1. Department of BiochemistryDuke University Medical Center, 307 Research Drive Durham NC 27710 USA
2. Department of ChemistryVassar College, 124 Raymond Avenue, Box 748 Poughkeepsie NY 12604‐0748 USA
3. Department of MicrobiologyUniversity of Pennsylvania Perelman School of Medicine, Johnson Pavilion, 3610 Hamilton Walk Philadelphia PA 19104‐6076 USA