1. Department of Chemistry and Biochemistry University of California San Diego La Jolla California 92093-0358 USA
2. Department of Chemistry and Biochemistry San Diego State University San Diego California 92182-1030 USA
3. Adolphe Merkle Institute University of Fribourg Chemin des Verdiers 4 1700 Fribourg Switzerland
4. Department of Biomedical Engineering University of Michigan Ann Arbor Michigan 48109-2110 USA