1. Departments of Pharmacology and Neuroscience, University of Florida, Gainesville, Florida 32610, Department of Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973-5000, and Department of Physics, University of Waterloo, Waterloo, Ontario N2L3G1, Canada