Amino acid hydrogen oxalate quasiracemates – hydrocarbon side chains

Abstract

Amino acid quasiracemates – generated from the assembly of pairs of chemically distinct amino acids of opposite handedness – continue to provide important opportunities to understand how self-assembly can be promoted despite using components with drastically different sizes and molecular shapes. Previous studies by Görbitz et al. and others cataloged 32 crystal structures of amino acid quasiracemates, with each showing the building blocks aligned with near inversion symmetry similar to their racemic counterparts. This investigation examined the impact of using a secondary coformer molecule, hydrogen oxalate, on the cocrystalline landscape of amino acid quasiracemates with hydrocarbon side chains. Eight racemic (4) and quasiracemic (4) hydrogen oxalate structures were generated. Crystal structures of these systems show the hydrogen oxalate moieties assembled into C(5) molecular columns by the construction of robust O–H⋯O⁻ hydrogen bonds with the amino acid enantiomers and quasienantiomers linked to these column motifs using a complex blend of N⁺–H⋯O⁻, O–H⋯O⁻, and N⁺–H⋯OC contacts. The racemates and quasiracemates form similar packing motifs; however, due to the chemically non-identical nature of the quasiracemic components, the outcome is that the amino acids organize with near inversion symmetry. Both the conformational similarity (χ_RMS) and degree of inversion symmetry (C_i) of related pairs of quasienantiomeric components have been systematically assessed using readily available structural tools. This study shows how coformer molecules such as hydrogen oxalate can provide new and critical insight into the molecular recognition process of quasiracemic materials.