Proton magnetic resonance spectra of amino-acids and peptides relevant to wool structure. Part III. Relative residence times of dipeptides of asparagine, aspartic acid, phenylalanine, and tyrosine
Iterative analyses have been carried out of the methine–methylene ABC spin systems in 60 MHz proton magnetic resonance (1H n.m.r.) spectra, over the temperature range 283–353 K, of alkaline deuteriated aqueous solutions of aspartic acid and asparagine and of dipeptides of aspartic acid, asparagine, tyrosine, and phenylalanine with glycine and alanine. With increase of temperature, the fractional rotamer populations, deduced from the observed averaged vicinal coupling constants, exhibit three kinds of behaviour: invariance in aspartic acid, asparagine, glycyltyrosine and alanyltrosine; a tendency towards equalization in glycylphenylalanine and alanylphenylalanine and, to some extent, in glycylaspartic acid and alanylaspartic acid; and a greater preference for one rotamer in tyrosine. In phenylalanine, increasing concentration of base causes linear downfield shifts of the methine and methylene resonances, while changes in coupling constants correspond to steady changes in relative rotamer populations.