Molecular aggregation in crystalline 1:1 complexes of hydrophobic D- and L-amino acids. I. The L-isoleucine series

Abstract

The amino acid L-isoleucine has been cocrystallized with seven selected D-amino acids including D-methionine [L-isoleucine–D-methionine (1/1), C6H13NO2.C5H11NO2S, amino-acid side chain R = —CH2—CH2—S—CH3] and a homologous series from D-alanine [L-isoleucine–D-alanine (1/1), C6H13NO2.C3H7NO2, R = —CH3] through D-α-aminobutyric acid [L-isoleucine–D-α-aminobutyric acid (1/1), C6H13NO2.C4H9NO2, R = —CH2—CH3] and D-norvaline [L-isoleucine–D-norvaline (1/1), C6H13NO2.C5H11NO2, R = —CH2—CH2—CH3] to D-norleucine [L-isoleucine–D-norleucine (1/1), C6H13NO2.C6H13NO2, R = —CH2—CH2—CH2—CH3] with linear side chains, and D-valine [L-isoleucine–D-valine (1/1), C6H13NO2.C5H11NO2, R = —CH—(CH3)2] and D-leucine [L-isoleucine–D-leucine (1/1), C6H13NO2.C6H13NO2, R = —CH2—CH—(CH3)2] with branched side chains. All the crystal structures are divided into distinct hydrophilic and hydrophobic layers. In the five complexes with amino acids with linear side chains the polar parts of the D- and L-amino acids are related by pseudo-glide-plane symmetry, and four of them have remarkably similar molecular arrangements. The D-valine and D-leucine complexes, on the other hand, are structurally quite different with the polar parts of the D- and L-amino acids related by pseudo-inversion. Differences in the hydrogen-bond pattern in the two molecular arrangements are discussed.