1. Department of Chemistry, University of North Carolina at Wilmington, Wilmington, North Carolina 28403-3297, Department of Microbiology and Immunology (M/C 790), E-704 Medical Sciences Building, 835 South Wolcott Avenue, University of Illinois at Chicago, Chicago, Illinois 60612, and Department of Chemistry, The Institute of Molecular Biology, University of Oregon, Eugene, Oregon 97403-1229