Oxovanadium(IV) complexes of peptides with non-co-ordinating side chains and related ligands; a spectroscopic study

Abstract

The systems VO ²⁺ + 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 = alanine) have been studied in the range pH 1.5–13 by a combination of spectroscopic methods (ESR, circular dichroism and visible absorption). Very extensive hydrolysis and precipitation of [VO(OH) ₂ ] occurs at pH > 4–5, even when using a L∶M ratio of 180∶1. Plausible isomeric structures are discussed and for dipeptides N _amide deprotonation/co-ordination occurs at pH > ≈7. Several hydrolysis products (monomeric and oligomeric) are formed at higher pH values. Spectroscopic studies 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 glycylglycylglycine with VO ²⁺ . For tripeptides, optically inactive oligomeric complexes are the major species; there is no evidence of amide deprotonation/co-ordination.