Crystal structures of hydrophobic amino acids .1. redeterminations of L-methionine and L-valine at 120 K

The crystal structures of L-methionine and L-valine have been redetermined by X-ray diffraction methods at 120 K. Both compounds crystallize in the monoclinic space group P2(1), Z=4, with lattice parameters a=9.493(2), b=5.201(2), c=14.831(3) Angstrom, beta=99.84(2)degrees for L-Met and a=9.682(2), b=5.247(1), c=11.930(2) Angstrom, beta=90.57(1)degrees for L-Val. The least squares refinements converged at R=0.037, R(W)=0.038 for L-Met and R=0.034, R(W)=0.038 for L-Val. The structure factors for L-Met were empirically corrected for absorption. The overall geometries agree with the original structure determinations, but the standard deviations for geometric parameters have been reduced by a factor between 4 and 10. Also, positional parameters for all hydrogen atoms have been refined, allowing a detailed study of the hydrogen bond patterns.