Acta Crystallographica Section B

Structural Science

Volume 58, Part 3 Number 2 (June 2002)

research papers

Acta Cryst. (2002). B58, 512-518 [ doi:10.1107/S010876810200277X ]

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

C. H. Görbitz

Abstract: 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 (P2₁, Z' = 8) in the shape of thin plates. Peptide molecules in these two structures occur in three basic conformations, termed c₁, c_2A and c_2B. c_2B has not been observed previously for dipeptides. Together with c₁ 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 -NH^-OOC- contacts in the structure are unusually short and the minimum NO distance of 2.649 (5) Å represents a new extreme limit for this type of hydrogen bond in peptide structures.

Keywords: turns; water cage formation; hydrogen bonding; peptides.