1. Laboratory of Chemical Physics; National Institute of Diabetes and Digestive and Kidney Diseases; Bethesda MD 20892-0520 USA
2. Present address: Department of Chemistry and Biochemistry; University of California, San Diego; La Jolla CA 92093 USA
3. Laboratory of Bioorganic Chemistry; National Institute of Diabetes and Digestive and Kidney Diseases; Bethesda MD 20892-0830 USA