Unequal populations of amino acid and tripeptide rotamers from 220 MHz PMR spectra

Abstract

The 220 MHz high-resolution PMR spectra in acidic aqueous solution of two amino acids, l-aspartic acid and l-cysteine, enable earlier analyses of deceptively simple ABX five-line spectra to be corrected. Use of a deuteriated analogue facilitates unequivocal assignment of fractional populations to rotamers about the C αC β bond of aspartic acid. Despite A 2X-like spectra at 60 MHz, the rotamer populations are unequal with a preponderance of the rotamer with the bulky groups gauche. Deceptive simplicity in some of the 220 MHz spectra of the tripeptides, Gly-L-Phe-L-Ala, L-Met-L-Phe-L-Met, L-Met-L-Phe-Gly, Gly-L-Tyr-Gly and L-Val-L-Tyr-L-Val is not indicative of equal rotamer populations; the least favoured rotamer is that with the aromatic ring gauche to both peptide bonds.