title: Kinetics and mechanism of Ca2+ binding to arsenazo III and antipyrylazo III.
abstract: Equilibrium and temperature-jump spectrophotometric measurements were carried out on arsenazo III (Ar) and antipyrylazo III (Ap) in order to establish the kinetic reaction schemes for complexing of these dyes with Ca2+. The reaction media contained 30 mM Na2HPO4 as the buffer salt, at pH 7.4. Dependence of the relaxation rate of arsenazo III on dye and Ca2+ concentrations indicates the presence of both CaAr and CaAr2 complexes, with the CaAr2 form being responsible for the slow, 10-20 ms relaxation of this dye. For antipyrylazo III, the relaxation rate is much faster, less than 1 ms, and the complexing kinetics can be covered with only a CaAp complex. Unlike arsenazo III, antipyrylazo III binding with Ca2+ is rate-limited by a slow structural transition in the dye, taking antipyrylazo III from a low- to a high-affinity structure for Ca2+.
mesh_terms: D001150:Arsenazo III / Q000378:metabolism*; D001391:Azo Compounds / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D066298:In Vitro Techniques; D007700:Kinetics; D008433:Mathematics; D009282:Naphthalenesulfonates / Q000378:metabolism*; D013053:Spectrophotometry; D013696:Temperature
chemical_list: D001391:Azo Compounds; D009282:Naphthalenesulfonates; D001150:Arsenazo III; C016238:antipyrylazo III; D002118:Calcium