1. Center for Biochemical and Biophysical Sciences and Medicine, Harvard Medical School, Seeley G. Mudd Building, 250 Longwood Avenue, Boston, Massachusetts 02115, Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, 240 Longwood Avenue, Boston, Massachusetts 02115, and New Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, England OX1 3QT