Photophysics of Mag-fura-2: a fluorescent indicator for intracellular Mg2+

Mag-fura-2 is the first commercial fluorescent indicator for the determination of intracellular Mg2+ concentrations. To investigate the reversible reaction of Mg2+ with this indicator in the ground and excited states, steady-state and time-resolved fluorescence measurements were done. The following values of the rate constants at room temperature in aqueous solution at pH 7.2 were recovered by global compartmental analysis from the fluorescence decay surface: k01=4×108 s−1, k21=8×108 M−1 s−1, k02=6×108 s−1, k12=4×107 s−1. k01 and k02 denote the deactivation rate constants of the Mg2+-free and bound forms of the indicator in the excited state, k21 the second-order rate constant of association of Mg2+ with Mag-fura-2 in the excited state, and k12 is the first-order rate constant of dissociation of the excited Mg2+:indicator complex. The excited-state complex-forming reaction does not interfere with the fluorimetric determination of Kd.