Vibrational analysis of cyclo(-Ala--Ala) in two crystalline forms. Effect of structure on peptide group and CH modes

Abstract

Cyclo(-Ala--Ala) has been found by X-ray diffraction to crystallize in two forms with different hydrogen bond patterns and strengths and different conformations of the C[alpha]H[alpha]C[beta]H3 group. We have done a comprehensive analysis of the Raman and i.r. spectra of both forms and their N-deuterated derivatives. Spectra were taken of oriented single crystals, polycrystalline powders, and aqueous solutions, allowing a virtually complete vibrational assignment. Systematic, distinct differences were observed in the modes of the CONH group and in the CH stretching and bending modes. These spectral differences have been correlated with the different molecular and crystal structures. In particular, the width and sub-band structure of the NH stretch mode and the splittings of the CO stretch are shown to be related to the hydrogen bond pattern, and the NH bend modes are found to be relatively as sensitive to the hydrogen bond strength as is the NH stretch. The differences in the CH[alpha] bend modes show that both conformations exist in solution. Ab initio molecular orbital calculations have been done to understand the frequency shifts of the NH and CH[alpha] stretch modes in the two forms. Normal mode calculations were also done.