1. Department of Chemistry and the Skaggs Institute for Chemical Biology; The Scripps Research Institute; La Jolla California 92037
2. Department of Molecular and Experimental Medicine and the Skaggs Institute for Chemical Biology; The Scripps Research Institute; La Jolla California 92037
3. Instituto de Bioquímica Médica, Programa de Biologia Estrutural, Universidade Federal do Rio de Janeiro; Rio de Janeiro 21941-590 Brazil
4. Department of Biochemical Engineering; Washington University in St. Louis; St. Louis Missouri 63130