1. Department of Biochemistry, Nanaline Duke Building, Duke University Medical Center, Durham, North Carolina 27710, Faculte des Sciences et Techniques, Laboratoire de Méthodologies RMN, BP 239, Université H. Poincaré, Boulevard des Aigillettes, F54506 Vandoeuvre-lés-Nancy Cedex, France, Department of Chemistry, East Carolina University, Greenville, North Carolina 27858