VIBRATIONAL STUDIES, NORMAL-COORDINATE ANALYSIS, AND INFRARED VCD OF ALANYLALANINE IN THE AMIDE-III SPECTRAL REGION

Detailed vibrational assignments and normal coordinate calculations for L-alanyl-L-alanine and L-alanyl-D-alanine and several deuteriated isotopomers in the spectral region between 1200 and 1700 cm-1 are reported. In this spectral region, the peptide amide I, II, and III vibrations occur. Large infrared vibrational circular dichroism (VCD) intensities are reported for the amide III vibration. These studies were undertaken to obtain a description correlating the atomic displacements to strong VCD intensities. We demonstrate that large VCD signals occur in either delocalized, coupled C-H/N-H deformations, or in C*-H deformations which are similar to those which produce large VCD in alanine itself.