Spectroscopic and electrochemical studies on

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J. Surface Sci. Technol., Vol 27, No. 3-4, pp. 1-14, 2011 © 2011 Indian Society for Surface Science and Technology, India.

Spectroscopic and electrochemical studies on cobaloximes and alkylcobaloximes in aqueous surfactant micelles as models of Vitamin B12 PRANJOLI DAS and O. K. MEDHI* Department of Chemistry, Gauhati University, Guwahati 781 014, Assam, India. Abstract — Various synthetic models of VitaminB12 are prepared using dimethylglyoxime, pyridine, imidazole and methyliodide as ligands and their spectroscopic and electrochemical results are compared with the natural analogues methylcobalamin and cyanocobalamin. The spectroscopic and electrochemical results of these compounds are studied in hydrophobic membrane mimicking systems such as aqueous systems of Sodium dodecyl sulfate (SDS), Cetyl trimethyl ammonium bromide (CTAB). The hydrophobic environment of aqueous surfactant micellar solutions around the metal complexes at the active site of the metalloproteins under physiological pH has considerable effect on the electronic spectrum of the model compounds. Platinum working electrode gives a more reliable result for vitamin B12 showing a reversible couple in the range from –0.3 to –0.6 V. Cobaloxime complexes show reversible couple in the range from –0.6 to –0.8 V with glassy carbon electrode whereas methylcobalamin shows positive redox potential values with glassy carbon electrode. Micellar systems stabilize the organometallic derivatives in aqueous solutions. Keywords : VitaminB12, Methylcobalamin, Cobaloxime, Alkylcobaloximes, electronic spectroscopy, infra-red spectroscopy, redox behaviour.

INTRODUCTION

VitaminB12 (cyanocobalamin) and methylcobalamin [1-4] are exclusively synthesized by certain microorganisms (bacteria) in our small intestine. Its deficiency can cause Pernicious Anemia. They mediate the catalysis of a distinctive class of biologically important reactions together with an appropriate enzyme system [5-7]. Coenzyme B12 is the naturally occurring organometallic compound as it contains the Co-C sigma *Corresponding author E-mail : [email protected] ; contact no.: 09954040638/ (0361- 2583052)

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bond formed by the replacement of cyanide ion by an adenosine group. Methylcobalamin is known to be operative in the catalysis of the methylation of homocysteine to methionine, thereby preventing heart disease; it is also involved in the biosynthesis of methane and reduction of carbon dioxide to acetic acid. Vitamin B12 coenzyme, with an appropriate apoenzyme, mediates the catalysis of adverse group of mutase or isomerase reactions [8]. VitaminB12 [9] structure consists of a cobalt atom linked by four N-atoms of a corrin ring, the fifth position is coordinated to the N-atoms of a benzimidazole base and the sixth by a ligand which can be varied but it is evidently absent when the cobalt atom is reduced to +1 state. The complexes that possess D-D ribofuranose-3-phosphate and terminal 5,6-dimethylbenzimidazole as an axial ligand are called cobalamin. The cobalt atom in vitaminB12 and its derivatives can exist in three main formal oxidation states Co(²²²), Co(²²) and Co(²), which display quite different chemical properties and hence oxidoreduction phenomena are very important in the chemistry of vitaminB12 [10-12] and its derivatives can be reduced in neutral or alkaline medium by NaBH4 or chromium acetate. In view of the important biochemical functions of the alkylcobalamins, it is highly desirable to study simple model compounds such as bis(dimethylglyoximato) [13-16] complexes of cobalt (cobaloximes) which chemically closely resemble vitaminB12 derivatives. Recently cyclic voltammetry seems to be the most versatile tool for evaluating the redox chemistry of vitaminB12 and its model complexes [4,7] because of its ability to reproduce the interconversion between the pertinent oxidation states of cobalt in these compounds. Therefore, a detailed electrochemical investigation has been undertaken to evaluate the suitability of these models as a mimic of coenzyme B12. The naturally occurring apoproteins, which provide relevant reaction sites for Vitamin B12, are considered to perform additional important roles that lead to desolvation and close association of reacting species. In this regard, modifications of cobalamin are required in order to demonstrate high affinity for hydrophobic microenvironments which are provided by bilayer membranes and macrocyclic hosts. Surfactant micelles are known to mimic biological membranes and are attractive systems to study electron transfer under physiological conditions [17-18]. We prepare model compounds of vitaminB 12 soluble in aqueous solution and study their spectroscopic and electrochemical properties in aqueous solution of hydrophobic membrane mimicking micellar systems of sodiumdodecylsulfate (SDS) and cetyltrimethylammoniumbromide (CTAB).

