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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.
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Amino acid hydrogen oxalate quasiracemates – sulfur containing side chains
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.
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- PAR ID:
- 10338135
- Date Published:
- Journal Name:
- CrystEngComm
- Volume:
- 23
- Issue:
- 46
- ISSN:
- 1466-8033
- Page Range / eLocation ID:
- 8061 to 8070
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D1CE01214B
