1. Department of Chemistry, University of Rochester, Rochester, New York 14627-0216, Department of Chemistry, State University of New York at Fredonia, Fredonia, New York 14063, Department of Biochemistry and Biophysics, University of Rochester, Rochester, New York 14642, MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB22QH, U.K., and Department of Structural Biology, Stanford University School of Medicine, Stanford, California 94305-5126