beta Turns, water cage formation and hydrogen bonding in the structures of L-valyl-L-phenylalanine.

L-Valyl-L-phenylalanine has been crystallized as an orthorhombic dihydrate (1) in the shape of needles and as a monoclinic trihydrate (2) with Z = 16 (P2(1), Z' = 8) in the shape of thin plates. Peptide molecules in these two structures occur in three basic conformations, termed c(1), c(2A) and c(2B). c(2B) has not been observed previously for dipeptides. Together with c(1) it forms a model pair for Type I and Type II beta-turns in protein structures. The crystal packing of (2) is remarkable in that some of the L-Val side chains are exposed to the solvent region of the crystal rather than being located in a hydrophobic layer. The crystal packing thus offers a unique and detailed view of hydrogen-bond cage formation around the hydrophobic groups by the 24 cocrystallized water molecules. The eight [bond]NH(3)(+)...(-)OOC[bond] contacts in the structure are unusually short and the minimum N...O distance of 2.649 (5) A represents a new extreme limit for this type of hydrogen bond in peptide structures.