Conformational aspects of L -histidine

Ab initio molecular orbital (MO) calculations at the STO-3G level are reported for selected conformations of L-histidine. The minimum energy conformations are chosen on the basis of force-field calculations on the χ1–χ2,1 plane of the side chain of histidine. The MO relative energies are observed to be substantially larger than the force-field values. With one exception both theoretical methods are consistent with the results of X-ray diffraction studies on L-histidine. The exception occurs in the case of the force-field calculations which suggest that the (60,80) conformation may be accessible contrary to the MO results and experiment. And finally the ab initio charge distribution is used to account for the fact that the Nε2 atom is always protonated in the crystal structures of L-histidine.