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.