1. Chemical Biology Program, Broad Institute of Harvard and MIT, Cambridge, Massachusetts 02142, Department of Chemistry, New York University, New York, New York 10003, Department of Chemistry and NUS MedChem Program of the Office of Life Sciences, National University of Singapore, Singapore 117543, and Laboratory of Bioimaging Probe Development, Singapore Bioimaging Consortium, Agency for Science, Technology and Research (A*STAR), Biopolis, Singapore 138667