NORMAL-MODE ANALYSIS OF ALANINE DIPEPTIDE IN THE CRYSTAL CONFORMATION USING A SCALED ABINITIO FORCE-FIELD

In a previous work (T.C. Cheam, J. Mol. Struct., 274 (1992) 289), scale factors for the ab initio 4-21 basis force constants of glycine dipeptide were refined to fit the IR data of Koyama and Shimanouchi (Biopolymers, 6 (1968) 1037) on the crystalline material and four deuterated derivatives. These scale factors have now been applied to the ab initio 4-21 force constants of a model complex of alanine dipeptide (ALA) hydrogen bonded to four water molecules. The optimized conformation of the ALA molecule, with (phi, psi) = (-83.6-degrees, 155.0-degrees), was close to that in the crystal. The normal modes given by the scaled force field were compared with the Raman and IR spectra of polycrystalline ALA; the IR intensities were also computed. The agreement between observed and calculated frequencies was very satisfactory, with an average error of 9 cm-1 in the 200-1800 cm-1 region. An essentially complete assignment was made of the modes above 200 cm-1. It has therefore been demonstrated that the refined scale factors can be transferred to other hydrogen-bonded peptides, allowing reliable spectral predictions with a relatively small basis set.