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 (Ala 3 ), 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 Ala 3 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 Ala 3 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.