THE STRUCTURE OF A DESIGNED PEPTIDE REFINED TO 2.1 Å RESOLUTION. Nancy L. Ogihara, Manfred S. Weiss, William F. DeGrado, and David Eisenberg, UCLA-DOE Laboratory of Structural Biology and Molecular Medicine, Box 951570, University of California, Los Angeles, Los Angeles, CA 90095-1570

The three-dimensional structure of the designed peptide Acetyl-E VEALEKK VAALESK VQALEKK VEALEHG- amide has been determined and refined to a crystallographic R-factor of 21.4% for all data from 10 to 2.1 Å, resolution. In the trigonal crystal, three molecules, related by a crystallographic 3-fold axis form a parallel three helix bundle. The bundles are stacked head-to- tail to form a continuous coiled coil along the z direction of the crystal. The contacts between neighboring helices within the coiled coiled are mainly hydrophobic; four layers of valine residues alternating with four layers of leucine residues form the core of the bundle. Mostly hydrophilic contacts mediate the interaction between trimers. Here, a total of 2 solvent mediated hydrogen bonds and 2 direct protein- protein hydrogen bonds are found. Based on the structure, we propose a rule for designing crystals of peptides containing continuous 2-, 3-, and 4-helix bundles.