Spectroscopic and electrochemical studies on cobaloximes and alkylcobaloximes

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EXPERIMENTAL

Materials : Cobalt(II)chloridehexahydrate (CoCl2·6H2O), sodiumborohydride (NaBH4) and imidazole were obtained from Merck. Dimethylglyoxime, Pyridine, Sodiumdodecylsulphate were purchased from Qualigen. 2-methylimidazole, Potassium dihydrogenphosphate, methanol, dichloromethane, acetonitrile, 1-imidazole, trishydroxyaminomethane, trishydrochloride, tetrabutylammoniumperchlorate and cetyltrimethylammoniumbromide were purchased from Sisco Research Laboratory Pvt. Ltd. Mumbai. Analytical grade chemicals and solvents are used without further purification. Deionized and MilliQ water were used throughout the experiments. Preparation of cobaloxime and alkylcobaloxime complexes The reaction schemes for the preparation of Cobaloxime and alkylcobaloxime complexes were based on the reported method [19-20] but with some modifications (as change in solvents, chemicals and reaction condition) as shown in Fig. 1. Bisdimethylglyoximatocobalt(III)L complex (L = pyridine, 1-imdazole and 1-

Fig. 1. A Schematic illustration for the preparation of cobaloxime and alkylcobaloxime complexes.

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methylimidazole) recrystallized from dichloromethane are obtained as brown colored crystals and are soluble in water and organic solvents. The starting material for the preparation of methylpyridinatocobaloxime complex was bisdimethylglyoximatocobalt(III)pyridinato complex. Methylpyridinatocobaloxime complex was obtained as orange crystals soluble in both water and organic solvents. Micellar solutions They were prepared by dissolving the surfactant (viz., SDS, CTAB) in distilled water or in appropriate buffer solution and warmed at 50°C to get a clear solution. The pH of the micellar solutions were 7.0 in phosphate buffer and 8.5 in tris buffer. Tetramethylammonium bromide (0.1 M) was added to the micellar solution to have a more rigid and compact structure [21]. To this micellar solution the cobaloxime complexes are added and warmed at 50°C on water bath for one or two hours. Characterization UV-visible spectroscopy was recorded in Hitachi U-3210 double beam spectrophotometer to analyze the Omax absorption band of the complexes. Fourier transform infrared (FT-IR) spectroscopy was recorded on Perkin Elmer 160 series FTIR to analyze the interactions between metal and ligands. Electrochemical measurements were performed on BAS100W Electrochemical Analyzer [22] (BioAnalytical system : USA) using three electrode assembly consisting of (i) Reference electrode (Ag/AgCl) (ii) Auxiliary electrode (Pt) (iii) Working electrode (usually glassy carbon). Sodium chloride or tetrabutylammonium perchlorates were used as supporting electrolyte to suppress the migration of charged species. Potentials were measured versus Ag/AgCl reference electrode and also with respect to the reference redox system ferrocene/ferrocinium ion. RESULTS AND DISCUSSION

Electronic spectroscopy : Fig.2 shows the electronic spectrum of model compounds CoCl(dmg)2Py (Fig. 2a), Co(dmg)2(imi)2 (Fig. 2b), Co(dmg)2(1-meimi)2, CH3Co(dmg)2py (Fig. 2c) and natural analogs vitaminB12 (Fig. 2d) and methylcobalamin (Fig. 2e) in water and aqueous surfactant i.e. CTAB, SDS micellar solution (1mM solution, pH 7.0/8.5, tris/phosphate buffer). Table 1 clearly shows that the coordinated ligands and hydrophobic environment of the surfactant in aqueous solutions around the metal complexes at the active site of metalloproteins under physiological pH have considerable effect on the electronic spectrum of the model compounds. It also gives the splitting pattern of energy levels and transitions involved in the electronic spectra of regular octahedral


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Fig. 2. Electronic Spectra of model compounds and natural analogs (a) Co(dmg)2Clpy in water and SDS (tris buffer pH 8.5), (b) Co(dmg) 2(Imi)2 in water (tris buffer pH 8.5), (c) Methylpyridinatocobaloxime in water, CTAB and SDS (phosphate buffer pH 7.0), (d) Vitamin B12 in water and CTAB (phosphate buffer pH 7.0) and (e) Methylcobalamin in SDS (phosphate buffer pH 7.0).

