1. Contribution from the Departments of Chemistry and Molecular Biology, The Scripps Research Institute, La Jolla, California 92307, The Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, California 92307, Department of Medical Biochemistry & Genetics, The Texas A&M University System Health Science Center, College Station, Texas 77843-1114, Department of Chemistry, Texas A&M University, College Station, TX 77843-3255