1. Graduate Group in Biophysics, University of California San Francisco, San Francisco, California 94143 Bruker AG, Industriestrasse 26 CH-8117 Fällanden, Switzerland Bruker Instruments, Inc., Manning Park Billerica, Massachusetts 01821 Rowland Institute for Science, 100 Edwin H. Land Boulevard Cambridge, Massachusetts 02142 Departments of Pharmaceutical Chemistry and Cellular & Molecular Pharmacology, University of California San Francisco, San Francisco, California 94143