1. Department of Chemistry, University of California, Berkeley, Berkeley, California 94720
2. Graduate Group in Biophysics, University of California, Berkeley and Life Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720
3. ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, U.K.
4. Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, Texas 75235-9038
5. Life Sciences Division & Physical Biosciences Division, Lawrence Berkeley, National Laboratory, Berkeley, California 94720