1. Center for Neurodegenerative Disease Research, Department of Pathology and Laboratory Medicine, Perlman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
2. Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, U.K.
3. Department of Chemistry, School of Arts and Sciences, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States