STRUCTURE AND CONFORMATION OF SEQUENTIALLY RELATED PEPTIDES: CRYSTAL STRUCTURE OF L-PHENYLALANYLGLYCYLGLYCINE (FGG). T. Srikrishnan and Seth Hoffman, Center for Crystallographic Research and Department of Biophysics, Roswell Park Cancer Institute, Buffalo N.Y. 14263
A systematic structural investigation of sequentially related peptides is of great importance for the elucidation of the structure-function relationship of peptides and in deducing the possible conformations of polypeptides. Although GGG has an extended antiparallel ß-structure, crystal structures of other tripeptides of the sequence GGX and XGG show a wider range of conformations ranging from the extended, many kinds of folded conformations to a few helical conformations. In this line of investigation, the crystal structure of FGG was undertaken in our laboratory. Crystals of FGG (C13H17N3O4), grown by slow evaporation from an aqueous ethanol solution, are orthorhombic, space group P212121, with the following cell dimensions: a= 5.459 (5), b=15.299 (6), c=16.047 (6) Å, V= 1340.2 Å3, Do= 1.38 g/c.c, Dc= 1.384 g/c.c and Z=4. Complete three dimensional data was collected on a CAD 4 diffractometer (2643 reflections, 2305>3[[sigma]]). The structure was solved by the application of direct methods and refined to a final R factor of 0.031. The molecule exists as a zwitterion in the crystal. The peptide units are trans planar ([[omega]]1= -178.6 and [[omega]]2 = 175.6 deg. ). The peptide backbone is folded with the torsion angles of [[Psi]]1 =
-116.7, [[omega]]1= -178.6, [[Phi]]2= 88.8, [[Psi]]2=29.4, [[omega]]2= 175.6, [[Phi]]3= -135.6 and [[Psi]]3= -8.2deg.. For the phenyalanine side chain, [[chi]]1= 123.4 and [[chi]]2= -56.3deg.. The molecules are linked together intermolecularly in an infinite sequence by head to tail hydrogen bonds, as is typical of charged peptides.
Work supported by the New York State Department of Health.