1. Contribution from the Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota Medical School, Minneapolis, Minnesota 55455, the Laboratory for Bioorganic Chemistry, Sloan-Kettering Institute for Cancer Research, 1275 York Avenue, New York, New York 10021, the Department of Chemistry, Columbia University, New York, New York 10027, and the Computational Biology Center, IBM Thomas J. Watson Research Center, Yorktown Heights, New York, New York 10598