New hydrophobic l-amino acid salts: Maleates of l-leucine, l-isoleucine and l-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 contain C₂²(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.