1. Department of Biochemistry, The University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, Texas 75390-9038, United States
2. Laboratoire de Chimie Organique, Université libre de Bruxelles (ULB), CP 160/06, 50 avenue F.D. Roosevelt, 1050 Brussels, Belgium
3. Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States