Symmetry, pseudosymmetry and packing disorder in the alcohol solvates of L-leucyl-L-valine.

The dipeptide L-Leu-L-Val can crystallize as a hydrate in the hexagonal space group P6(5) [Görbitz & Gundersen (1996). Acta Chem. Scand. 50, 537-543], but forms 1:1 solvates when methanol, ethanol and 2-propanol are used as precipitating agents. The structures of these complexes can be divided into hydrophobic and hydrophilic layers. The alcohol molecules are embedded in the hydrophobic layers, but with the hydroxyl groups as crucial parts of the hydrogen-bonding network. L-Leucyl-L-valine-methanol (1/1) crystallizes in the space group P2(1) with Z = 2. L-Leucyl-L-valine-ethanol (1/1) has very special systematic absences, and the structure could not be solved with direct methods. Unraveling the actual build-up of the crystal was an unusual process involving modeling with molecular graphics programs. The solution shows a structure which formally belongs to the space group P2(1), with four dipeptide molecules and four solvent molecules in the asymmetric unit (Z = 8). Additional non-crystallographic symmetry in three dimensions fixes all cell angles, including beta, to 90 degrees. L-Leucyl-L-valine-2-propanol (1/1) is structurally closely related to the ethanol solvate, but owing to a rare type of packing disorder the length of the a axis is halved (Z = 4, P2(1)2(1)2(1)). The hydrogen-bonding pattern is still the same as in the ethanol solvate, which means that the hydrogen-bond periodicity along the a axis in the 2-propanol solvate is two unit-cell lengths.