Synthesis, characterization and urease inhibitory

Inorganic Chemistry Communications 14 (2011) 636–640

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Inorganic Chemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i n o c h e

Synthesis, characterization and urease inhibitory activity of oxovanadium(V) complexes with similar Schiff bases Zhong-Lu You a,⁎,1, Yong-Ming Cui b,1, Yu-Ping Ma a, Che Wang a, Xiao-Shuang Zhou a, Kun Li a a b

Department of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, PR China Engineering Research Center for Clean Production of Textile Printing, Ministry of Education, Wuhan Textile University, Wuhan 430073, PR China

a r t i c l e

i n f o

Article history: Received 31 December 2010 Accepted 28 January 2011 Available online 3 March 2011 Keywords: Schiff base Oxovanadium complex Crystal structure Urease inhibition

a b s t r a c t A series of structurally similar oxovanadium(V) complexes with analogous Schiff bases have been prepared and structurally characterized. The urease inhibitory activities of the complexes were investigated. The shorter the terminal groups of the Schiff base ligands are, the stronger the urease inhibitory activities of the oxovanadium(V) complexes. © 2011 Elsevier B.V. All rights reserved.

Urease is a nickel-containing metalloenzyme that catalyzes the hydrolysis of urea to form ammonia and carbamate [1,2]. The resulting carbamate spontaneously decomposes to yield a second molecule of ammonia and carbon dioxide. A high concentration of ammonia arising from the reaction, as well as the accompanying pH elevation, has important negative implications in medicine and agriculture [3–5]. Control of the activity of urease through the use of inhibitors could counteract these negative effects. In recent years, urease inhibitors play an important role in the treatment of the infections caused by urease producing bacteria [6]. Inhibitors of urease can be broadly classified into two fields: (1) organic compounds, such as acetohydroxamic acid, humic acid, and 1,4-benzoquinone [7–9]; and (2) heavy metal ions, such as Cu2+, Zn2+, Pd2+, and Cd2+ [10,11]. Considering that the metal complexes are a kind of versatile enzyme inhibitors [12], we have recently reported the urease inhibitory activities of a number of complexes with Schiff bases [13–15]. The results show that the Schiff base copper(II) complexes have potential urease inhibitory activities. During the search of literature, we found that the vanadium(IV) complexes also possess interesting urease inhibitory activities [16]. Vanadium complexes have been widely investigated in biological chemistry, especially for their insulinenhancing activities [17–19]. In this paper, a series of oxovanadium(V) complexes, [VO2L1] (1), [VO2L2] (2), [VO2L3] (3), and 2[VO2L4]∙CH3OH (4) (HL 1 = 1-[(2-dimethylaminoethylimino)methyl]naphthalen-2-ol, HL2 =1-[(2-diethylaminoethylimino)methyl]naphthalen-2-ol, HL3 = 1[(2-piperidin-1-ylethylimino)methyl]naphthalen-2-ol, HL4 = 1-[(2morpholin-4-ylethylimino)methyl]naphthalen-2-ol; Scheme 1) were ⁎ Corresponding author. E-mail address: [email protected] (Z.-L. You). 1 These authors contributed equally to this work. 1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2011.01.038

synthesized and structurally characterized. The urease inhibitory activities of the complexes were investigated both from the experimental and from the docking analysis using the AUTODOCK 4.0 program [20]. The Schiff bases and the oxovanadium(V) complexes were readily prepared [21,22]. The molar conductance values of the complexes measured in methanol at the concentration of 10− 3 M are in the range 10–18 Ω− 1 cm2 mol− 1, indicating the non-electrolytic nature of the complexes [23]. The molecular structures of the complexes 1–4 are shown in Figs. 1–4, respectively. X-ray crystallography [24] reveals that the four complexes are similar mononuclear oxovanadium(V) compounds. The difference among the complexes is the variety of the Schiff base ligands. The Schiff bases serve as tridentate ligands to form five- and six-membered chelate rings in the complexes. The coordination geometry around the metal center in each of the complexes can be best described as a distorted square pyramid,

Scheme 1. The Schiff bases.

