 h stirring, they were collected on membrane filters (JG 0.2 μm), washed with ethanol and water, and dried in vacuo (yield>70 %). Cu(LI)2 complex. For elemental analyses data of Cu(LI)2 see Ref. [15]. Co(H2O)2LI complex. For elemental analyses data of Co(H2O)2LI see Ref. [12]. Ni(LI)2 complex. For elemental analyses data of Ni(LI)2 see Ref. [15]. Zn(LI)2 complex. For elemental analyses data of Zn(LI)2 see Ref. [15]. Fe(LI)2 complex. Blue. Yield: 73–75 %. Anal Calc. for C32H46S2O8Fe: C 56.61, H 6.84, S 9.45, Fe8.23. Found: C 56.54, H 6.82, S 9.38, Fe 8.16. Mn(LI)2 complex. White. Yield: 75–80 %. Anal. Calc. for C32H46S2O8Mn: C 56.69, H 6.84, S 9.47, Mn 8.11. Found: C 56.62, H 6.78, S 9.41, Mn 8.05. Cu(LII)2 complex. For elemental analyses data of Cu(LII)2 see Ref. [15]. Co(LII)2 complex. For elemental analyses data of Co(LII)2 see Ref. [15]. Ni(LII)2 complex. For elemental analyses data of Ni(LII)2 see Ref. [15]. Zn(LII)2 complex. For elemental analyses data of Zn(LII)2 see Ref. [15]. Fe(LII)2 complex. Blue. Yield: 70–72 %. Anal. Calc. for C32H46S2O10Fe: C 54.06, H 6.53, S 9.03, Fe 7.86. Found: C 55.98, H 6.43, S 8.92, Fe 7.77. Mn(LII)2 complex. White. Yield: 75–80 %. Anal. Calc. for C32H50S2O12Mn: C 51.54, H 6.71, S 8.59, Mn 7.38. Found: C 51.49, H 6.77, S 8.54, Mn 7.32. Antibacterial assays The following test microorganisms (collection of Department of Microbiology, Belarusian State University) were used: Escherichia coli, Pseudomonas aeruginosa, Serratia marcescens, Salmonella typhimurium, Bacillus subtilis, Sarcina lutea, Staphylococcus saprophyticus, Staphylococcus aureus, Mycobacterium smegmatis. Antibacterial activity of 201

New Trends in Coordination, Bioinorganic, and Applied Inorganic Chemistry Edited by M. Melník, P. Segľa, M. Tatarko Slovak University of Technology Press, Bratislava © 2011

the compounds was estimated by a minimum inhibitory concentration (MIC, μg·ml–1) as described elsewhere [8, 17]. The compounds being tested were dissolved in dimethyl sulfoxide (DMSO). A twofold serial dilution from 200 to 3.1 μg·ml–1 was used. The absence of microbial growth after an incubation period of 48 h at 37 °C for bacteria was taken to be a criterion of effectiveness. In every case MIC was determined as the lowest concentration of the compound under study which inhibits the visible microbial growth, compared with the control system in which the microorganisms were grown in the absence of any test compound. The amount of DMSO in the medium was 1% and did not affect the growth of the tested microorganisms. There were three replicates for each dilution. Results were always verified in three separate experiments. Reduction of cytochrome c Bovine heart cytochrome c (Sigma) was used. Spectrophotometric experiments were performed with a Shimadzu UV-1202 spectrophotometer using quartz cuvette with 1 cm optical path. Cyt с concentration was determined on its interaction with excess sodium dithionite, using the absorption coefficient ε550 = 21 mmol-1·l·cm-1 [18]. Ar-saturated acetonitrile solutions of the ligands and complexes under study and Cyt с (7 µmol·l-1) were used. Experiments were performed in 10 mmol·l-1 sodium phosphate buffer (pH 7.4) at 20 °C. Aliquotes of the compounds under study were added to Cyt с solution up to the final concentrations 35.0 or 17.4 µmol·l-1. The initial rate of Cyt с reduction (υ) was evaluated by the slope of the kinetic curve A550 vs time according to [19, 20]. The results were confirmed in three independent experiments. RESULTS AND DISCUSSION Physico-chemical characterization. The solid products resulting from the interaction of Fe(II) and Mn(II) ions with HLI and HLII ligands were well characterized by means of elemental analysis, TG/DTA, FT-IR, UV-vis, ESR, magnetic susceptibility and conductance measurements. The elemental analyses data for the Mn(II) and Fe(II) complexes with the ligand HLI and the Fe(II) complex with the ligand HLII are in agreement with the general formula ML2 and for Mn(II) complex with the ligand HLII – Mn(H2O)2(LII)2. Thermal analysis performed in nitrogen atmosphere with identification of the final products by X-ray powder diffraction has shown the complexes Fe(LI)2, Mn(LI)2 and Fe(LII)2 to be anhydrous and unsolvated, because their DTA curves lack any endothermic peaks over a wide range from 70 to 160 °C. Coordination core of Mn(II) complex with the ligand HLII comprises two water molecules as suggested by endothermic 202

