Optical control of calcium affinity in a spiroamido-rhodamine based calcium chelator.

An optically controlled Ca(2+)-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 Ca(2+) (K(d): 509 nM) with excellent selectivity over Mg(2+) (K(d): 19 mM). The open isomer had a 350-fold lower Ca(2+) affinity (K(d): 181 μM), while the Mg(2+) affinity was not significantly affected (K(d): 14 mM).