1. School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich NR4 7TJ, U.K., Departments of Chemistry and Biochemistry and Structural Biology Graduate Program, Stony Brook University, Stony Brook, New York 11794-3400, and Rutherford Appleton Laboratory, Central Laser Facility, CCLRC, Didcot, Oxon, OX11 0QX U.K.