1. Contribution from the Department of Chemistry and Biochemistry, University of Maryland Baltimore County, Baltimore, Maryland 21250, Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, and Department of Biochemistry, Uppsala University, Biomedical Center, Box 576, SE-751 23 Uppsala, Sweden