1. Departments of Biological Sciences and Biochemistry, Purdue University, West Lafayette, Indiana 47907, Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, 30239 Krakow, Poland, Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02167, Department of Chemistry, University of Texas Southwest Medical Center, Dallas, Texas 75235, and Department of Biochemistry, Cornell University, Ithaca, New York 14853