Occurrence and geometrical features of head-to-tail sequences involving amino acids in crystal structures

A careful study of the crystal structures of commonly occurring amino acids, and their racemates and complexes reveals that each hydrogen bond connecting the alpha-amino and the alpha-carboxylate groups and its symmetry equivalents generally give rise to an infinite head-to-tail sequence in which the two groups are periodically brought into close proximity. Such sequences, which have earlier been suggested to be of probable relevance to prebiotic polymerisation, appear to be an almost universal feature of amino acid aggregation in the solid state. These sequences belong to two main categories in terms of the geometrical arrangement of amino acid molecules in them. The sequences in the first category consist of straight chains of molecules related mostly by the shortest cell translation in the crystals. The sequences of the second category form hydrogen bonded two fold helices centred around crystallographic 2(1) screw axes. The sequences can be further sub-divided into different types on the basis of the geometrical features of the hydrogen bonds involved in them. A few sequences involving both L and D isomers have also been observed in the crystal structures of some DL-amino acids. The shortest cell translation in most crystals under consideration has a value in the neighbourhood of 5.3 A and corresponds to the periodicity of a straight head-to-tail sequence or, less frequently, that of a helical sequence or both. The crystal structures of amino acids and their complexes can be classified in terms of the occurrence and the geometrical disposition of different types of head-to-tail sequences in them.