New Trends in Coordination, Bioinorganic, and Applied Inorganic Chemistry Edited by M. Melník, P. Segľa, M. Tatarko Slovak University of Technology Press, Bratislava © 2011

peak in DTA curve at 107 °C. The agreement between the experimental and theoretical weight losses for the above processes confirms the above-mentioned general formula of the metal complexes. Cu(LI)2 complex. For physical and spectral characteristics of Cu(LI)2 see Ref. [15]. Co(H2O)2LI complex. For physical and spectral characteristics of Co(H2O)2LI see Ref. [15]. Ni(LI)2 complex. For physical and spectral characteristics of Ni(LI)2 see Ref. [15]. Zn(LI)2 complex. For physical and spectral characteristics of Zn(LI)2 see Ref. [15]. Fe(LI)2 complex. Molar conductivity (in acetonitrile): Λmol=6.9 Ω–1cm2mol–1. TG/DTA data: no weight loss was observed until decomposition which began about 170 °C, with an exothermic peak at 200 °C and an endotermic one at 335 °C, ultimately leaving FeO as the residue. The maximal weight loss of 88.6 % corresponds to the loss of two ligand molecules in the Fe(L)2 complex (Calc. 89.4 %). UV-Vis data (acetonitrile) (λmax, nm (lg ε)): 665 (3.34), 600 (3.29), 420 (3.32), 310 (3.72), 300 (3.65), 240 (3.94), 225 (3.99). µeff (RT, BM)=0. Mn(LI)2 complex. Molar conductivity (in acetonitrile): Λmol=10.3 Ω–1cm2mol–1. TG/DTA data: no weight loss was observed until decomposition which began about 210 °C, with an endothermic peak at 285 °C, ultimately leaving MnO as the residue. The maximal weight loss of 88.3 % corresponds to the loss of two ligand molecules in the Mn(LI)2 complex (Calc. 89.5 %). UV-Vis data (acetonitrile) (λmax, nm (lg ε)): 660 (3.39), 595 (3.55), 490 (3.56), 410 (3.65), 315 (3.74), 295 (3.79), 245(3.96). ESR data: giso=2.015 (ΔH=270 G). µeff (RT, BM)=1.75. Cu(LII)2 complex. For physical and spectral characteristics of Cu(LII)2 see Ref. [15]. Co(LII)2 complex. For physical and spectral characteristics of Co(LII)2 see Ref. [15]. Ni(LII)2 complex. For physical and spectral characteristics of Ni(LI)2 see Ref. [15]. Zn(LII)2 complex. For physical and spectral characteristics of Zn(LI)2 see Ref. [15]. Fe(LII)2 complex. Molar conductivity (in acetonitrile): Λmol=4.9 Ω–1cm2mol–1. TG/DTA data: no weight loss was observed until decomposition which began about 165 °C, with a wide exothermic peak at 230 °C, leaving FeO as the residue. The maximal weight loss of 88.9 % corresponds to the loss of two ligand molecules in the Fe(L)2 complex (Calc. 89.9 %). UV-Vis data (acetonitrile) (λmax, nm (lg ε)): 650 (3.31), 560 (3.51), 338 (3.58), 305 (3.67), 255 (3.71), 225 (3.85). µeff (RT, BM)=0. Mn(H2O)2(LII)2 complex. Molar conductivity (in acetonitrile): Λmol=11.0 Ω–1cm2mol–1. TG/DTA data: no weight loss was observed until decomposition which began about 65 °C, with an endothermic peak at 107°C which corresponds to the loss of two water molecules (Found 4.27 %, Calc. 4.83 % ), and with a wide exothermic peak in the temperature range 180– 300 °C, ultimately leaving MnO as the residue. The maximal weight loss of 89.5 % corresponds to the loss of two ligand molecules in the Mn(H2O)2(LII)2 complex (Calc. 90.5 %). 203

New Trends in Coordination, Bioinorganic, and Applied Inorganic Chemistry Edited by M. Melník, P. Segľa, M. Tatarko Slovak University of Technology Press, Bratislava © 2011

UV-Vis data (acetonitrile) (λmax, nm (lg ε)): 645 (3.39), 590 (3.55), 343 (3.56), 315 (3.74), 297 (3.79), 258(3.88), 228 (3.96). ESR data: giso=2.02 (ΔH=390 G). µeff (RT, BM)=1.72. These complexes were insoluble in water, ethanol, methanol, diethyl ether, but they were soluble in acetonitrile and dimethyl sulfoxide. The conductivity data indicate their being essentially non-electrolytes in acetonitrile [21], and suggest that the two bidentate ligands may be coordinated to Fe(II) and Mn(II) ions as monoanionic species. All the complexes were characterized by X-ray patterns of their own, differing significantly from those of the ligands. However, a full structural analysis could not be performed because no single crystals suitable for X-ray diffraction studies were obtained. Because of the wellknown difficulties in direct X-ray investigations of polycrystalline metal complexes with sterically hindered ligands [22, 23], the geometrical arrangement of the ligating atoms in the metal complexes was investigated by several spectroscopic and magnetochemical methods. To specify the coordination cores in the Fe(II) and Mn(II) complexes, we used IR spectroscopy. T he IR parameters of the Mn(II) complexes are presented in Table 1. Table 1 IR spectral assignments (cm–1) of the ligands HLI and HLII and their metal complexes

Compound HLI

ν(O–H) 3545 s 3447 w 3349 s

Fe(LV)2

3429 m

ν(C=C) 1487 m

ν(COO–) –

1473 m

1383 m

I

Mn(L )2

3446 w

1474 w

1395 m

HLII

3510 s 3463 s 3348 s 3625 s 3391 s 3286 m 3625 s 3419 m

1602 m



1485 m

1577 m 1390 m

1490 m

1575 m

Fe(LII)2 Mn(H2O)2(LII)2

ν(C–O) 1163 m 1117 w 1027 w 1188 s 1113 s 1025 m 1189 m 1175 m 1115 m 1026 m 1167 s 1134 m

