The mean conformation of N-acetylamino acid N'-methylamides in dimethyl sulfoxide solution

The mean conformation of a series of N-acetylamino acid N'-methylamides in dimethyl sulfoxide (DMSO) was determined by 1H-NMR and 13C-NMR methods.The series investigated consisted of derivatives of DL-Ala, DL-Asn, Gln, His, DL-Ile, DL-Leu, Met, DL-Pro, DL-Phe, Ser, Thr, Trp, Tyr, DL-Val, DL-Nva, and dl-Nle. It was found that the conformational equilibria in DMSO are dominated by C5 and C7eq forms.The amounts of these forms in the equibbria were found to be roughly proportional to the Boltzmann probabilities for the occurrence of a definite form, as calculated theoretically by Vasquez et al.(Macromolecules 16 (1983) 1043).Exceptions to this rule were DL-Pro and, to a lesser extent, Ser, Asn and Trp derivatives.