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, C 6 H 14 NO 2 + ·C 4 H 3 O 4 − , (I), L-isoleucenium hydrogen maleate hemihydrate, C 6 H 14 NO 2 + ·C 4 H 3 O 4 − ·0.5H 2 O, (II), and L-norvalinium hydrogen maleate–L-norvaline (1/1), C 5 H 11 NO 2 + ·C 4 H 3 O 4 − ·C 5 H 12 NO 2 , (III)], were obtained. The new structures contain C 2 2 (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.