1. MRC Centre for Protein Engineering, Department of Chemistry, Lensfield Road, Cambridge, CB2 1EW, U.K., Department of Chemistry and Biological Chemistry, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, and Laboratoire de Chimie Biophysique, ISIS/ULP, 8 allée Gaspard Monge, BP 70028, F-67083 Strasbourg Cedex, France