Synthetic analogues for oxovanadium(IV)-glutathione interaction: An EPR, synthetic and structural study of oxovanadium(IV) compounds with sulfhydryl-containing pseudopeptides and dipeptides

Valuable analogues of the (VO2+)-O-IV - glutathione complex have been synthesized and characterized. The reaction of [(VO)-O-IV(CH3COO)(2)(phen)] (phen = 1,10-phenanthroline) with the sulfhydryl-containing pseudopeptides (scp) N-(2-mercaptopropionyl)cysteine (H(4)m(2)pc), and N-(3-mercaptopropionyl) cysteine (H(4)m(3)pc) in the presence of triethylamine gives the oxovanadium(rv) compounds [(Et3NH)(2)][VO(m(2)pc)] (1) and [(Et3NH)(2)][VO(m(3)pc)] (2), while reaction of [VOCl2(phen)] with the sep N-(2-mercaptopropionyl)glycine (H(3)mpg) and the dipeptides glycylglycine (H(3)glygly) and glycyl-(L)-alanine (H(2)glyala) in the presence of triethylamine results in the formation of the compounds [Et3NH][VO (mpg) (phen)] (3), [VO(glygly)(phen)] -2CH(3)OH (4 2CH(3)OH), and [VO(glyala)(phen)] CH,OH (5 CH,OH). Complex [VOCl2(phen)(CH3OH)] (7) was prepared by the reaction of [VOCl2(thf)(2)] with phen in a methanolic solution. The X-ray structure of 3 shows that the vanadium(rv) atom is ligated to a tridentate mpg(3-) Ligand at the S-thiolato, N-peptide and O-carboxylato atoms. The X-ray structure of 7 is also reported. The optical, infrared, magnetic, electron paramagnetic resonance, and electrochemical properties of compounds 1-5 CH,OH and 7 were studied. Combination of the correlation plots of the EPR parameters g(z) versus A(z), or the ground state orbital population (beta*)(2) versus A(z), together with the additivity relationship, A(z,calcd) = Sigma n(i)A(zi)/4, were shown to provide a powerful tool for probing the equatorial donor atoms in an oxovanadium(Iv) compound and consequently in biomolecules. Thus, these methods provide valuable evidence for the assignment of the equatorial donor atoms for the (VO2+)-O-IV center of the (VO2+)-O-IV-glutathione system at various pH values. Model NMR studies (interaction of vanadium(v) with H(3)mpg) showed that there is a possibility of vanadium(v) ligation to glutathione. The contribution of a deprotonated peptide(amide) nitrogen to A, is not a fixed quantity tit varies from 29 to 43 x 10(-4) cm(-1)), but is influenced by the presence df the three other donor atoms in the equatorial plane and, in particular, their charge.