1. Contribution from the Cambridge Centre for Molecular Recognition, Department of Biochemistry, University of Cambridge, Tennis Court Road, Cambridge CB2 1QW, U.K., the Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K., the European Molecular Biology Laboratory, Meyerhofstrasse 1, 69012 Heidelberg, Germany, and the Du Pont Merck Pharmaceutical Company, Box 80328, Wilmington, Delaware 19880-0328