1. Department of Chemistry; University of Illinois; 505 South Mathews Avenue Urbana IL 61801 USA
2. Department of Chemistry; University of Richmond; 28 Westhampton Way Richmond VA 23173 USA
3. Department of Infectious Diseases; St. Jude Children's Research Hospital; 262 Danny Thomas Place Memphis TN 38105 USA
4. Present address: Department of Biotechnology; All India Institute of Medical Sciences; Ansari Nagar New Delhi Delhi 110029 India