X-ray studies on crystalline complexes involving amino acids and peptides. Part XIV. Closed conformation and head-to-tail arrangement in a new crystal form of L-histidine L-aspartate monohydrate

Abstract

A new form of L-histidine L-aspartate monohydrate crystallizes in space group P2₁ with a = 5·131(1), b = 6·881(1), c= 18·277(2) Å, β= 97·26(1)° and Z = 2. The structure has been solved by the direct methods and refined to an R value of 0.044 for 1377 observed reflections. Both the amino acid molecules in the complex assume the energetically least favourable allowed conformation with the side chains staggered between the α-amino and α-carboxylate groups. This results in characteristic distortions in some bond angles. The unlike molecules aggregate into alternating double layers with water molecules sandwiched between the two layers in the aspartate double layer. The molecules in each layer are arranged in a head-to-tail fashion. The aggregation pattern in the complex is fundamentally similar to that in other binary complexes involving commonly occurring L amino acids, although the molecules aggregate into single layers in them. The distribution of crystallographic (and local) symmetry elements in the old form of the complex is very different from that in the new form. So is the conformation of half the histidine molecules. Yet, the basic features of molecular aggregation, particularly the nature and the orientation of head-to-tail sequences, remain the same in both the forms. This supports the thesis that the characteristic aggregation patterns observed in crystal structures represent an intrinsic property of amino acid aggregation.