1. Contribution from the Biochemistry and Biotechnology Group, CST-4, MS C345, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, Department of Chemistry and The James Franck Institute, University of Chicago, 5735 S. Ellis Avenue, Chicago, Illinois 60637, Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, The Rockefeller University, 1230 York Avenue, New York, New York 10021, and Department of Chemistry, University of California, Riverside, California 92521