Z.-L. You et al. / Inorganic Chemistry Communications 14 (2011) 636–640

Fig. 1. A perspective view of the molecular structure of 1 with the atom labeling scheme. The thermal ellipsoids are drawn at the 30% probability level. Selected bond lengths (Å) and bond angles (°): V1―O1 1.909(2), V1―O2 1.608(2), V1―O3 1.619(2), V1―N1 2.126(2), V1―N2 2.183(2); O2―V1―O3 110.7(1), O2―V1―O1 104.4(1), O3―V1―O1 97.7(1), O2―V1―N1 107.7(1), O3―V1―N1 140.2(1), O1―V1―N1 82.5(1), O2―V1―N2 96.7(1), O3―V1―N2 88.6(1), O1―V1―N2 154.1(1), N1―V1―N2 76.8(1).


with τ parameters of 0.23 for 1, 0.38 for 2, 0.22 for 3, 0.36 and 0.22 for 4, respectively (τ = 0 for an ideal square pyramid; τ = 1 for an ideal trigonal bipyramid). The three donor atoms of the Schiff base ligand and one terminal oxo O atom define the basal plane of the square pyramidal coordination, and the other oxo O atom occupies the apical position. The V atoms lie 0.490(2) Å for 1, 0.494(2) Å for 2, 0.489(2) Å for 3, and 0.492(2) for 4, respectively, from the least-squares planes of the basal donor atoms, in the direction of the axial oxo ligands. The bond distances between the V atoms and the oxo O atoms indicate that they are typical double bonds. The corresponding bond

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distances and bond angles in the complexes are very close to each other, and also comparable to those observed in other oxovanadium (V) complexes with Schiff bases [25–27]. The measurement of Helicobacter pylori urease inhibitory activity was carried out for three parallel times. Helicobacter pylori (ATCC 43504; American Type Culture Collection, Manassas, VA) was grown in brucella broth supplemented with 10% heat-inactivated horse serum for 24 h at 37 °C under microaerobic conditions (5% O2, 10% CO2, and 85% N2). The method of preparation of H. pylori urease by Mao [28] was followed. Briefly, broth cultures (50 mL, 2.0×108 CFU mL− 1) were centrifuged (5000 g, 4 °C) to collect the bacteria, and after washing twice with phosphate-buffered saline (pH 7.4), the H. pylori precipitation was stored at −80 °C. H. pylori was returned to room temperature, and after addition of 3 mL of distilled water and protease inhibitors, sonication was performed for 60 s. Following centrifugation (15,000 g, 4 °C), the supernatant was desalted through SephadexG-25 column (PD-10 columns, Amersham–Pharmacia Biotech, Uppsala, Sweden). The resultant crude urease solution was added to an equal volume of glycerol and stored at 4 °C until use in the experiment. The mixture, containing 25 μL (4U) of H. pylori urease and 25 μL of the test compound, was pre-incubated for 3 h at room temperature in a 96-well assay plate. Urease activity was determined by measuring ammonia production using the indophenol method as described by Weatherburn [29]. The inhibition rates (%) with the concentration of 100 μM for the complexes are 82.32 ± 2.17 (1), 72.05 ± 2.64 (2), 64.50 ± 1.55 (3), and 48.91 ± 3.46 (4). The acetohydroxamic acid was used as a reference [6] with the inhibition rate (%) of 87.30 ± 3.35. The results are depicted in Fig. 5. The IC50 values (27.32 ± 2.10 μM for 1, 38.05 ± 2.03 μM for 2, 47.89 ± 3.34 μM for 3) were determined since they have strong urease inhibitory activities, which are superior or comparable to the acetohydroxamic acid with an IC50 value of 46.27 ± 0.73 μM, and also stronger than the vanadyl sulfate with an IC50 value of 207.13 ± 3.10 μM. Considering that the difference of the structures is mainly the terminal groups of the Schiff base ligands, viz. N,Ndimethyl for 1, N,N-diethyl for 2, piperidine for 3, and morpholine for 4, it is easy to conclude that the shorter the terminal groups of the Schiff base ligands are, the stronger the urease inhibitory activities of the oxovanadium(V) complexes. Molecular docking of the complexes into the three-dimensional X-ray structures of H. pylori urease structure (entry 1E9Z in the Protein Data Bank) was carried out using the AUTODOCK 4.0 software as implemented through the graphical user interface AUTODOCKTOOLS (ADT 1.4.6). The graphical user interface AUTODOCKTOOLS was employed to setup the enzymes: all hydrogens were added, Gasteiger charges were calculated and nonpolar hydrogens were merged to carbon atoms. The Ni initial parameters are set as r = 1.170 Å, q = + 2.0, and van der Waals well depth of 0.100 kcal/mol [30]. The 3D structures of ligand molecule were saved in Mol2 format with the aid of the program MERCURY. The partial charges of Mol2 file were further modified by


