Contribution
of Weak S–H···O
Hydrogen Bonds to the Side Chain Motions in d,l-Homocysteine
on Cooling
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Abstract
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 <i>N</i>-acetyl-l-cysteine on increasing pressure and is related
to the strengthening of a three-centered bifurcated S–H···O
hydrogen bond