ν(S=O) –



ν(C–S) 775 w 677 w 665 m 757 w 668 m

ν(Me–O) – 582 m 532 m 582 m 531 w 509 w



668 m

1005s

687 m 612 m



1168 s, 1131 m

1023 m

688 m

1171 m 1135 m 1085 m 1045 w

1027 w

696 m 617 m

562 m 584 m 472 m 581 m 554 m 503 m

In the spectrum of the ligand HLI there are three broad bands in the range from 3545 cm–1 to 3349 cm–1, which are evidence of intermolecular hydrogen bonds involving phenol hydroxyl groups (Table 1) [24, 25]. IR spectral assignments of the ligand HLI, Fe(LI)2 and Mn(LI)2 complexes are indicative of metal(II) being coordinated with hydroxyl of the ligand HLI: only 204

New Trends in Coordination, Bioinorganic, and Applied Inorganic Chemistry Edited by M. Melník, P. Segľa, M. Tatarko Slovak University of Technology Press, Bratislava © 2011

one sharp band (3446 cm–1 or 3429 cm–1) is present in the spectra of metal complexes, and the absorption bands corresponding to the stretching С–О bond vibrations are shifted to lower frequency region, suggesting that the ligand is coordinated in the form of monoanion [24, 25]. The shift of the bands at 775 cm–1 and 677 cm–1 (assigned to C–S bond vibrations) in the spectra of Fe(LI)2, Mn(LI)2 complexes toward low-frequency region suggests that sulphur is involved in the complexation. Instead of the characteristic bands of COOH group stretching vibrations at 1760–1740 сm–1 new bands appear in the spectra of these metal complexes at 1600–1573 cm–1, characteristic of СОО–-anion coordinated to a metal ion. It should be noted that in the spectra of Fe(LI)2 and Mn(LI)2 complexes there are new bands in the region from 580 to 470 cm–1, which may be assigned to the stretching vibrations of M–O bonds [24]. M–S stretching vibration frequencies are registered in the region 350–330 cm–1. In the IR spectrum of the ligand HLII there are three broad bands at 3510, 3463 and 3348 cm–1, which undergo the following changes in the spectra of its metal complexes: the first one becomes sharper and stronger, and shifts to high-frequency region, the other two also change their positions (Table 1). It may be due to a free and a hydrogen-bond bound hydroxyl groups in metal complex molecules [25]. A broad band in the range 3391–3200 cm– 1

and a band at 899 cm–1 assigned to М–ОН2 vibrations present in the spectrum of

Mn(H2O)2(LII)2 complex substantiates the participation of water molecules in forming the coordination core of the complex and agrees with the elemental analysis data for this complex. In the spectra of metal complexes for the ligand HLII neither the bands resulting from the stretching vibrations of С–S bond change their position or intensity, nor the vibration bands in the range 380–300 cm–1 characteristic of М–S bond appear, thus suggesting that there is no coordination binding of sulphur atom to metal ion. Beside, a band at 1580–1575 cm–1 characteristic of stretching vibrations of СОО– group appears in their spectra instead of the band at 1715 cm–1 (that of stretching vibrations of COOH group) present in the spectrum of the parent HLII ligand. In the spectra of Fe(LII)2 and Mn(H2O)2(LII)2 complexes a shift of the characteristic band of S=O group is observed, which is due to coordination binding of oxygen atom of this group. The strong bands in the range 590–415 cm–1 in the spectra of these complexes belong to М–O bonds [24]. The changes in the frequencies of (C=C) stretching vibrations of aromatic ring in the spectra of all metal complexes compared to those of the ligand HLI (1487 cm–1) or the ligand HLII (1602 cm–1) also are evidence in favour of the coordination bond formation. The above-listed facts suggest that sulphanyl, sulphinyl and carboxylate groups as well as deprotonated hydroxyls take part in forming the MS2O4 or MO4 coordination cores in the metal(II) complexes under study. 205

New Trends in Coordination, Bioinorganic, and Applied Inorganic Chemistry Edited by M. Melník, P. Segľa, M. Tatarko Slovak University of Technology Press, Bratislava © 2011

The ESR spectra of the Mn(LI)2 and Mn(H2O)2(LII)2 complexes show a broad isotropic signal (270 and 390 G, respectively) at liquid nitrogen temperature with giso values of 2.015 and 2.020, respectively. According to [26, 27], such parameters are characteristic of Mn(II) complexes with a distorted octahedral core and with virtually no interaction between manganese atoms. No signal of stabilized radicals (benzosemiquinons) present in ESR spectra. In the electronic absorption spectra of the Fe(LI)2, Mn(LI)2, Fe(LII)2 and Mn(H2O)2(LII)2 complexes in acetonitrile solution the absorption maxima in the high-energy region 225–305 nm belong to intraligand transitions. The absorption maxima appearing in the spectra of the Fe(LI)2 and Mn(LI)2 at 310–315 nm and 410–490 nm are indicative of the ligand-to-metal(II) charge transfer transitions S→МII and Ophenolate→МII, Ocarboxylate→МII, respectively [28]. The spectra of the Fe(LII)2 and Mn(H2O)2(LII)2 complexes exhibit absorption bands at 315–345 nm which can be attributed to Ocarboxylate→ МII and OS=O → МII charge transfer transitions [28, 29]. According to the literature [28, 30–32], the absorption maxima observed in the spectra of complexes Fe(LI)2 (600 nm) and Fe(LII)2 (560 nm) are due to d-d transitions characteristic of low-spin Fe(II) complexes with the octahedral and square planar geometry of FeS2O4 and FeS2O4 coordination cores, respectively [32]. This conclusion is in agreement with ESRspectroscopic data and values of effective magnetic moments of Fe(LI)2 and Fe(LII)2 complexes, indicative of their diamagnetism. Hence, the field of the ligands HLI and HLII is strong enough for spin coupling in Fe(II) ion, and they form low-spin complexes with this ion. Spin coupling is also characteristic of Mn(II) ion in Mn(LI)2 and Mn(H2O)2(LII)2 complexes, which is supported by the values of effective magnetic moment µeff of these complexes. Note that the absorption spectra of low-spin Mn(II) complexes with redox-active ligands have d-d bands at about 590–610 nm, which are characteristic of the octaheadral coordination core [26, 28, 33]. These bands were found in the spectra of Mn(LI)2 and Mn(H2O)2(LII)2 complexes, which may be indicative of the octahedral geometry of their MnS2O4 and MnO6 chromophores. The absorption maxima appearing in the spectra of the Fe(LI)2, Mn(LI)2, Fe(LII)2 and Mn(H2O)2(LII)2 complexes at 640–665 nm are indicative of the metal(II)-to-ligand charge transfer transitions MII→(πO), which are characteristic of low-spin of complexes of redoxactive ligands with readily oxidizadle ions of transition metals, specifically, Fe(II) and Mn(II) [28].

