1. Department of Physics & Astronomy, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands; Krebs Institute for Biomolecular Research and Robert Hill Institute for Photosynthesis, Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield S0 2TN, U.K; and Department of Chemistry, Stanford University, Stanford, California 94305-5080