complexes. The methyl derivative has an additional band near 440 nm and the organometallic compound is quite stable in aqueous solutions of surfactant micelles. The differences between the spectra of the natural systems vitaminB 12 and methylcobalamin are due to the electronic transitions within the corrin ligands [2325]. In CoCl(dmg)2Py (Fig. 2a), Co(dmg)2(imi)2 (Fig. 2b) and Co(dmg)2(1-meimi)2 level split very slightly at very low energy giving a broad shoulder at Omax 1g around 673.3, 675.0 and 674.4 nm due to 1A 1T1 transition while 1T2g level split

1T

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TABLE 1. Electronic spectroscopy of Vitamin B12 and its model compounds in aqueous and aqueous surfactant media COMPLEX

Omax (nm)

SOLVENT 1

1 A1 T1 A1T2 1A 1E 1A 1A 1A 1B 1A 1E 1 1 2 1 2 1

CoCl(dmg)2py

1

H2O(tris buffer pH = 8.5)

673.3(sh)

309.2

259.2

SDS(tris buffer pH = 8.5)

675.6(sh)

323.0

260.8

H2O(tris buffer pH = 8.5)

675.0(sh)

296.0

249.2

Co(dmg)2(1-meimi)2 H2O(tris buffer pH = 8.5)

674.4(sh)

309.0

247.1

Co(dmg)2(imi)2

CH3Co(dmg)2py

VitaminB12

Methylcobalamin

H2O(phosphate buffer pH 7.0)

442.0

382.0

224.0

SDS(phosphate buffer pH 7.0)

440.0

379.0

225.2

CTAB(phosphate buffer pH 7.0)

436.9

378.0

223.4

H2O(phosphate buffer pH 7.0)

549.6

531.0

361.2

321.2

CTAB (phosphate buffer pH 7.0) 546.8

519.6

359.6

324.0

H2O (phosphate buffer pH 7.0)

530.8

439.2

350.4

301.6

SDS (phosphate buffer pH 7.0)

524.4

430.0

351.2

266.8

dmg = dimethylglyoxime, py = pyridine, im = imidazole, 1-meim=1-methylimidazole

markedly giving two transitions 1A 1B2 and 1A 1E at absorption maxima (309.2, 259.2) nm; (296.0, 249.2) nm and (309.0, 247.1) nm respectively in water. On binding methyl group (-CH3) to the cobalt atom, the energy of 1T2g level splits increases drastically and splits into 1E and 1A2 level and hence the bands at absorbance maxima (442.0, 382.0); (436.9, 378.0) and (440.4, 379.2) nm for H2O, CTAB and SDS are due to 1A1 1E and 1A1 1A2 (Fig. 2c). Thus it is concluded that 1T1g level is more sensitive to the strength of the axial ligand field. In vitaminB12 (Fig. 2d) the absorption spectra has two low energy peaks at ëmax (549.2, 531.0), (546.8, 519.6) nm for H2O, CTAB due to splitting of 1T1g level into 1E and 1A2 level giving two transitions 1A1 1E and 1A1 1A2 arising at low energy giving two absorption peaks while 1T2g level split markedly giving two transitions 1A1 1B2 and 1A1 1E arising at absorption maxima (361.6, 321.1) and (359.6, 324.0) nm for H O and 2 CTAB. The absorption spectra of methylcobalamin (Fig. 2e) in water and aqueous


