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

The physico-chemical properties of several Ca2+-selective, photolabile chelators are described. These molecules have been developed as part of an effort to produce a caged Ca2+ that improved upon the Ca2+ 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 Kd for Ca2+ 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 Ca2+ release kinetics (70% released with a rate of about 48,000 s−1 and 30% at 1.5 s−1) and photoproducts that bind Ca2+ with very low affinity (Kd in the range of 2 mM, pH 7.2), hence most of the bound Ca2+ is released rapidly and efficiently upon photolysis. Thus, DMNPE-4 has a unique combination of properties that make it an extremely effective Ca2+ cage.