Biologically Useful Chelators That Take Up Ca(2+) upon Illumination

Article abstract of DOI:10.1021/ja00202a042

Two approaches were explored toward the goal of synthesizing physiologically useful Ca(2+)-selective chelators whose Ca(2+) affinities increase markedly upon photolysis.In the first approach, the known Ca(2+)-selective chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) was masked with a variety of photoremovable protecting groups on one of its four carboxyl groups, reducing its affinity for Ca(2+) to ca. 1E5 M^-1.Upon irradiation around 356 nm, free chelator with an affinity constant ca. 1E7 M^-1 was regenerated but with very low quantumefficiencies (<=0.007).A more successful approach was to manipulate one of the coordinating nitrogens by remote inductive effects.Addition of an electron-withdrawing diazoacetyl substituent to one ring of BAPTA, para to the amino group, resulted in "diazo-2", a chelator with a Ca(2+) affinity of 4.5E5 M^-1.Photochemical rearrangement of the diazoacetyl group converted it into an electron-donating carboxymethyl group, causing the Ca(2+) affinity to increase 30-fold to 1.4E7 M^-1.The photolysis of Ca(2+)-free diazo-2 had a quantum efficiency with 365-nm light (λ_max 370 nm, ε ca. 22000 M^-1cm^-1) of ca. 0.03 and generated the high-affinity chelator with rate constants of 2300 s^-1 after a flash.Ca(2+) was then bound with association and dissociation rate constants of 8.0E8 M^-1s^-1 and 58 s^-1, respectively.Diazo-2 was incorporated into rat fibroblasts either by microinjection or by incubation as the membrane-permeable, enzymatically labile tetrakis(acetoxymethyl) ester and, when flash-photolyzed, caused a drop in intracellular free to or below resting values of ca. 1E-7 M.An even larger increase in affinity(1600-fold) was obtained by substituting both phenyl rings of BAPTA with diazoacetyl substituents.Therefore, these chelators can be used to generate controlled fast decrements in intracellular free to mimic or ablate a host of important cellular responses, especially in nerve and muscle.