Contribution of weak S-H · · · O hydrogen bonds to the side chain motions in D,L-homocysteine on cooling.

Sulfhydryl groups play an important role in the formation of native structures of proteins and their biological functions. In the present work, we report for the first time the crystal structure of D,L-homocysteine and the results of a detailed study of the dynamics of its sulfhydryl group on cooling by precise single-crystal X-ray diffraction combined with polarized Raman spectroscopy of oriented single crystals. Although the crystal structures of both D,L-cysteine and D,L-homocysteine are layered, hydrogen bonds formed by -SH groups differ. In contrast with the crystal structure of D,L-cysteine with weak S-H · · · S hydrogen bonds between layers, D,L-homocysteine resembles the structures of amino acids with hydrophobic aliphatic side chains with no hydrogen bonds between the layers. The side chain of D,L-homocysteine forms a three-centered S-H · · · O hydrogen bond with carboxylate groups of two neighboring zwitterions. On cooling down, despite the shortening of the two S · · · O distances in the bifurcated S-H · · · O hydrogen bond, the wavenumber of the stretching vibrations of -SH groups increases. The same effect was also observed previously for other sulfhydryl containing amino acids, L-cysteine, and N-acetyl-L-cysteine on increasing pressure and is related to the strengthening of a three-centered bifurcated S-H · · · O hydrogen bond.