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New Trends in Coordination, Bioinorganic, and Applied Inorganic Chemistry Edited by M. Melník, P. Segľa, M. Tatarko Slovak University of Technology Press, Bratislava © 2011

This view of coordination cores agrees with the data obtained by physico-chemical methods for other metal(II) complexes of the ligands HLI and HLII, investigated previously [15]. The conclusions made on the basis of spectroscopic data are substantiated by the results of quantum-chemical calculations of geometric parameters of Cu(II), Co(II), Ni(II), Zn(II), Fe(II) and Mn(II) complexes of these ligands. In the light of the spectral data, magnetic moment, analytical results and quantum-chemical calculations the mode of bonding in the metal(II) complexes can be represented as shown below (Scheme 1):

OH

OH O

OH

O

H

C

CH2

S

S

O

O

H2C

O

H

O

O OH

O C

O

CH2

C

OH2

O

M

M O

C

O

O

M S

S

C

CH2

O

OH CH2

O

OH

O C

S

HO HO

M=Cu(II), Ni(II),

M=Cu(II), Co(II),

Fe(II), Mn(II)

Ni(II), Fe(II)

O

OH2 CH2 HO

O S

HO

M=Mn(II)

The data on redox properties of compounds HLI and HLII their metal complexes and 3,5-di tert-butylpyrocatechin (HLIII) we used to elucidate the nature of the redox conversions taking place, are presented in Table 2. On anodic potential scan o-diphenols HLI, HLII and HLIII undergo electrochemical oxidation reactions. Comparing electrochemical data for these o-diphenols, one can note a characteristic anodic peak in the region 1.2÷1.33 V, which is characteristic for them and has a counterpart on the reverse scan, that is, a cathodic peak corresponding to reduction of oxidation products in the range of 0.45÷0.64 V. Controlled electrolysis of solutions of these o-diphenols demonstrated that upon the anodic oxidation process corresponding to the peak at 1.20÷1.33 V, the quantity of electricity passed corresponds to two electrons per molecule. In the absorption spectra of solutions of their electrochemical oxidation products there are bands at 400–470 nm which can be assigned to absorption of respective substituted о-benzoquinones [34].

207

New Trends in Coordination, Bioinorganic, and Applied Inorganic Chemistry Edited by M. Melník, P. Segľa, M. Tatarko Slovak University of Technology Press, Bratislava © 2011 Table 2 Cyclic voltammetry data (anodic scan) for the ligands HLI, HLII and their metal(II) complexes

Compound

Еpa, V

Еpc, V

E1/2, V

HLI Cu(LI)2

0.41

0.14

0.28

Еpa, V

Еpc, V

E1/2, V

Еpa, V

Еpa, V

1.26

0.64

0.95

1.54*

1.88*

1.20

0.57

0.88

1.76*

Co(H2O)2LI

1.77*

Ni(LI)2 I

Zn(L )2

1.39

0.64

1.02

1.39

0.80

1.10

I

Fe(L )2

1.76* 1.76* *

1.81*

1.54*

1.83*

1.55

I

Mn(L )2

0.79

0.30

0.55

HLII

1.22

0.48

0.85

1.33

0.51

0.92

Cu(LII)2

0.68

0.18

0.43

1.33

0.69

1.01

Co(LII)2

0.94

0.17

0.55

1.33

0.68

1.01

Ni(LII)2

1.02

0.24

0.63

1.33

0.69

1.01

1.81

0.50

1.16

0.65

0.90

II

Zn(L )2 II

Fe(L )2

1.06

0.34

0.70

Mn(H2O)2(L )2

0.80

0.50

0.65

1.30

HLIII

1.20

0.45

0.83

1.40

II

1.77*

* Irreversible process, it is just the anodic peak that is observed.

