A stopped-flow investigation of calcium ion binding by ethylene glycol bis(β-aminoethyl ether)- N,N′-tetraacetic acid

The kinetics of calcium ion complexation by ethylene glycol bis(β-aminoethyl ether)- N,N′-tetraacetic acid (EGTA) were investigated using the stopped-flow technique. This study was performed within the pH range 5.8 to 8.4. The reaction was found to be first order in EGTA and complex order in calcium, with an observed second-order rate constant (pH 7, T = 25°C, ionic strength = 0.1 m) of about 1.5 × 10 6 m −1 s −1. The rate constant was independent of hydrogen ion concentration between pH 5.8 and 7.3; above pH 7.3 it increased in magnitude with increasing pH, and was 2.0 × 10 8 m −1 s −1 at pH 8.4. The rate constant at 16 and 38°C (pH 6.8) was found to be 0.9 and 7 × 10 6 m −1 s −1, respectively. These data imply that calcium ion buffering by EGTA will require times on the order of milliseconds when EGTA is present in millimolar concentrations.