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Fig. 4. A perspective view of the molecular structure of 4 with the atom labeling scheme. The thermal ellipsoids are drawn at the 30% probability level. Selected bond lengths (Å) and bond angles (°): V1―O1 1.914(2), V1―O3 1.603(2), V1―O4 1.613(2), V1―N1 2.114(3), V1―N2 2.231(3), V2―O5 1.906(2), V2―O7 1.599(2), V2―O8 1.626(2), V2―N3 2.123(3), V2―N4 2.196(3); O3―V1―O4 110.1(1), O3―V1―O1 103.6(1), O4―V1―O1 96.2(1), O3―V1―N1 113.8(1), O4―V1―N1 135.3(1), O1―V1―N1 81.9(1), O3―V1―N2 94.0(1), O4―V1―N2 91.2(1), O1―V1―N2 157.1(1), N1―V1―N2 77.7(1), O7―V2―O8 110.3(1), O7―V2―O5 105.0(1), O8―V2―O5 97.9(1), O7―V2―N3 107.7(1), O8―V2―N3 140.5 (1), O5―V2―N3 82.2(1), O7―V2―N4 96.6(1), O8―V2―N4 87.9(1), O5―V2―N4 153.9(1), N3―V2―N4 77.4(1).

using the ADT package (version 1.4.6) so that the charges of the nonpolar hydrogen atoms would be assigned to the atom to which the hydrogen is attached. The resulting file was saved as pdbqt file. The AUTODOCKTOOLS program was used to generate the docking input files. In all docking a grid box size of 60 × 60 × 60 pointing in x, y, and z directions was built, the maps were centered on the Ni3001 atom in the catalytic site of the protein. A grid spacing of 0.375 Å and a distances-dependent function of the dielectric constant were used for the calculation of the energetic map. Ten runs were generated by using Lamarckian genetic algorithm searches. Default settings were used with an initial population of 50 randomly placed individuals, a maximum number of 2.5 × 106 energy evaluations, and a maximum number of 2.7 × 104 generations. A mutation rate of 0.02 and a crossover rate of 0.8 were chosen. The results of the most favorable free energy of binding were selected as the resultant complex structures. The enzyme surface models for the complexes 1–4 were shown in Figs. 6–9, which revealed that the complex molecules were well filled 100 90 80 70 60 50 40 30 20 10 0

in the active pocket of the urease. All the complexes form hydrophobic interactions with Ala169 and Ala365 of the urease, with binding energies of −5.9 eV for 1, −5.6 eV for 2, − 6.0 eV for 3, and −6.2 eV for 4, respectively. The result of the molecular docking study could explain the inhibitory activity of the complexes against urease. In summary, the present paper reports the synthesis, structures and urease inhibitory activities of a series of structurally similar oxovanadium(V) complexes with analogous Schiff bases. The structure–activity relationship indicates that the shorter the terminal groups of the Schiff base ligands are, the stronger the urease inhibitory activities of the oxovanadium(V) complexes. Considering that the oxovanadium complexes have interesting biological activities and

1 2 3 4 5

1

2

3

4

5

Fig. 5. Inhibition rate of the tested materials (100 μM) to the urease. 5 represents the acetohydroxamic acid.

Fig. 6. Binding mode of 1 with Helicobacter pylori urease. The enzyme is shown as Surface. The complex is shown as sticks.


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Appendix A. Supplementary Material Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre (CCDC — 805718 for 1, 805719 for 2, 805720 for 3, and 805721 for 4). Copies of this information can be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336 033; e-mail: deposit@ccdc. cam.ac.uk or www: http://www.ccdc.cam.ac.uk). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.inoche.2011.01.038. References



have been widely used in medicine [17–19,31,32], the complexes may be used in the treatment of infections caused by urease producing bacteria.

Acknowledgment This work was financially supported by the Natural Science Foundation of China (Project No. 20901036).