Thus, it can be concluded that at the potentials of the first peak the o-diphenol derivatives HLI, HLII and HLIII undergo two successive one-electron oxidation processes to give о-benzoquinones, the cathodic peak corresponding to their reduction being observed in the range 0.45÷0.64 V on the reverse scan. As this redox process for HLI, HLII and HLIII involves phenolic hydroxyls, the potentials of respective peaks are in a narrow range (Table 2), and it is only the compound HLII that has a slightly different potential (1.33 V), which may be due to the acceptor effect of the sulfoxide group [35]. For the compounds HLII and HLIII the abovementioned peaks at the potentials Еpa1 and Еpс1 are the the only ones, while for the compound HLI there are several anodic peaks more (Table 3). Taking into account that the o-diphenol HLIII does not comprise sulphur, it may be suggested that in the case of o-diphenol derivative of thioglycolic acid HLI at 1.54 V and 1.88 V successive oxidation processes take place, involving sulphur atom and/or carboxylic group. As opposed to HLI the compound HLII has a sulfoxide lateral substitute in the side chain and a redox process involving sulphur atom of this group isn’t realized any more. This results in only one redox process involving hydroxyl groups of o-diphenol being observed for HLII in the range under

208

New Trends in Coordination, Bioinorganic, and Applied Inorganic Chemistry Edited by M. Melník, P. Segľa, M. Tatarko Slovak University of Technology Press, Bratislava © 2011

study (Table 3). No peaks corresponding to reduction of the compounds HLI, HLII and HLIII is observed upon cathodic polarization (down to –2.0 V). Peaks in the regions 1.26÷1.39 V and 1.76÷1.88 V are registered in voltammograms of Сu(LI)2, Ni(LI)2 and Zn(LI)2 complexes, their potentials being close to respective peaks for HLI ligand. However, there is no peak of the ligand about 1.54 V in voltammograms of metal complexes (Table 3). In the voltammogram of the complex Cu(LI)2 an additional anodic peak is registered at 0.41 V, a cathodic peak at 0.12 V being its counterpart on the reverse scan. The complex Cu(LI)2 is reduced upon cathodic polarization (the peak at –0.64 V). Oxidation of the reduction products is observed on the reversed scan as a double anodic peak at – 0.13 ÷ –0.07 V. Taking into accounts that this double peak comprises a sharp anodic peak characteristic of the metal being dissolved, and a shoulder at more positive potentials, one may suggest that the processes Cu(II)→Cu(I)→Cu(0) are realized on the reduction of the Cu(LI)2 complex, while the double anodic peak at –0.13 ÷ (–0.07) V corresponds to reverse processes. The anodic peaks at 1.54÷1.55 V and 1.81÷1.83 V are present in voltammograms of Fe(LI)2 and Mn(LI)2 complexes. These peaks are close to respective ligand peaks, but there is no peak at 1.26 V (for Fe(LI)2 complex) or it is small (for Mn(LI)2 complex) (Table 3). Upon anodic polarization a small peak at 0.79 V (with a cathodic counterpart at 0.30 V upon the reverse scan) is present in voltammogram of Mn(LI)2 complex along with the abovementioned peaks. No electrochemical process related to Fe(LI)2 and Mn(LI)2 reduction are observed on cathodic polarization down to –2.0 V. Only a peak of oxidation at 1.77 V, close to the respective ligand peak, is present in the voltammogram of the complex Co(H2O)2LI (Table 3). Relating the electrochemical findings for the ligand HLI and its metal complexes to the composition of their coordination cores, one can make the following conclusions. In the molecules of Cu(LI)2, Ni(LI)2, Mn(LI)2 and Zn(LI)2 complexes there is a hydroxyl group which is not bound in the coordination core, a peak in the potential range 1.20–1.40 V corresponding to its oxidation (Table 3). It should also be noted that coordination binding of sulphur atom of thioether group may hinder its oxidation. This suggestion is consistent with the peak at 1.54 V being absent in voltammograms of the complexes Cu(LI)2, Ni(LI)2, Co(H2O)2LI and Zn(LI)2. In the potential range of the peak 1.76÷1.88 V the process of oxidation can involve the sulphur atom. Redox process for the complex Cu(LI)2 (the respective anodic peak at 0.41 V) may be related either to Cu(II)↔Cu(III) transformation or the oxidation of one of HLI molecule to give o-semiquinone (the one-electron nature of the redox process being conformed by the results of controlled electrolysis of Cu(LI)2) solution). We have found before [15] that the complex Co(H2O)2LI is polymeric in its structure where the functional groups of 209

New Trends in Coordination, Bioinorganic, and Applied Inorganic Chemistry Edited by M. Melník, P. Segľa, M. Tatarko Slovak University of Technology Press, Bratislava © 2011

HLI ligand (hydroxyl and thioether ones), capable of being electrochemically oxidized in the molecule of the ligand not bound with the metal ion, are coordinated. These oxidation processes are responsible for the peaks at 1.26 and 1.54 V being present in HLI ligand voltammogram while they are absent in that of its Co(H2O)2LI complex. The anodic peak at 1.54 V being present in the voltammogram of Fe(LI)2 and Mn(LI)2 complexes is indicative of a characteristic feature of their electrochemical behaviour which may be due to octahedral low-spin complexes Fe(II) and Mn(II) being less stable and to a soft Lewis base (the sulphur atom of thioether group) weakly bound to hard Lewis acids (Fe(II) and Mn(II) ions) being able to be oxidized [35, 36]. The small anodic peaks present in the voltammogram of Mn(LI)2 complex at 1.26 and 0.79 V may be due to redox processes involving the free ligand in solution. The investigation carried out showed that the ligand HLI and its metal complexes can be arranged into a sequence according to their reducing ability, the formal potential of the

redox I

system I

I

E11/2 I

being I

used

as

I

its

criterion

(according

to

[57]):

