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 D 2 O solution. The intense positive VCD band attributed to the C α 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 … OC hydrogen‐bonded ring between the C‐terminal carboxyl OH and peptide CO groups at low pH. In the N‐terminal alanyl residue, the peptide CO 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.