1. Department of Chemistry, University of Minnesota, Minneapolis, MN 55455, USA.
2. Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, MN 55455, USA.
3. Department of Pharmacology, University of California, San Diego, La Jolla, CA 92093, USA.
4. Department of Chemistry and Biochemistry, University of California at San Diego, La Jolla, CA 92093, USA.