Vibrational spectra and crystal structure of the di-amino acid peptide cyclo(L-Met-L-Met): comparison of experimental data and DFT calculations

Experimental Raman and FT-IR spectra of solid-state non-deuterated and N-deuterated samples of cyclo(L-Met-L-Met) are reported and discussed. The Raman and FT-IR results show characteristic amide I vibrations (Raman: 1649 cm(-1), infrared: 1675 cm(-1)) for molecules exhibiting a cis amide conformation. A Raman band, assigned to the cis amide 11 vibrational mode, is observed at similar to 1493 cm(-1) but no IR band is observed in this region. Cyclo(L-Met-L-Met) crystallises in the triclinic space group P1 with one molecule per unit cell. The overall shape of the diketopiperazine (DKP) ring displays a (slightly distorted) boat conformation. The crystal packing employs two strong hydrogen bonds, which traverse the entire crystal via translational repeats. B3-LYP/cc-pVDZ calculations of the structure of the molecule predict a boat conformation for the DKP ring, in agreement with the experimentally determined X-ray structure. Copyright (C) 2009 John Wiley & Sons, Ltd.