1. Contribution from the Department of Biochemistry, Molecular Biology and Cell Biology, Northwestern University, Evanston, Illinois 60208, Structural Biology NMR Facility, Northwestern University, Evanston, Illinois 60208, Departments of Chemical and Biological Engineering and Chemistry, Northwestern University, Evanston, Illinois 60208, Department of Chemistry, New York University, New York, New York 10003, and Chiron Technologies, Chiron Corporation, Emeryville, California 94608