I

Cu(L )2>Mn(L )2>HL >Ni(L )2≥Zn(L )2≥Fe(L )2≥Co(H2O)2L . (The formal potential is calculated as the average potential of the peaks found by the cyclic voltammetry method: E11/2=(Еpa1+Еpc1)/2). The electrochemical findings for HLII ligand and its metal complexes are presented in Table 3. The anodic peak at 1.33 V (with the respective cathodic one at 0.50–0.69 V on the reverse scan) is present in voltammograms both for the ligand and its metal complexes (except Fe(LII)2). This peak, as has been shown above, is due to o-diphenol HLII oxidizing to give o-benzoquinone. For the complex Zn(LII)2 the potential of the anodic peak (1.81 V) is significantly shifted in anodic direction compared to that of the ligand. Besides, in voltammograms of the complexes Сu(LII)2, Сo(LII)2, Ni(LII)2, Mn(H2O)2(LII)2 and Fe(LII)2 there are anodic peaks in the potential range 0.66–1.06 V (Table 3), which do not belong to the ligand HLII and can be assigned to the processes of oxidation involving metal, M(II)→M(III). For the complex Fe(LII)2 redox process involving the ligand HLII is unlikely to be realized, as no respective anodic peak at 1.33 V is observed in the voltammogram of this complex. As noted above, when interpreting these data it should be taken into account that phenolic hydrohyls do not take part in forming coordination core of the complex Fe(LII)2 and thus could be oxidized at potentials about 1.33 V. But this process is preceded by Fe(II)→Fe(III) oxidation which is realized at more cathodic potential values (Table 3). This transformation results in the structural rearrangement of catecholate complexes of Fe(II) and Fe(III) [32], as they are distinguished by various coordination polyhedra. These processes result in the hydroxyls of HLII o-diphenol becoming coordinated in the Fe(III) complex formed and being unable to undergo oxidation. For the complex Сu(LII)2 the reduction of Cu(II) is observed 210

New Trends in Coordination, Bioinorganic, and Applied Inorganic Chemistry Edited by M. Melník, P. Segľa, M. Tatarko Slovak University of Technology Press, Bratislava © 2011

upon cathodic polarization: Cu(II)→Cu(I)→Cu(0) (a double peak –0.53 ÷ (–0.67) V corresponding to it, while a double anodic peak in the range –0.13 ÷ (–0.08) V corresponds to processes Cu(0)→Cu(I)→Cu(II) on the reverse scan). No processes of reduction are observed on cathodic polarization for other metal complexes. On the basis of the electrochemical findings the ligand HLII and its metal complexes can be graded in their reducing ability as follows: Cu(LII)2>Сo(LII)2>Mn(H2O)2(LII)2∼Ni(LII)2≥ Fe(LII)2>LII>Zn(LII)2. The electrochemical findings were used in interpreting the results of the biological evaluation of the compounds synthesized ones. Biological evaluation Antibacterial activity MIC values of the ligands HLI and HLII and their metal(II) complexes are listed in Table 4. For the ligands and their Cu(II), Co(II), Ni(II) and Zn(II) complexes, continuing our previous study [15], we have expanded the spectrum of bacteria, and it was for the first time that the antibacterial activity of Fe(II) and Mn(II) complexes of these ligands was investigated. Commonly used antibiotics streptomycin, tetracycline and chloramphenicol were tested as positive controls. Evaluating the antibacterial activity of the ligands and their metal(II) complexes, we can note that they demonstrated a low inhibiting ability toward Gram-negative bacteria: MIC>100 μg⋅ml–1 (Table 3. Antibacterial activity (Staphylococcus saprophiticus, Bacillus subtilis, Sarcina lutea) of the ligands HLI and HLII and their metal(II) complexes against Gram-positive bacteria is well higher than their activity against Gram-negative bacteria, and in some tests it is comparable to that of standard antibiotics (Table 3). The activity of the complexes Cu(LI)2, Cu(LII)2 and Co(LII)2 against most of Gram-positive bacteria is higher than that of the ligands and other metal complexes under study; in particular, the high activity of these Cu(II) and Co(II) complexes against Mycobacterium smegmatis is worthy of particular notice. The pronounced change in activity resulting from complexation can be due to the growth in their lipophilicity (Table 4), the latter promoting the transmembrane transfer of metal complexes into cells and action of metal ions on the cellular structures and processes [1].

211

New Trends in Coordination, Bioinorganic, and Applied Inorganic Chemistry Edited by M. Melník, P. Segľa, M. Tatarko Slovak University of Technology Press, Bratislava © 2011 Table 3 Antibacterial activity of the free ligands and their metal(II) complexes evaluated by minimum inhibitory concentration (MIC, μg⋅ml–1)

Compound

Pseudomonas aeruginosa

HL

I

>100

a

Serratia

Salmonella

marcescens typhimurium >100

a

Escherichia Bacillus Sarcina Staphylococcus Staphylococcus Mycobacterium coli

subtilis

lutea

a

a

>100

>100

a

>100

100

saprophyticus 100

aureus

smegmatis

a

100

100

Cu(LI)2

>100 a

>100 a

>100

>100 a

25 a

6.2 a

12.5 a

6.2

6.2

Co(H2O)2LI

>100 a

>100 a

>100

>100 a

25 a

12.5 a

12.5 a

25

12.5

Ni(LI)2

100 a

>100 a

>100

100 a

50 a

12.5 a

50 a

50

25

Zn(LI)2

>100 a

>100 a

>100

>100 a

>100 a

12.5 a

12.5 a

25

25

I

>100

>100

>100

>100

100

50

50

100

50

I

>100

>100

>100

>100

50

50

50

25

50

50

Fe(L )2 Mn(L )2 HL

II

>100

a

>100

a

>100

>100

a

>100

12,5 a

12.5

a

50

a

Cu(LII)2

>100 a

>100 a

>100

>100 a

12.5 a

6.2 a

12.5 a

6.2

6.2

Co(LII)2

100 a

>100 a

>100

100 a

12.5 a

12.5 a

12.5 a

12.5

6.2

Ni(LII)2

100 a

>100 a

>100

100 a

100 a

12.5 a

50 a

50

50

Zn(LII)2

>100 a

>100 a

>100

>100 a

>100 a

12.5 a

25 a

50

50

>100

>100

>100

>100

>100

50

50

50

50

Mn(H2O)2(L )2

>100

>100

>100

>100

25

12,5

50

50

50

Streptomycin

>100

6.2

12.5

3.1

6.2

12.5

6.2

6.2

3.1

Tetracycline





6.2

3.1

6.2

6.2

6.2

3.1



Chloramphenicol

12.5



6.2

6.2

3.1



6.2

6.2

12.5

Fe(LII)2 II

a

Ref. [15].