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SDS micellar solution show absorption maxima at (350.4, 301.6) nm; (351.2, 266.8) nm for H2O, SDS which is due to 1A1 1B2 and 1A1 1E transitions whereas bands at Omax 530.8 nm and 524.4 nm for H2O and SDS at a very low energy is due to 1A 1T transition. 1 Infra-red spectroscopy : Table 2 summarizes the FT-IR spectra of model compounds, vitaminB-complex, vitaminB12 and methylcobalamin. The Infra red spectra of the cobaloxime and alkylcobaloxime complexes show a broad band of the O-H….O bridging group around 1740 cm–1. The C=N stretch occurs between 1536 and 1570 cm–1 and generally decreases with interaction of the base component with the cobalt atom. In complexes, the Co-N(axial ligands) (cobaloxime complexes with nitrogen-containing axial ligands) stretching vibrations were observed between 520 and 437 cm–1 and the symmetric and asymmetric Co-C stretching modes were observed between 432 and 420 cm–1. The typical hydrogen-bonded stretching frequency for the dimethylglyoxime complexes occurs at 1700 cm–1 and the band at 3100 cm–1 is probably due to the water of crystallization [26-28]. The cyano complexes can be easily identified since they exhibit sharp Q(CN) band at 2200–2000 cm–1 [24]. The Q(CN) band of free CN– ligand is 2082.2 cm–1, while the Q(CN) in Vitamin B12 and vitamin B-complex appears at 2151.8 cm-1 and 2157.1 cm–1 respectively for symmetric stretching and 2127.4 cm–1 and 2133.3 cm– 1 respectively for assymmetric stretching. Thus upon coordination to a metal, the Q(CN) band shift to higher frequencies. This is because CN– ion acts as an V-donor by donating electrons to the metal and also as a S-acceptor by accepting electrons from the metal. V-donation tends to raise the Q(CN), since electrons are removed from the 5V orbital, which is weakly antibonding, while S-back bonding tends to decrease the Q(CN) because the electrons enter into the antibonding 2pS* orbital. In general, CN– is a better V-donor and a poorer S-acceptor. Thus the Q(CN) band of the complexes are generally higher than the value for the free CN–. Again the difference in Q(CN) of cyanocobalamin and vitamin-Bcomplex is due to the fact that Q(CN) changes with change in trans axial ligands. Comparison of the spectra of K3[Co(CN)6], it may be assumed that the band arising at 2157.1 cm–1 in case of vitamin-B complex is due to the symmetric stretching of Co-CN bond, whereas the band arising at 2133.3 cm–1 is due to the asymmetric stretching of Co-CN bond. The IR spectra of vitaminB complex matches with that of cyanocobalamin where the bands arising at 2157.1 cm– 1 (A ) and 2132.9 cm–1(E ) are due to symmetric stretching and the bands arising 1g g at 2133.3 cm–1(F1u) is due to the asymmetric stretching of Co-CN bond, which are also present in cyanocobalamin.

1240.8

1241.2

L = Dimethylglyoxime

1567.8

434.4

515.9

1558.1

453.0

512.4

1238.7

1568.1

434.0

513.1

2151.8 2136.0 2127.4

2132.9 2133.3

1556.3

VitaminB12

2157.1

424.5

422.4

1557.2 1234.0

1565.0

Methylcobalamin

1235.9

1558.7

451.8

516.1

complex

CoCl(L)2py Co(L)2(imi)2 Co(L)2(1meimi)2 CH3Co(L)2py Vitamin-B

Band Observed (cm–1)

Infra-red spectroscopy of Vitamin B12 and its model compounds

Q(CN)F1u(Anti symm)

Q(CN)Eg(Symm)

Q(CN)A1g(Symm)

QCo-CH3

QN-O

QC=N

QCo-N(imi)

QCo-N(py)

QCo-N(dmg)

Assignment

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TABLE 2

8 Das and Medhi


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Electrochemical studies (Cyclic voltammetry) : Fig. 3-5 shows the cyclic voltammetry of the model compounds, cyanocobalamin and methylcobalamin in water, organic and aqueous surfactant micellar solutions in buffer

Fig. 3. Cyclic Voltammogram of cobaloximes, in tris buffer pH 8.5, 1 mM solution, supporting electrolyte NaNO3, Working electrode carbon electrode (a) Co(dmg)2Clpy, (b) Co(dmg)2(imi)2 and (c) Co(dmg)2(1-meimi)2.

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Fig. 4. Cyclic voltammetry of methylpyridinatocobaloxime complex in Acetonitrile 1 mM solution, supporting electrolyte NaNO3, Working electrode carbon electrode.

