Structural Feature and Molecular Interaction of Basic Amino Acid-Picric Acid Complexes by X-Ray Crystal Analyses.

Abstract

As a part of elucidating the structural features of a host molecule necessary for the recognition of basic amino acids, the crystal structures of the picrates of DL-arginine (1), L-arginine (2), L-lysine (3) and L-ornitine (4) have been determined by X-ray analyses. The molecular packing pattern is found to be common in these crystal structures. The picric acids themselves form layers perpendicular to a crystallographic axis, and respective basic amino acids are packed into these layers, where the amino acids themselves also form singly or doubly arranged layers in a head-to-tail fashion and are stabilized by hydrogen bonds between the α-carboxyl and guanidyl or terminal amino groups. The picric acid interacts with the amino acid by three to seven NH(amino acid)...O(picric acid) hydrogen bonds. A notable feature of the molecular interaction commonly found in complexes I-A is the simultaneous fixation at three portions of the amino acid (α-amino, α-carboxyl and terminal amino or guanidyl groups) by a hydrogen bond and/or electrostatic interactions. In conclusion, it was shown that both the crystal packing and the molecular interaction modes are important for the complex formation between the basic amino acid and picric acid molecules