Amino-Acids, their Hydrochlorides and Salts, and Amido-Acids

Abstract

Edsall’s studies on the Raman spectra of amino-acids provided the first direct optical evidence [1, 2, 60] for the dipolar structure of these materials, and his findings have been fully confirmed by similar later studies [3–5]. The first infra-red studies were made by Freymann, Freymann and Rumpf [6] working in the overtone region on the NH stretching frequencies, and they also obtained evidence for the presence of a quaternary nitrogen atom in the neutral amino-acids. Further data in the fundamental region were obtained by a number of other workers, mostly in studies in the 3000 cm⁻¹ region [7–10]. Wright [11, 12] was also amongst the early workers in this field, and he was the first to point out that the spectrum of the racemic form of cystine in the solid state differs from that of either of the pure optical isomers. This fact has been fully substantiated by later workers on other amino-acids [13, 14], and it appears to be true also of meso-tartaric acid [15]. Some fifty pairs of d- and l-forms of amino-acids have been studied by Koegel et al. [32]. As with the polypeptides examined by Ellenbogen [33], the individual d- and l-isomers have identical spectra, but these can differ appreciably from those of the racemate. When two asymmetric carbon atoms are present, as in normal and allo-forms, differences become possible between the spectra of the optical isomers.