1. Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794-3400, Biological Engineering Division and Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Room NE-725, Cambridge, Massachusetts 02139-4307, and Graduate Program in Biophysics and Graduate Program in Biochemistry and Structural Biology, State University of New York at Stony Brook, Stony Brook, New York 11794