Abstract

The complexation of the oxovanadium(IV) ion with five dipeptides containing L-histidine or L-cysteine (GlyHis, HisHis, HisGly, CysGly, GlyCys) was studied. L-histidinamide (HisNH₂) was assumed as a model system for dipeptides with L-histidine in the N-terminal position. The study was performed in aqueous solution through the combined application of potentiometric and spectroscopic (electronic absorption and EPR) techniques. The results indicate that simple dipeptides lacking a strong anchoring group can form mono- and bischelated complexes with the V^IVO ion if a suitable donor is present in the chain. The ligands behave like amino acids in the acidic and neutral pH range, inhibit the precipitation of hydroxides and suppress the formation of hydrolytic species if at least a fivefold molar excess of ligand is used. In alkaline media all the ligands, except CysGly, promote the deprotonation and N-coordination of the amide group. CysGly forms a bischelated complex with a [2 (NH₂, S^-)] donor set. The contribution of the deprotonated amide group to the ⁵¹V hyperfine coupling constant, A_z, as a function of the total equatorial charge of oxovanadium(IV) ion, is discussed. The results have general validity and are useful to predict the geometry and donor set of complexes involving the bonding of the V^IVO ion to the deprotonated amide group.

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