[1] P.A. Karplus, M.A. Pearson, R.P. Hausinger, 70 years of crystalline urease: what have we learned? Acc. Chem. Res. 30 (1997) 330–337. [2] J.B. Sumner, The isolation and crystallization of the enzyme urease, J. Biol. Chem. 69 (1926) 435–441. [3] L.E. Zonia, N.E. Stebbins, J.C. Polacco, Essential role of urease in germination of nitrogen-limited arabidopsis-thaliana seeds, Plant Physiol. 107 (1995) 1097–1103. [4] C.M. Collins, S.E.F. DÓrazio, Bacterial ureases — structure, regulation of expression and role in pathogenesis, Mol. Microbiol. 9 (1993) 907–913. [5] C. Montecucco, R. Rappuoli, Living dangerously: how Helicobacter pylori survives in the human stomach, Nat. Rev. Mol. Cell Biol. 2 (2001) 457–466. [6] B. Krajewska, I. Ureases, Functional, catalytic and kinetic properties: a review, J. Mol. Catal. B Enzym. 59 (2009) 9–21. [7] Z. Amtul, Atta-ur-Rahman, R.A. Siddiqui, M.I. Choudhary, Chemistry and mechanism of urease inhibition, Curr. Med. Chem. 9 (2002) 1323–1348. [8] W. Zaborska, M. Kot, K. Superata, Inhibition of jack bean urease by 1, 4benzoquinone and 2, 5-dimethyl-1, 4-benzoquinone. Evaluation of the inhibition mechanism, J. Enzym. Inhib, Med. Chem. 17 (2002) 247–253. [9] M.A. Pearson, L.O. Michel, R.P. Hausinger, P.A. Karplus, Structures of Cys319 variants and acetohydroxamate-inhibited Klebsiella aerogenes urease, Biochemistry 36 (1997) 8164–8172. [10] W. Zaborska, B. Krajewska, Z. Olech, Heavy metal ions inhibition of jack bean urease: potential for rapid contaminant probing, J. Enzyme Inhib. Med. Chem. 19 (2004) 65–69. [11] W. Zaborska, B. Krajewska, M. Leszko, Z. Olech, Inhibition of urease by Ni2+ ions — analysis of reaction progress curves, J. Mol. Catal. B Enzym. 13 (2001) 103–108. [12] A.Y. Louie, T.J. Meade, Metal complexes as enzyme inhibitors, Chem. Rev. 99 (1999) 2711–2734. [13] Z.-L. You, L. Zhang, D.-H. Shi, X.-L. Wang, X.-F. Li, Y.-P. Ma, Synthesis, crystal structures and urease inhibitory activity of copper(II) complexes with Schiff bases, Inorg. Chem. Commun. 13 (2010) 996–998. [14] Z.-L. You, L.-L. Ni, D.-H. Shi, S. Bai, Synthesis, structures, and urease inhibitory activities of three copper(II) and zinc(II) complexes with 2-{[2-(2-hydroxyethylamino) ethylimino]methyl}-4-nitrophenol, Eur. J. Med. Chem. 45 (2010) 3196–3199. [15] P. Hou, Z.-L. You, L. Zhang, X.-L. Ma, L.-L. Ni, Synthesis, crystal structures, and urease inhibitory activities of two azido-bridged polynuclear copper(II) complexes with Schiff bases, Transition Met. Chem. 33 (2008) 1013–1017. [16] R. Ara, U. Ashiq, M. Mahroof-Tahir, Z.T. Maqsood, K.M. Khan, M.A. Lodhi, M.I. Choudhary, Chemistry, urease inhibition, and phytotoxic studies of binuclear vanadium(IV) complexes, Chem. Biodivers. 4 (2007) 58–71. [17] M.J. Pereira, E. Carvalho, J.W. Eriksson, D.C. Crans, M. Aureliano, Effects of decavanadate and insulin enhancing vanadium compounds on glucose uptake in isolated rat adipocytes, J. Inorg. Biochem. 103 (2009) 1687–1692. [18] M.-J. Xie, X.-D. Yang, W.-P. Liu, S.-P. Yan, Z.-H. Meng, Insulin-enhancing activity of a dinuclear vanadium complex: 5-chloro-salicylaldhyde ethylenediamine oxovanadium (V) and its permeability and cytotoxicity, J. Inorg. Biochem. 104 (2010) 851–857. [19] J.J. Smee, J.A. Epps, K. Ooms, S.E. Bolte, T. Polenova, B. Baruah, L.Q. Yang, W.J. Ding, M. Li, G.R. Willsky, A. la Cour, O.P. Anderson, D.C. Crans, Chloro-substituted dipicolinate vanadium complexes: synthesis, solution, solid-state, and insulin-enhancing properties, J. Inorg. Biochem. 103 (2009) 575–584. [20] R. Huey, G.M. Morris, A.J. Olson, D.S. Goodsell, A semiempirical free energy force field with charge-based desolvation, J. Comput. Chem. 28 (2007) 1145–1152. [21] The yellow gummy product of the four Schiff bases were prepared by reaction of equimolar quantities of 2-hydroxy-1-naphthaldehyde with analogous primary amines in methanol and subsequent evaporation of the solvent. Yields: 93–98%. Anal. Calc. for C15H18N2O (HL1): C 74.4, H 7.5, N 11.6%. Found: C 74.1, H 7.6, N 11.7%. Anal. Calc. for C17H22N2O (HL2): C 75.5, H 8.2, N 10.4%. Found: C 75.3, H 8.2, N 10.3%. Anal. Calc. for C18H22N2O (HL3): C 76.6, H 7.8, N 9.9%. Found: C 76.8, H 7.9, N 9.7%. Anal. Calc. for C17H20N2O2 (HL4): C 71.8, H 7.1, N 9.8%. Found: C 71.5, H 7.3, N 9.8%. Selected IR data (KBr, cm-1): HL1, ν 1646 (s, C=N); HL2, ν 1647 (s, C=N); HL3 and HL4, ν 1648 (s, C=N). [22] A methanol solution (20 mL) of the Schiff base (0.5 mmol) was added with stirring to a methanol solution (20 mL) of VO(acac)2 (0.5 mmol, 0.133 g). The mixtures were stirred at room temperature for 30 min to give yellowish-green solutions. Yellowish-green block-shaped single crystals suitable for X-ray diffraction were formed by slow evaporation of the solutions in air for a few days. Yield: 73% for 1, 81% for 2, 87% for 3, and 72% for 4. For 1: Anal. Calc. for C15H17N2O3V: C 55.6, H 5.3, N 8.6%. Found: C 55.2, H 5.5, N 8.5%. IR data: 1624 (s), 1606 (m), 1543 (m), 1510 (w), 1461 (m), 1422 (m), 1395 (w), 1366 (w), 1343 (m), 1306 (w), 1281 (w), 1258 (w), 1194 (m), 1145 (m), 1096 (w), 1011 (w), 947 (s), 931 (s), 898 (w), 831 (m), 787 (w), 768 (m), 579 (m), 518 (w), 493

