Amino acid hydrogen oxalate quasiracemates – sulfur containing side chains

Abstract

New additions to quasiracemic materials have been developed by cocrystallizing a ternary component – hydrogen oxalate – with pairs of amino acid quasienantiomers where at least one of the side-chain R groups contains a sulfur atom. Of the eight quasiracemates investigated, six exhibit crystal packing that drastically deviates from the expected centrosymmetric alignment present in the racemic counterparts and the extant database of quasiracemic materials. These structures were quantitatively assessed for conformational similarity (CCDC-Mercury structure overlay) and the degree of inversion symmetry (Avnir's Continuous Symmetry Measures) for each quasienantiomeric pair. Despite the variance in quasienantiomeric components, these structures exhibit a high degree of isostructurality where the principal components assemble by a complex blend of common N⁺–H⋯O and O–H⋯O⁻ interactions. These charge-assisted hydrogen-bonded networks form thermodynamically favored crystal packing that promotes cocrystallization of a structurally diverse set of quasienantiomeric components.