OXOVANADIUM(IV) COMPLEXES OF PEPTIDES WITH NONCOORDINATING SIDE-CHAINS AND RELATED LIGANDS - A SPECTROSCOPIC STUDY

The systems VO2+ + L (L = Gly-Gly, Gly-Gly-Gly, L-Ala-Gly, Gly-L-Ala, L-ALa-L-Ala, Gly-Gly-L-Ala or L-Ala-Gly-Gly; Gly = glycine, Ala = alan ine) have been studied in the range pH 1.5-13 by a combination of spec troscopic methods (ESR, circular dichroism and visible absorption). Ve ry extensive hydrolysis and; precipitation of [VO(OH2] occurs at pH > 4-5, even when using a L:M. ratio of 18.0:1. Plausible isomeric struct ures are discussed and for dipeptides N-amide deprotonation/co-ordinat ion occurs at pH > approximate to 7. Several hydrolysis products (mono meric and oligomeric) are formed at higher pH values Spectroscopic stu dies with Gly-Sar (glycyl-N-methylglycine), Gly-L-Pro. (proline) and N -acetyl-L-alanine support the proposed co-ordination : geometries. The results-are compared with those for glycylglycine and glycylglycylgly cine with VO2+. For tripeptides, optically inactive oligomeric complex es are the major species; there is no evidence of amide deprotonation/ co-ordination.