212

New Trends in Coordination, Bioinorganic, and Applied Inorganic Chemistry Edited by M. Melník, P. Segľa, M. Tatarko Slovak University of Technology Press, Bratislava © 2011 Table 4 Octanol/water partition сoefficients (logPow) of the ligands and metal complexes

a

Compound

logРоw

Compound

logРоw

HLI

2.31 a

HLII

–2.07 a

Cu(LI)2

5.42 a

Cu(LII)2

4.79 a

Co(H2O)2LI

2.85 a

Co(LII)2

4.24 a

Ni(LI)2

4.44 a

Ni(LII)2

4.07 a

Zn(LI)2

3.79 a

Zn(LII)2

3.79 a

Fe(LI)2

5.26

Fe(LII)2

5.39

Mn(LI)2

3.06

Mn(H2O)2(LII)2

3.08

Ref. [15].

The Cu(LI)2, Cu(LII)2 and Co(LII)2 complexes may be considered as potential antimicrobial agents, particularly when their activity is comparable with the inhibiting effect of streptomycin, tetracycline and chloramphenicol (Table 3). It has been found that the antibacterial activity of the metal(II) complexes synthesized does not correlate with the toxicity of the metal(II) ions to the bacteria tested, because none of the starting metal salts acts against bacteria up to the dose of 200 μg·ml–1. The antibacterial activity of the compounds examined follows the order: 1) Сu(LI)2>Mn(LI)2>HLI>Ni(LI)2∼Zn(LI)2≥Fe(LI)2>Co(H2O)2LI; 2) Сu(LII)2>Co(LII)2>Mn(H2O)2(LII)2>Ni(LII)2>Fe(LII)2∼Zn(LII)2≥HLII;

their

reducing

ability

(determined electrochemically) follows the same order, as it was shown above (Table 2 ). For many of these metal complexes their lipophylicity decreases in a similar order (Table 4). It should be noted that their redox properties affect the antibacterial activity to the greater extent than the lipophylicity does. Thus, Cu(LI)2, Cu(LII)2 and Co(LII)2 complexes characterized by a strong antibacterial activity demonstrate a much higher reducing ability than that of the ligands and the rest of the metal complexes. It is indicative that in many tests redox-active Mn(II) complexes with low logРоw values show a higher antibacterial activity as compared to that of highly lipophylic Fe(II) complexes with a lower reducing ability.

213

New Trends in Coordination, Bioinorganic and Applied Inorganic Chemistry Edited by M. Melník, P. Segľa, M. Tatarko Press of Slovak University of Technology, Bratislava © 2011

Reduction of cytochrome c The results of spectrophotometric investigation of redox interaction of oxidized form of Cyt с with the HLI and HLII ligands and their Cu(II), Co(II), Ni(II), Zn(II), Mn(II), and Fe(II) complexes are given in Table 6. The characteristic absorption bands at 550 and 520 nm appearing when the ligands or their metal complexes are added to the solution of the oxidized Cyt с bear witness to the ability of these compounds to reduce Cyt с in vitro, thus substantiating the previous findings [37, 38]. Table 5 Rates of reduction of Cyt с (υ) with the ligands HLI, HLII, and their metal(II) complexes*

Compound

υ, nmol·min-1

Compound

υ, nmol·min-1

HLI

1.0

HLII

1.1

Сu(LI)2

6.1

Сu(LII)2

3.1

Co(H2O)2LI

2.1

Сo(LII)2

5.4

I

Ni(L )2 I

Zn(L )2 I

0.5

II

3.0

II

3.5

II

Ni(L )2

0.8

Zn(L )2

Fe(L )2

0.5

Fe(L )2

2.2

Mn(LI)2

4.4

Mn(H2O)2(LII)2

2.3

*The final concentrations of Cyt с and the complex (or the ligand) were respectively 7 and 35 µmol·l–1.

We have found that the rate of Cyt с reduction with HLII ligand corresponds to that with HLI ligand (Table 2). Taking into account the findings presented in [37, 38], it can be suggested that the most probable route of oxidation of the ligands HLI and HLII in the system of interest in vitro under anaerobic conditions involves single-electron stages of their monoanionic or molecular forms being oxidized to о-benzoquinones upon interacting with Cyt с via intermediate formation of respective о-benzosemiquinone. Almost equal rates of Cyt с reduction with HLI and HLII ligands can be due first of all to similar reducing abilities of these compounds (Table 2). Together with this fact, their ionization constants, characterizing their ability to form anions by hydroxyls being deprotonated, differ slightly pK(HLI)=8.3 and pK(HLII)=7.9 [15]). Cu(II), Co(II), Ni(II), Zn(II), Mn(II), and Fe(II) complexes of the ligands HLI and HLII also reduce Cyt с. It should be taken into account that metal complexes can interact with Cyt с both in their molecular form and via phenolate-ligand and metal ions formed upon their dissociation. In this connection attention should be paid to the values of stability constants of these metal complexes. The values of stability constants for the complexes under investigation were previously determined in our studies [15] The steadiest among the metal complexes of the ligands HLI and HLII are Сu(LI)2 (lgβ=7.1) и Сu(LII)2 (lgβ=7.3). Ni(LI)2, 214

