IR vibrational CD in alanyl tripeptide: indication of a stable solution conformer.

Infrared vibrational CD (VCD) of a small peptide, L-alanyl-L-alanyl-L-alanine (Ala3), and a peptide model, N-acetyl-L-alanine-N'-methyl-amide (AAMA), in the 1550-1750-cm-1 region has been observed. The "coupled oscillator" VCD feature observed for Ala3 in the amide I region is interpreted in terms of a solution structure stabilized by the electrostatic interaction of the zwitterionic groups. No such interactions are possible in basic aqueous solution of Ala3 nor in AAMA in neutral solution. Thus, the coupled oscillator features are lost in the latter two cases, indicating the absence of a simple stabilized conformation.