1. Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, Institute of Biochemistry, Humboldt University, 10098 Berlin, Germany, Department of Chemistry, Brandeis University, Waltham, Massachusetts 02454, Institut für Angewandte Chemie, Berlin-Adlershof, 12489 Berlin, Germany, and Gesellschaft für Biotechnologische Forschung, 38124 Braunschweig, Germany carpino@chem.umass.edu