X-ray studies on crystalline complexes involving amino acids and peptides. XXIII. Variability in ionization state, conformation and molecular aggregation in the complexes of succinic acid with DL- and L-lysine

Abstract

Crystalline complexes of succinic acid with DL- and L-lysine have been prepared and analysed by X-ray diffraction. DL-Lysine complex: C₆H₁₅N₂O₂⁺·-½ C₄H₄O₄^2-·½C₄ H₆O₄, M_r = 264.2, P1̅, a = 5.506 (4), b = 8.070 (2), c = 14.089 (2) Å, α = 92.02 (1), β = 100.69 (3), γ = 95.85 (3)°, Z = 2, D_x = 1.44 g cm^-3, R = 0.059 for 2546 observed reflections. Form I of the L-lysine complex: C₆H₁₅N₂O₂⁺ ·C₄H₅O₄, M_r = 264.2, P1, a = 5.125 (2), b = 8.087 (1), c = 8.689 (1) Å, α = 112.06 (1), β = 99.08 (2), γ = 93.77 (2) °, Z = 1, D_m = 1.34 (3), D_x = 1.34 g cm^-3, R = 0.033 for 1475 observed reflections. Form II of the L-lysine complex: C₆H₁₅N₂O₂⁺. ¹₄C₄H₄O₄^{2 1}₄C₄H₆¹₄C₄ H₆O₄.₄¹(C₄H₅O₄···H···C₄H₄O₄)², M_r = 264.2, P1, a = 10.143 (4), b = 10.256 (2), c = 12.916 (3) Å, α = 105.00 (2), β = 99.09 (3), γ = 92.78 (3)°, Z = 4, D_m = 1.37 (4), D_x = 1.38 g cm^-3, R = 0.067 for 2809 observed reflections. The succinic acid molecules in the structures exhibit a variety of ionization states. Two of the lysine conformations found in the complexes have been observed for the first time in crystals containing lysine. Form II of the L-lysine complex is highly pseudosymmetric. In all the complexes, unlike molecules aggregate into separate alternating layers. The basic element of aggregation in the lysine layer in the complexes is an S2-type head-to-tail sequence.