New Trends in Coordination, Bioinorganic and Applied Inorganic Chemistry Edited by M. Melník, P. Segľa, M. Tatarko Press of Slovak University of Technology, Bratislava © 2011

Zn(LI)2, Сo(LII)2, Ni(LII)2, Zn(LII)2, Fe(LII)2 and Mn(H2O)2(LII)2 complexes have intermediate values of the overall stability constant (lgβ=6.4÷6.8), while Fe(LI)2 and Mn(LI)2 complexes are less stable (lgβ=5.6÷5.7) and Co(H2O)2LI complex is the most ready to dissociate (lgβ=3.5). In the series of metal complexes with HLI ligand it is Сu(LI)2 and Mn(LI) complexes that show the highest rate of Cyt с reduction, substantially higher than the rate of redox process envolving the free ligand (Table 5). It should be emphasized that according to electrochemical data these complexes are characterized by more cathodic (Еp1a+Еp1c)/2 values and are the most active reducing agents in the series of metal complexes with the ligand HLI (Table 2). The Ni(LI)2, Zn(LI)2 and Fe(LI)2 complexes are characterized by low and virtually comparable rates of Cyt с reduction (Table 5). This may be due to the fact that their reducing abilities determined electrochemically, are also comparable and are substantially lower than those of Сu(LI)2 and Mn(LI)2 complexes (Table 2). It should be taken into account that their stability constants and water solubility have similar values as well [15]. Co(H2O)2LI complex can reduce Cyt с in the system under study in vitro, faster than Ni(LI)2, Zn(LI)2, Fe(LI)2 complexes and HLI ligand (Table 5), although, according to the results of the electrochemical investigation it has the lowest reducing ability among the complexes of this series (Table 2). It should be noted that Co(H2O)2LI complex is the most water-soluble among the abovementioned complexes [15] and dissociates more readily than these complexes to give ligand ions which, according to [38], are the most probable reducing agents for Cyt с. In the series of complexes with the ligand HLII all the metal complexes reduce Cyt с at a rate several times higher than that of the redox process involving the free ligand (Table 5). These results, in general, correspond to electrochemical data, according to which, metal(II) complexes with the ligand HLII are characterized by more cathodic (Еp1a + Еp1c)/2 values and are the more active reducing agents than HLII ligand (Table 2). The rate of Cyt с reduction with Zn(LII)2 complex in the system under study in vitro is 1,5–1,7 times higher than that of Ni(LII)2 and Fe(LII)2 complexes (Table 5), although, according to the results of electrochemical investigation, it has the lowest reducing ability among the complexes with ligand HLII (Table 2). But this complex has a lower value of the n-octanol/water partition coefficient (Pow) (Table 4), which, according to [16], can facilitate its interaction with protein in solution. The low rate of Cyt с reduction with Mn(H2O)2(LII)2 complex do not agree with its reducing ability and Pow value in the series of metal complexes with HLII ligand, which can be due to two water molecules present in axial positions of its coordination polyhedron. Among their complexes it is Co(LII)2 complex that is characterized by the highest Cyt с reduction, thus being one of the strongest reducing agents in this series of complexes. Cu(LII)2 complex, in spite of the most cathodic (Еp1a+Еp1c)/2 value among these 215

New Trends in Coordination, Bioinorganic and Applied Inorganic Chemistry Edited by M. Melník, P. Segľa, M. Tatarko Press of Slovak University of Technology, Bratislava © 2011

compounds, ranks below Co(LII)2 and Zn(LII)2 complexes in the rate of Cyt с reduction (Table 5), which may be due to it being more stable to dissociation as compared to the abovementioned complexes. The results obtained evidence that ionization of metal complexes and their water solubility, the other characteristics being comparable, can play the crucial part in the process of Cyt с reduction (Fig. 2).

Fig. 2 Scheme of the reduction of сytochrome c with the anion form of a sterically hindered odiphenolic derivative of thioglycolic acid; Fe(III)-Cyt c and Fe(II)-Cyt c – oxidized and reduced form of cytochrome c, respectively; the process of disproportionation of the derivative is depicted in dash line

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B. Wen, K. J. Coe, P. Rademacher, W. L. Fitch, M. Monshouwer, S. D. Nelson, Chem. Res. Toxicol. , 21 (2008) 2393 216

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J. Simon, FEMS Microbiol. Rev. , 26 (2002) 285

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J. Hendriks, A. Oubrie, J. Castresana, A. Urbani, S. Gemeinhardt, M. Saraste, Biochim. Biophys. Acta 1459 (2000) 266

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L. De Smet, S. N. Savvides, E. Van Horen, G. Pettigrew, J. J. Van Beeumen, J. Biol. Chem. , 281 (2006) 4371

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N. V. Loginova, T. V. Koval’chuk, R. A. Zheldakova, A. A. Chernyavskaya, N. P Osipovich, G. K. Glushonok, G. I. Polozov, V. L. Sorokin, O. I. Shadyro, Centr. Eur. J. Chem. , 4 (2006) 440

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T. V. Koval’chuk, N. V. Loginova, G. I. Polozov, A. A. Chernyavskaya, D. A. Gursky, O. I. Shadyro, Vestnik BSU. Ser. 2. Khim. , Biol. , Geogr. , 2 (2008) 25 (In Russian).

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