Cocrystallizing agents for amino acids.: II.: The crystal structures of L-histidine•4,5-imidazoledicarboxylic acid (1:1) and L-lysine•4,5-imidazoledicarboxylic acid (1:1)

The crystal structure of L-histidine (L-His) 4,5-imidazoledicarboxylic acid (IDC) (1:1) has been refined to a final R (F-o) = 0.035 for 2857 observed reflections (I > 2.5 sigma I). A C-H . . . O hydrogen bond between the charged L-His side chain and an IDC carboxylate is among the shortest of this type ever observed. The dimensions are d(H . . . O) 2.11(2) Angstrom, d(C . . . O) = 2.912(2)Angstrom and alpha(C-H . . . O) = 147(2)degrees. The crystal structure of L-lysine (L-Lys) IDC (2:2) (two L-Lys cations and two IDC anions in the asymmetric unit) has been refined to a final R(F-o) = 0.052 for 4153 observed reflections. One of the two L-Lys molecules has a unique eclipsed side chain conformation with a 128.9(5)degrees C beta C gamma C delta-C epsilon torsion angle. The crystals were prepared as part of a program aimed at finding suitable cocrystallization agents for amino acids and peptides. For basic amino acids IDC is the best complementary organic acid found in this search. The crystal packing of the amino acids in the structures adapts to the need for stacking of the IDC molecules in layers with 3.3 Angstrom spacing.