Calcium behaves as a chemical messenger in a wide variety of cells, and local variations in its

concentration play a key role in many biological processes. 1,2 The understanding of how calcium

affects cellular functions necessitates the precise modulation of calcium concentration with high

spatial and temporal resolution. Tremendous efforts have led to the development of molecules

designed to encapsulate calcium, and more importantly, to capture and release calcium under

external physicochemical stimuli such as photons. The design of such molecules must fulfill strict

requirements dictated by biological constraints (water solubility, physiological pH, remarkable

selectivity for calcium over all other competing cations, and biological inertness). Tsien's group

has developed and thoroughly studied a series of calcium chelators based on BAPTA

(1,2-bis(o-aminophenoxy)ethane-N,N,-N′,N′-tetraacetic acid)-type ligands, which …