1. Department of Biochemistry, Duke University Medical Center, Durham, North Carolina 27710, Stanford Synchrotron Radiation Laboratory, SLAC, Stanford University, P.O. Box 4349, MS 69, Stanford, California 94309, Exxon Research and Engineering Company, Route 22 East, Annandale, New Jersey 08801, and Department of Biochemistry and Molecular Biology, College of Medicine and H. Lee Moffitt Cancer Center and Research Institute, University of South Florida, Tampa, Florida 33612