1. Bioscience Division and Physics Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, Department of Biochemistry, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, Piscataway, New Jersey 08854, Howard Hughes Medical Institute, Chevy Chase, Maryland 20815-6789, and Physical Biosciences, Lawrence Berkeley National Laboratory, Berkeley, California 94720