1. Department of Chemistry; Duke University; 124 Science Drive Durham NC 27708-0346 USA
2. Department of Biological Chemistry and Molecular Pharmacology; Harvard Medical School; 240 Longwood Avenue Boston MA 02115 USA
3. Department of Computer Science; Texas A&M University; College Station TX 77843 USA
4. Department of Biochemistry and Biophysics; Texas A&M University; College Station TX 77843 USA