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(w), 419 (w), 334 (w). For 2: Anal. Calc. for C17H21N2O3V: C 58.0, H 6.0, N 8.0%. Found: C 57.7, H 6.1, N 8.3%. IR data: 1624 (s), 1609 (m), 1547 (m), 1508 (w), 1459 (m), 1434 (m), 1415 (w), 1399 (w), 1365 (w), 1346 (m), 1307 (w), 1261 (w), 1195 (m), 1149 (w), 1092 (w), 1034 (w), 983 (w), 935 (s), 873 (m), 839 (m), 747 (m), 647 (w), 573 (m), 503 (w), 475 (w), 443 (w), 414 (w), 315 (w). For 3: Anal. Calc. for C18H21N2O3V: C 59.3, H 5.8, N 7.7%. Found: C 59.6, H 5.9, N 7.5%. IR data: 1625 (s), 1607 (m), 1544 (m), 1509 (w), 1460 (m), 1434 (w), 1421 (m), 1396 (w), 1366 (w), 1344 (m), 1309 (w), 1271 (w), 1251 (w), 1190 (m), 1141 (m), 1029 (w), 947 (s), 831 (m), 750 (m), 622 (w), 577 (m), 519 (w), 501 (w), 420 (w), 338 (w), 314 (w). For 4: Anal. Calc. for C35H42N4O9V2: C 55.0, H 5.5, N 7.3%. Found: C 54.8, H 5.7, N 7.1%. IR data: 3367 (w), 1624 (s), 1606 (m), 1543 (m), 1457 (w), 1421 (m), 1395 (w), 1364 (m), 1343 (m), 1307 (w), 1264 (w), 1191 (m), 1143 (w), 1113 (m), 1051 (w), 1034 (w), 938 (s), 832 (m), 756 (m), 579 (w), 519 (w), 504 (w), 484 (w), 421 (w), 342 (w). [23] W.J. Geary, The use of conductivity measurements in organic solvents for the characterisation of coordination compounds, Coord. Chem. Rev. 7 (1971) 81–122. [24] The X-ray single crystal diffraction measurement was carried out at 298(2) K on a Bruker Smart 1000 CCD area diffractometer. The unit cell parameters and data collection was performed with MoKα (λ=0.71073 Å) radiation. The crystal data and structural parameters for 1: C15H17N2O3V, M = 324.2, monoclinic, space group P21/c, a= 13.418(4), b =9.852(3), c = 12.358(3) , β =114.752(3)o, V= 1483.7(7) 3, Z=4, ρcalcd=1.452 g cm-3, T=298(2) K, μ(Mo Kα)=0.679 mm–1, R1=0.0372, wR2=0.0922. The crystal data and structural parameters for 2: C17H21N2O3V, M=352.3, monoclinic, space group P21/c, a=12.821(3), b=6.902(2), c=17.842(3) , β=95.311(3)o, V=1572.1(6) 3, Z=4, ρcalcd=1.488 g cm-3, T=298(2) K, μ(Mo Kα)=0.648 mm–1, R1=0.0504, wR2=0.1103. The crystal data and structural parameters for 3: C18H21N2O3V, M = 364.3, monoclinic, space group P21/c, a=13.358(2), b=9.8474(16), c=12.597(2) , β=92.226(2)o, V=1655.8(5) 3,

