ANALYSIS OF VIBRATIONAL STRUCTURE IN NEAR-ULTRAVIOLET CIRCULAR DICHROISM AND ABSORPTION SPECTRA OF PHENYLALANINE AND ITS DERIVATIVES

The circular dichroism (CD) and absorption of L-phenylalanine, N-acetyl-L-phenylalanine amide, and N-acetyl-L-phenylalanine methyl and ethyl esters were recorded at 298 and 77 °K. The signs and intensities of the CD bands varied greatly among these compounds. However, all CD spectra revealed similar vibrational perturbations arising from the benzyl moiety. 0-0 and 0 + 520 cm-1 transitions occurred in both CD and absorption spectra, whereas a 0 + 180 cm-1 band appeared only in CD. These three transitions started progressions with 930-cm-1 spacing to shorter wavelengths. The 930-cm-1 vibration did not affect either the sign or intensity of CD. In contrast, the 180- and 520-cm-1 vibrations always altered the CD intensity relative to that of the 0-0 band. In many cases, these vibrations also reversed the CD sign. By using toluene as a model for the benzyl chromophore and assuming a local symmetry of C2v, the symmetry species of the phenylalanine vibrational modes were identified: 930 cm-1, A1; 520 cm-1, B2; 180 cm-1, either A2 or B1. Thus only the nontotally symmetrical vibrations appear to alter the CD of phenylalanine compounds. The total rotatory strength increased three- to eightfold when the temperature was reduced from 298 to 77 °K. This finding suggests that a, conformational equilibrium exists at 298°K. © 1969, American Chemical Society. All rights reserved.