Raman Spectra of Amino Acids and Related Compounds I. The Ionization of the Carboxyl Group

Abstract

Raman spectra have been determined for four amino acids and their hydrochlorides; for three fatty acids and chloracetic acid, and their sodium salts; and for several related compounds. The influence of the solvent water on the Raman spectra of aqueous solutions is discussed. The ``carbonyl'' frequency, lying near 1670 cm<sup>—1</sup> in the pure fatty acids, shifts to 1720 cm<sup>—1</sup> when they are dissolved in water. No shift is found for the corresponding frequency in methyl acetate or acetone. The presence of a charged NH<sub>3</sub><sup>+</sup> group on the carbon α to the carboxyl increases this frequency by about 20 cm<sup>—1</sup>. On ionization of the carboxyl group it is found that: (1) The ``carbonyl'' frequency vanishes in all cases investigated. The behavior of the amino acids in this respect is entirely consistent with their structure as zwitterions. (2) A group of lines in the region 1200–1420 cm^—1 undergoes characteristic changes in position and intensity. (3) In most cases there is a powerful line in the region 750–930 cm^—1 whose frequency increases by 20–40 cm^—1 on ionization. The frequency of this line is decreased by about 50 cm^—1 for each additional methyl group on the carbon atom adjoining the carboxyl. (4) Ionization markedly decreases the C–H frequency in formic acid, and also certain strong frequencies in methyl and ethyl amine.