[25]

[26] [27]

[28]

[29] [30] [31]

[32]

Z=4, ρcalcd=1.461 g cm-3, T=298(2) K, μ(Mo Kα)=0.618 mm–1, R1=0.0371, wR2=0.0908. The crystal data and structural parameters for 4: C35H42N4O9V2, M=764.6, triclinic, space group P-1, a=10.005(3), b=12.403(2), c=15.028(3) , α = 73.358(2)o, β = 73.968(2)o, γ = 89.021(2)o, V = 1713.4(7) 3, Z = 2, ρcalcd = 1.482 g cm-3, T = 298(2) K, μ(Mo Kα) = 0.607 mm–1, R1= 0.0575, wR2=0.1588. L.M. Mokry, C.J. Carrano, Steric control of vanadium(V) coordination geometry: a mononuclear structural model for transition-state-analog RNase inhibitors, Inorg. Chem. 32 (1993) 6119–6121. G. Romanowski, M. Wera, A. Sikorski, {2-[1-(2-Amino-2-methylpropylimino)ethyl] phenolato-κ3N, N′, O}dioxidovanadium(V), Acta Crystallogr. E65 (2009) m190. E. Kwiatkowski, G. Romanowski, W. Nowicki, M. Kwiatkowski, K. Suwinska, Chiral dioxovanadium(V) complexes with single condensation products of 1, 2-diaminocyclohexane and aromatic o-hydroxycarbonyl compounds: synthesis, characterization, catalytic properties and structure, Polyhedron 26 (2007) 2559–2568. W.J. Mao, P.C. Lv, L. Shi, H.Q. Li, H.L. Zhu, Synthesis, molecular docking and biological evaluation of metronidazole derivatives as potent Helicobacter pylori urease inhibitors, Bioorg. Med. Chem. 17 (2009) 7531–7536. M.W. Weatherburn, Phenol-hypochlorite reaction for determination of ammonia, Anal. Chem. 39 (1967) 971–974. B. Krajewska, W. Zaborska, Jack bean urease: the effect of active-site binding inhibitors on the reactivity of enzyme thiol groups, Bioorg. Chem. 35 (2007) 355–365. J.K. Jackson, W. Min, T.F. Cruz, S. Cindric, L. Arsenault, D.D. VonHoff, D. Degan, W.L. Hunter, H.M. Burt, A polymer-based drug delivery system for the antineoplastic agent bis(maltolato)oxovanadium in mice, Br. J. Cancer 75 (1997) 1014–1020. Z.H. Chohan, S.H. Sumrra, Some biologically active oxovanadium(IV) complexes of triazole derived Schiff bases: their synthesis, characterization and biological properties, J. Enzyme Inhib. Med. Chem. 25 (2010) 599–607.