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 synth esized and characterized. The reaction of [(VO)-O-IV(CH3COO)(2)(phen)] (phe n = 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 oxovanadi um(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)gl ycine (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 form ation of the compounds [Et3NH][VO (mpg) (phen)] (3), [VO(glygly)(phen)] -2C H(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 phe n in a methanolic solution. The X-ray structure of 3 shows that the vanadiu m(rv) atom is ligated to a tridentate mpg(3-) Ligand at the S-thiolato, N-p eptide and O-carboxylato atoms. The X-ray structure of 7 is also reported. The optical, infrared, magnetic, electron paramagnetic resonance, and elect rochemical properties of compounds 1-5 CH,OH and 7 were studied. Combinatio n of the correlation plots of the EPR parameters g(z) versus A(z), or the g round state orbital population (beta*)(2) versus A(z), together with the ad ditivity relationship, A(z,calcd) = Sigma n(i)A(zi)/4, were shown to provid e 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 valu es. Model NMR studies (interaction of vanadium(v) with H(3)mpg) showed that there is a possibility of vanadium(v) ligation to glutathione. The contrib ution of a deprotonated peptide(amide) nitrogen to A, is not a fixed quanti ty tit varies from 29 to 43 x 10(-4) cm(-1)), but is influenced by the pres ence df the three other donor atoms in the equatorial plane and, in particu lar, their charge.