A 1H nuclear magnetic resonance study of the interactions of vancomycin with N-acetyl-D-alanyl-D-alanine and related peptides

Abstract

We have assigned the 270 MHz ¹H n.m.r. spectrum of vancomycin hydrochloride in aqueous solution and measured the changes in chemical shifts caused by its complexation with N-acetyl-D-Ala, N-acetyl-Gly-D-Ala, N-acetyl-D-Ala-D-Ala, and diacetyl-L-Lys-D-Ala-D-Ala. We have also measured the ¹H chemical shifts for the complex of diacetyl-L-Lys-D-Ala-D-Ala and vancomycin in [²H]dimethyl sulphoxide where the NH protons can be directly observed. The observed shifts strongly support the model proposed by Sheldrick and his co-workers for D-Ala-D-Ala peptide binding to vancomycin. On peptide binding we observe large downfield chemical shifts for the NH proton in the Ala-Ala peptide bond and also for the vancomycin NH proton labelled c₄: this is strong evidence for the proposed hydrogen bonds involving these protons in the model. Ring-current chemical-shift calculations of the shifts in the bound peptides are consistent with the proposed model. We also present evidence for some induced conformational changes in the vancomycin molecule when the peptides bind. In particular, the flexible side-chain of the N-methyl-leucine residue probably undergoes considerable conformational rearrangement to allow the positively charged N-methyl group to interact favourably with the terminal carboxylate anion of the bound peptide.