1. Department of Physics, University of Wisconsin—Madison, 1150 University Avenue, Madison, Wisconsin 53706, Center for Biomolecular Materials Spectroscopy, Laboratory for Chemical Physics, New York University, 345 East 24th Street, New York, New York 10010, Department of Molecular and Cell Biology, University of California, Berkeley, California 94720, and Synchrotron Radiation Center, 3731 Schneider Drive, Stoughton, Wisconsin 53589