Vibrational CD studies of the solution conformation of simple alanyl peptides as a function of pH

The CH-stretching vibrational CD (VCD) spectra of glycyl-L-alanine, L-alanylglycine, and L-alanyl-L-alanine have been studied at neutral, high, and low pH in D2O solution. The intense positive VCD band attributed to the C alpha H stretch of the alanyl residue in glycyl-L-alanine at neutral pH is absent in L-alanylglycine. In contrast to the VCD spectra of L-alanine, the positive methine-stretching VCD band in glycyl-L-alanine and L-alanyl-L-alanine is still present at pH 2. Based on the ring current mechanism, the VCD spectra are consistent with the presence of a five-membered CO...HN intramolecular hydrogen-bonded ring between the C-terminal carboxylate and peptide NH groups at neutral and high pH; and a seven-membered COH...O = C hydrogen-bonded ring between the C-terminal carboxyl OH and peptide C = O groups at low pH. In the N-terminal alanyl residue, the peptide C = O group is hydrogen bonded to the NH trans to the methine bond. The CH-stretching VCD spectra of L-alanyl-L-alanyl-L-alanine at neutral pH are consistent with two intramolecularly hydrogen-bonded conformations for the central alanyl residue.