Precision Neutron Diffraction Structure Determination of Protein and Nucleic Acid Components. IV. the Crystal and Molecular Structure of the Amino Acid L-histidine.

A neutron diffraction study of L-valine · HCl has been carried out: space group P21, a = 10.382(2), b = 7.066(1), c = 5.4407(9) Å, β = 91.40(2)°, Z = 2. The structure has been refined by full-matrix least-squares techniques with anisotropic temperature factors for all atoms and with a Type II anisotropic extinction correction, leading to a conventional R value of 0.031. All hydrogen atoms have been located with a precision of 0.005 Å. The structure is stabilized by a three-dimensional network of one O–H … Cl and three N–H … Cl hydrogen bonds, one for each hydrogen atom that is expected to participate in hydrogen bonding. The potential energy barrier for torsional motion of the ammonium group is estimated to be 6.4 kcal mole−1, or about 3 times larger than those found for the methyl groups. This difference reflects the effects of hydrogen bonding.