1. Contribution from the Department of Chemistry, University of California, Davis, California 95616, Experimental Station, E.I. duPont de Nemours Company, Wilmington, Delaware 19880-0173, Department of Biochemistry, Imperial College of Science, Technology and Medicine, London SW7 2AY, United Kingdom, and DRFMC/SCIB/3MC, CEA-Grenoble, 38054 Grenoble Cedex 9, France