1. Contribution from the Department of Chemistry, Stanford University, Stanford, California 94305, Department of Microbiology and Enzymology, Technical University Delft, 2628 BC Delft, The Netherlands, Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, 2300 RA Leiden, The Netherlands, and Stanford Synchrotron Radiation Laboratory, SLAC, Stanford University, Stanford, California 94309