Fig. 5. Cyclic Voltammogram of (a) VitaminB12 and (b) Methylcobalamin in phosphate buffer (pH 7.0), 1 mM solution, supporting electrolyte NaNO3, working electrode platinum electrode.

at pH 7.0 and 8.5 using carbon or platinum electrode and NaNO3 or TBAP as supporting electrolyte. Redox potential values are summarized in Table 3. The electrostatic and hydrophobic interactions of surfactants have significant influence on


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TABLE 3. Cyclic Voltammetry of Vitamin B12 and their models with concentration of the solution 10–3M, supporting electrolyte NaNO3 and TBAP, working electrode glassy carbon and platinum, buffer solution of pH 7.0, 8.5 and acetonitrile, dichloromethane solvents. Oysteryoung Square wave voltammetry (OSWV) results are shown for confirmation of the results obtained from cyclic voltammetry COMPOUND

CoCl(dmg)2py

Co(dmg)2(imi)2

Co(dmg)2(1-meimi)2

CH3Co(dmg)2py Vitamin B12 Methylcobalamin

SOLVENT

E1/2(mV)

'Epc(mv)

ip / ip c a

CV

OSWV

CTAB

–782

–780

59

1.2

H2O

–815

–820

70

1.3

SDS

–883

–882

80

1.4

CTAB

–625

–624

86

0.9

H2O

–623

–626

60

1.2

SDS

–642

–648

80

1.5

CTAB

–709

–712

86

1.4

H2O

–733

–736

63

1.3

SDS

–882

–884

75

0.8

CH2Cl2

930

935

66

1.2

CH3CN

997

1007

84

1.1

H2O

–391

–388

80

1.2

H2O

590

585

80

0.7

the mid-point potentials of the complexes in surfactant micelles [16-18] and induce a positive shift in CTAB micelles for the complexes. The E1/2 value of the complexes (Table 3) are found in the order SDS < H2O < CTAB. The positive shift in the potential is due to the effect of a hydrophobic environment around the metal complexes [29-32]. The mid-point potential attains a constant value at critical micellar concentration (CMC) of the surfactant. The values measured by OSWV are within experimental error (±10 mv) from those measured by cyclic voltammetry. The voltammograms are reversible as well as quasi-reversible ('Ep = 59–86 mV, ipc/ ipa = 0.9–1.4). The E1/2 measured in this work for vitaminB12 are in agreement with those reported previously; the potentials are due to the redox couple Co(III)/Co(II). Cyclic voltammograms of model compounds CoCl(dmg)2Py [Fig. 3a (i), (ii),

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(iii)], Co(dmg)2(imi)2 [(Fig. 3b (i), (ii), (iii)], Co(dmg)2(1-meimi)2 [(Fig. 3c (i), (ii), (iii)] (tris buffer pH 8.5, 1 mM solution, 1 M NaNO3, G.C.E., scan rate 100 mV s–1) show quasi-reversible process at –815 mV, –623 mV, –733 mV (H2O); –782 mV, –625 mV, –709 mV (CTAB) and –883 mV, –642 mV, –884 mV (SDS) in water and aqueous solutions surfactant micelles. The large negative shift in redox potential for the model systems is due to the ligands which stabilizes Co(III) state preferentially over the Co(II) state. Since Co(III) is now difficult to reduce, the potential shifts in the negative direction. Unsubstituted imidazole stabilize the Co(III) state preferentially over the Co(II) state. Similarly, cyclic voltammogram of VitaminB12 (Fig. 5a) and methylcobalamin (Fig. 5b) (phosphate buffer pH 7.0, 1 mM solution, 1 M NaNO3, G.C.E., scan rate 100 mVs–1) show quasi-reversible process at –391 mV, 590 mV. CH3Co(dmg)2Py (Fig. 4) in acetonitrile show quasi-reversible process at 997 mV. In methyl cobalamin and CH3Co(dmg)2py, the oxidation of the cobalt is likely to be +1 state. The large positive shifts are not previously reported. The values of ÄEp and ratio of peak currents indicate that these are two-electron redox process. Non polar solvents of low dielectric constant have a profound effect on the measured redox potential. Further work in micellar aqueous solutions are on progress. CONCLUSION

It is seen that use of modern spectroscopic and electrochemical techniques have shown further insight into the nature of the oxidation state of cobalt atom in vitaminB12 and its model compounds. Aqueous micellar solution is an excellent media for electrochemical studies of vitaminB12 and its model compounds. The hydrophobic environment of surfactants around the metal complexes and vitaminB12 has shown a considerable influence on the visible absorption bands and the redox potentials in comparison to that in water solution. The micelle encapsulated Cobalt complexes were provided with a large macromolecular structure where the interactions within the system are strictly hydrophobic. Thus cobalt complexes encapsulated in micelles are likely to be good functional models for vitaminB12. This shows that further studies on these systems are likely to provide valuable information on the redox chemistry of cyanocobalamin and methylcobalamin. ACKNOWLEDGEMENT

We thank UGC, New Delhi for financial support under a Major Research Project.


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