1. Center for Bio/Molecular Science and Engineering, U.S. Naval Research Laboratory, Code 6900, Washington, D.C. 20375, Division of Optical Sciences, U.S. Naval Research Laboratory, Code 5611, Washington, D.C. 20375, Department of Chemistry, University of Albany-SUNY, 1400 Washington Avenue, Albany, New York 12222, and Departments of Cell Biology & Chemistry and the Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, California 92037