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Diperoxovanadate is effective only in presence of free vanadate in vanadium-dependent bromoperoxidation at physiological pH. Peroxide in the form of bridged divanadate complex (VOOV-type), but not the bidentate form as in diperoxovanadate, is proposed to be the oxidant of bromide. In order to obtain direct evidence, peroxo-divanadate complexes with glycyl-glycine, glycyl-alanine and glycyl-asparagine as heteroligands were synthesized. By elemental analysis and spectral studies they were characterized to be triperoxo-divanadates, [V2O,(O2)3(peptide)3] x H2O, with the two vanadium atoms bridged by a peroxide and a heteroligand. The dipeptide seems to stabilize the peroxo-bridge by inter-ligand interaction, possibly hydrogen bonding. This is indicated by rapid degradation of these compounds on dissolving in water with partial loss of peroxide accompanied by release of bubbles of oxygen. The 51V-NMR spectra of such solutions showed diperoxovanadate and decavanadate (oligomerized from vanadate) as the products. Additional oxygen was released on treating these solutions with catalase as expected of residual diperoxovanadate. The solid compounds when added to the reaction mixtures showed transient, rapid bromoperoxidation reaction, but not oxidation of NADH or inactivation of glucose oxidase, the other two activities shown by a mixture of diperoxovanadate and vanadyl. This demonstration of peroxide-bridged divanadate as a powerful, selective oxidant of bromide, active at physiological pH, should make it a possible candidate of mimic in the action of vanadium in bromoperoxidase proteins.

Novel dinuclear peroxo complexes of tungsten with coordinated cystine of the type A(2)[W(2)O(3)(O(2))(4)(cystine)].4H(2)O, A = Na (1) or K (2) have been synthesized from the reaction of A(2)WO(4,)cysteine and 30% H(2)O(2)at pH 2.5. The synthesized compounds were characterized by elemental analysis, spectral and physico-chemical methods. The two W(VI) centres with side-on bound peroxo groups of the dinuclear complex species are bridged by an oxo group and a cystine ligand, formed from the oxidation of cysteine. Cystine occurring as zwitterion binds the metal centers of the complex ion through O(carboxylate) atoms leading to hepta co-ordination around each W(VI). The compounds exhibit high stability toward decomposition in solution of acidic as well as physiological pH and serve as weak substrates to catalase, undergoing degradation in presence of the enzyme at a rate much slower relative to H(2)O(2). The compounds efficiently oxidized GSH to GSSG, a reaction in which only two of the peroxide groups of the complex species were found to participate. The compounds induce strong inhibitory effect on alkaline phosphatase activity with a potency higher than that of the free cystine, tungstate, or peroxotungstate.

A set of new diperoxovanadate(V) complexes with the dipeptides glycyl-glycine or glycyl-leucine as ancillary ligands of the type, A[VO(O 2 ) 2 (peptide)(H 2 O)] Á H 2 O, A = Na or K have been synthesized and characterized by elemental analysis, thermal analysis, magnetic susceptibility and spectral studies. The complexes contain side-on bound peroxo groups and a dipeptide zwitterion as co-ligand, binding the metal center unidentately through O (carboxylate) atom. The compounds are highly stable toward decomposition in solutions of acidic as well as physiological pH and serve as weak substrates to catalase, undergoing degradation in the presence of the enzyme at a rate much slower than H 2 O 2 . The compounds stoichiometrically oxidize GSH to GSSG. On comparing the GSH oxidizing ability of these compounds with those of previously reported peroxotungsten compounds containing similar co-ligands, a significant difference was noted. The compounds induce a strong inhibitory effect on alkaline phosphatase activity with a potency higher than that of the free peptides, vanadate, or peroxovanadate.