Proton magnetic resonance spectra of amino-acids and peptides relevant to wool structure. Part II. Relative residence times of sulphur-containing α-amino-acids

Abstract

High resolution ¹H n.m.r. spectra at 60 MHz of alkaline deuterium oxide solutions of eight sulphur-containing amino-acids have been examined over the temperature range 280–355 K. Iterative analyses of the ABC spin systems of the methylene and methine resonances yield coupling constants from which apparent fractional populations of the individual rotamers have been calculated. For djenkolic acid there is a slight tendency for equalization of population with increase in temperature; for cysteine sulphinic acid and, to a small extent, S-carboxymethyl-cysteine, the dominance of one or two rotamers increases with temperature; for lanthionine, cysteine, and S-benzylcysteine, the relative lifetimes of the three rotamers are almost unchanged; while in the case of S-methyl- and S-allyl-cysteine, the dominant rotamers alter with temperature. For six sulphur-containing amino-acids examined in acidic solution at 303 K, the coupling constants derived are remarkably consistent; the geminal coupling, J_AB, is ca. –15·0 Hz (modulus about 2 Hz larger than for alkaline solution), and the vicinal couplings, J_AC and J_BC, are ca. 7·9 and 4·4 Hz, respectively.