1. W. R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, Department of Physics, Washington State University, 2710 University Drive, Richland, Washington 99352, School of Molecular Biosciences, Washington State University, Pullman, Washington 99164, Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227, and Department of Biological Chemistry, John Innes Centre, Norwich, NR4 7UH, United Kingdom