1. Department of Materials Science and Engineering, 201 DuPont Hall, University of Delaware, Newark, Delaware 19716
2. Current address: Merck Research Laboratories, Summit, NJ 07901
3. Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716
4. Current address: Glaxosmithkline Biopharm R&D, 709 Swedeland Road, King of Prussia, PA 19406
5. Current address: Chemical Biology Laboratory, Center for Cancer Research, National Cancer Institute, Frederick, MD 21702-1201