1. Department of Chemistry and Biochemistry, University of California, 607 Charles E. Young Drive East, Los Angeles, CA 90095, USA.
2. Crump Institute for Molecular Imaging, Department of Molecular and Medical Pharmacology, David Geffen School of Medicine, University of California, 700 Westwood Plaza, Los Angeles, CA 90095, USA.
3. Department of Physiology, David Geffen School of Medicine, University of California, 700 Westwood Plaza, Los Angeles, CA 90095, USA.
4. Department of Radiology and Bio-X Program, Molecular Imaging Program at Stanford (MIPS), Stanford University School of Medicine, Stanford, CA 94305, USA.