1. Ames LaboratoryUSDOE and Department of Chemistry, Iowa State University, Ames, Iowa 50011, Department of Chemistry and Biochemistry, Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe, Arizona 85287-1604, and Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, G128 QQ, U.K.