The effect of temperature and ionic strength on the apparent Ca-affinity of EGTA and the analogous Ca-chelators BAPTA and dibromo-BAPTA.

The apparent calcium association constants (K'Ca) of ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) and 1,2-bis 2-bis(o-amino-5-bromophenoxy)ethane-N,N,N',N'-tetraacetic acid (dibromo-BAPTA) were measured using the method described by Bers (Am. J. Physiol. 242 (1982) C404-408). The purity of the three ligands determined from the chi-intercept of Scatchard plots were 96.3%, 79.0% and 97.3% for EGTA, BAPTA and dibromo-BAPTA, respectively. The impurity of BAPTA was found to be water by drying several samples to constant weight. Increasing temperature from 1 to 36 degrees C led to an increase in K'Ca which was of similar magnitude for the three ligands. Increasing ionic strength from 0.104 to 0.304 M led to a reduction of K'Ca in all cases, though EGTA was affected much less than BAPTA or dibromo-BAPTA. Experimental results were compared with values of K'Ca calculated from the individual association constants of the ligands for calcium and protons which were modified for the experimental conditions using the Debye-Hückel limiting law and the Van't Hoff Isochore to correct for ionic strength and temperature, respectively. The experimental values of K'Ca of EGTA agree well with those in the literature and with the calculated values. Good agreement was also found between the experimental and calculated values of K'Ca for the temperature and ionic strength dependence of BAPTA and dibromo-BAPTA.