X-ray studies on crystalline complexes involving amino acids and peptides XXXIV. Novel mode of aggregation, interaction patterns and chiral effects in the maleic acid complexes of DL- and L- arginine

New 1:1 and 2:1 salts in the `DL-norvaline–maleic acid' system as an example of assembling various crystal structures from similar supramolecular building blocks

Molecular salts and cocrystals of amino acids have potential applications as molecular materials with nonlinear optical, ferroelectric, piezoelectric, and other various target physical properties. The wide choice of amino acids and coformers makes it possible to design various crystal structures. The amino acid–maleic acid system provides a perfect example of a rich variety of crystal structures with different stoichiometries, symmetries and packing motifs built from the molecular building blocks, which are either exactly the same, or differ merely by protonation or as optical isomers. The present paper reports the crystal structures of two new salts of the DL-norvaline–maleic acid system with 1:1 and 2:1 stoichiometries, namely DL-norvalinium hydrogen maleate, C5H12NO2 +·C4H3O4 −, (I), and DL-norvalinium hydrogen maleate–DL-norvaline, C5H12NO2 +·C4H3O4 −·C5H11NO2, (II). These are the first examples of molecular salts of DL-norvaline with an organic anion. The crystal structure of (I) has the same C 2 2(12) structure-forming motif which is common for hydrogen maleates of amino acids. The structure of (II) has dimeric cations. Of special interest is that the single crystals of (I) which are originally formed on crystallization from aqueous solution transform into single crystals of (II) if stored in the mother liquor for several hours.

Structure-forming units of amino acid maleates. Case study ofL-valinium hydrogen maleate

A new salt of L-valinium hydrogen maleate was used as an example to study structure-forming units in amino acid maleates. This compound was crystallized, its structure solved from single-crystal X-ray diffraction data, and the phase purity of the bulk powder sample confirmed by X-ray powder diffraction and FT–IR spectra. The stability of the new salt was analyzed using density functional theory andPIXELcalculations with focus on theC22(12) structure-forming crystallographic motif. This motif was of particular interest as it is common for almost all maleates. The exceptionally high ability of maleic acid to form salts with various amino acids was rationalized.

New hydrophobicL-amino acid salts: maleates ofL-leucine,L-isoleucine andL-norvaline

Crystals of maleates of three amino acids with hydrophobic side chains [L-leucenium hydrogen maleate, C6H14NO2+·C4H3O4−, (I), L-isoleucenium hydrogen maleate hemihydrate, C6H14NO2+·C4H3O4−·0.5H2O, (II), and L-norvalinium hydrogen maleate–L-norvaline (1/1), C5H11NO2+·C4H3O4−·C5H12NO2, (III)], were obtained. The new structures containC22(12) chains, or variants thereof, that are a common feature in the crystal structures of amino acid maleates. The L-leucenium salt is remarkable due to a large number of symmetrically non-equivalent units (Z′ = 3). The L-isoleucenium salt is a hydrate despite the fact that L-isoleucine is a nonpolar hydrophobic amino acid (previously known amino acid maleates formed hydrates only with lysine and histidine, which are polar and hydrophilic). The L-norvalinium salt provides the first example where the dimeric cation L-Nva...L-NvaH+was observed. All three compounds have layered noncentrosymmetric structures. Preliminary tests have shown the presence of the second harmonic generation (SGH) effect for all three compounds.

X-ray studies on crystalline complexes involving amino acids and peptides. XXXVI. Crystal structures of hydrated glycyl-L-histidine and L-histidyl-L-alanine complexes with oxalic acid

The crystal structures of the complexes of oxalic acid with glycyl-L-histidine and L-histidyl-L-alanine were determined. The three crystallographically independent peptide molecules in the complexes have closed conformations. The terminal carboxyl group of the dipeptide and the oxalate ion in the Gly-His complex exhibit unusual ionization states and are connected by a symmetric O- - -O hydrogen bond. The peptide aggregation in the complex is almost identical to that in the corresponding semisuccinate complex and is similar to one of the predicted aggregation patterns for Ala-Ala, demonstrating that dipeptide aggregation is controlled primarily by main-chain interactions and is substantially unaffected by disturbing influences such as those arising from polar side chains, ions and water molecules. The peptide molecules in the highly pseudosymmetric crystals of the His-Ala complex, however, exhibit a hitherto unobserved aggregation pattern. Thus, in spite of the repeated occurrence of a few patterns, the possibility of the existence of new patterns needs to be taken into account.