1. CST-9, BN-2, MS M888, Chemical Science & Technology Division, Bioscience Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87544, Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706-1396, and Department of Biological Chemistry, School of Medicine, University of California at Davis, Davis, California 95616