Optical Control of Calcium Affinity in a Spiroamido-rhodamine Based Calcium Chelator

An optically controlled Ca2+-chelator 1 was developed to mimic natural calcium oscillations. Compound 1, a spiroamido-rhodamine derivative of 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), underwent cycles of reversible transitions between a colorless closed state and a fluorescent open form. The closed-state exhibited a high affinity for Ca2+ (K-d: 509 nM) with excellent selectivity over Mg2+ (K-d: 19 mM). The open isomer had a 350-fold lower Ca2+ affinity (K-d: 181 mu M), while the Mg2+ affinity was not significantly affected (K-d: 14 mM).