Molecular self-assembly in substituted alanine derivatives: XRD, Hirshfeld surfaces and DFT studies

The molecular assemblage in the crystal structures of three modified chiral amino acids, two of which are isomeric d- and l-pairs boc-l-benzothienylalanine (BLA), boc-d-benzothienylalanine (BDA) and the other boc-d-naphthylalanine (NDA) differing from this pair very slightly in the chemical modification introduced, is accurately described. The aggregation of amino acid molecules is similar in all the crystals and may be described as a twisted double helical ladder in which two complementary long helical chains formed through O—H ⋯ O hydrogen bonds are interconnected through the characteristic head-to-tail N—H ⋯ O hydrogen bonds. Thus the molecular aggregation enabled through classical hydrogen bonds may be regarded as a mimic of the characteristic double helical structure of DNA. Also, precise structural information involving these amino acid molecules with lower symmetry exhibiting higher trigonal symmetry in their self-assembly is expected to throw light on the nature and strength of intermolecular interactions and their role in self-assembly of molecular aggregates, which are crucial in developing new or at least supplement existing crystal engineering strategies. Single crystal X-ray analysis and their electronic structures were calculated at the DFT level with a detailed analysis of Hirshfeld surfaces and fingerprint plots facilitating a comparison of intermolecular interactions in building different supramolecular architectures.