Acta Crystallographica Section B

Structural Science

Volume 55, Part 3 (June 1999)

research papers

Acta Cryst. (1999). B55, 424-431 [ doi:10.1107/S0108768198013494 ]

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

B. Dalhus and C. H. Görbitz

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- [alpha] -aminobutyric acid [L-isoleucine-D- [alpha] -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.