1. D. E. Cane, Department of Chemistry, Box H, Brown University, Providence, RI 02912–9108, USA. C. T. Walsh, Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, MA 02115, USA. C. Khosla, Departments of Chemical Engineering, Chemistry, and Biochemistry, Stanford University, Stanford, CA 94305–5025, USA.