1. a Department of Chemistry , University of Southern California , Los Angeles, CA 90089–0744, USA
2. b Department of Chemistry , University of Southern California , Los Angeles, CA 90089–0744, USA
3. c Department of Chemistry , University of Southern California , Los Angeles, CA 90089–0744, USA
4. d Department of Pharmaceutical Chemistry , University of California San Francisco , San Francisco, CA 94143, USA and IMARX Pharmaceuticals, 1635 E. 18th Street, Tucson, AZ 85719, USA
5. e Department of Pharmaceutical Chemistry , University of California San Francisco , San Francisco, CA 94143, USA and IMARX Pharmaceuticals, 1635 E. 18th Street, Tucson, AZ 85719, USA