1. a Department of Chemistry and Biochemistry , University of California San Diego , 9500 Gilman Drive, La Jolla , CA , 92093-0340 , USA
2. b The Scripps Research Institute , 10666 N. Torrey Pines Road., La Jolla , CA , 92037 , USA
3. c Howard Hughes Medical Institute, Division of Cellular and Molecular Medicine, Department of Pharmacology, University of California San Diego , 9500 Gilman Drive, La Jolla , CA , 92093-0683 , USA
4. d Department of Chemistry and Biochemistry , University of California San Diego , 9500 Gilman Drive, La Jolla , CA , 92093-0340 , USA