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

Phase transitions of L-histidinium hydrogen oxalate crystal under high pressures investigated by Raman spectroscopy

L-histidinium hydrogen oxalate (L-HisH)(HC₂O₄) crystal is formed from amino acid. L-histidine with oxalic acid whose vibrational high pressures behavior have not yet been investigated in the literature. Here we synthesized (L-HisH)(HC₂O₄) crystal by slow solvent evaporation method in a 1:1 ratio of L-histidine and oxalic acid. In addition, a vibrational study of (L-HisH)(HC₂O₄) crystal as a function of pressure was performed via Raman spectroscopy in the pressure range of 0.0-7.3 GPa. From analysis of the behavior of the bands within 1.5-2.8 GPa, characterized by the disappearance of lattice modes, the occurrence of a conformational phase transition was noted. A second phase transition, now from structural type, close to 5.1 GPa was observed due to the incidence of considerable changes in lattice and internal modes, mainly in vibrational modes related to imidazole ring motions.

The effect of amino acid backbone length on molecular packing: crystalline tartrates of glycine, β-alanine, γ-aminobutyric acid (GABA) and DL-α-aminobutyric acid (AABA)

We report a novel 1:1 cocrystal of β-alanine with DL-tartaric acid, C₃H₇NO₂·C₄H₆O₆, (II), and three new molecular salts of DL-tartaric acid with β-alanine {3-azaniumylpropanoic acid-3-azaniumylpropanoate DL-tartaric acid-DL-tartrate, [H(C₃H₇NO₂)₂]⁺·[H(C₄H₅O₆)₂]^-, (III)}, γ-aminobutyric acid [3-carboxypropanaminium DL-tartrate, C₄H₁₀NO₂⁺·C₄H₅O₆^-, (IV)] and DL-α-aminobutyric acid {DL-2-azaniumylbutanoic acid-DL-2-azaniumylbutanoate DL-tartaric acid-DL-tartrate, [H(C₄H₉NO₂)₂]⁺·[H(C₄H₅O₆)₂]^-, (V)}. The crystal structures of binary crystals of DL-tartaric acid with glycine, (I), β-alanine, (II) and (III), GABA, (IV), and DL-AABA, (V), have similar molecular packing and crystallographic motifs. The shortest amino acid (i.e. glycine) forms a cocrystal, (I), with DL-tartaric acid, whereas the larger amino acids form molecular salts, viz. (IV) and (V). β-Alanine is the only amino acid capable of forming both a cocrystal [i.e. (II)] and a molecular salt [i.e. (III)] with DL-tartaric acid. The cocrystals of glycine and β-alanine with DL-tartaric acid, i.e. (I) and (II), respectively, contain chains of amino acid zwitterions, similar to the structure of pure glycine. In the structures of the molecular salts of amino acids, the amino acid cations form isolated dimers [of β-alanine in (III), GABA in (IV) and DL-AABA in (V)], which are linked by strong O-H...O hydrogen bonds. Moreover, the three crystal structures comprise different types of dimeric cations, i.e. (A...A)⁺ in (III) and (V), and A⁺...A⁺ in (IV). Molecular salts (IV) and (V) are the first examples of molecular salts of GABA and DL-AABA that contain dimers of amino acid cations. The geometry of each investigated amino acid (except DL-AABA) correlates with the melting point of its mixed crystal.