1. Biocrystallography Laboratory, Veterans Affairs Medical Center, P. O. Box 12055, University Drive C, Pittsburgh, Pennsylvania 15240, USA, Department of Pharmacology, University of Pittsburgh School of Medicine, 1340 BSTWR, Pittsburgh, Pennsylvania 15261, USA, Department of Chemistry, Rutgers University, Newark, New Jersey 07102, USA, and Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield, S100 2TN, United Kingdom