Tuning caged calcium: photolabile analogues of EGTA with improved optical and chelation properties

The physico-chemical properties of several CA(2+)-selective, photolabile chelators are described. These molecules have been developed as part of an effort to produce a caged CA(2+) that improved upon the CA(2+) chelation properties and light absorption capability of nitrophenyl-EGTA (NP-EGTA). Four dimethoxy-ortho-nitrophenyl derivatives of EGTA (called DMNPE-1 through -4), and one analogue of EGTA (DMNPE-5) have been characterized, each of which is bisected upon irradiation. One of these cages has a higher affinity than NP-EGTA: DMNPE-4 has a K(d) for CA(2+) of 48 nm at pH 7.2 (19 nM at pH 7.4). Furthermore, this cage has a large extinction coefficient of 5120 M(-1)cm(-1) at 350 nm (cf. 975 M(-1)cm(-1) for NP-EGTA). The Other physico-chemical properties of DMNPE-4 are: quantum yield of photolysis of 0.09; bipasic CA(2+) release kinetics (70% released with a rate of about 48,000 s(-1) and 30% at 1.5s(-1)) and photoproducts that bind CA(2+) with very low affinity (K(d) in the range of 2mM, pH 7.2), hence most of the bound CA(2+) is released rapidly and efficiently upon photolysis. Thus, DMNPE-4 has a unique combination of properties that make it an extremely effective CA(2+) cage.