1. Center for Advanced Research in Biotechnology, University of Maryland Biotechnology Institute, Rockville, Maryland 20850, Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131, Biological, Chemical, and Physical Science Department, Illinois Institute of Technology, Chicago, Illinois 60616, and School of Biology and Biochemistry, Medical Biology Centre, The Queens's University of Belfast, Belfast BT9 7BL, Northern Ireland