Energy-linked protonation of quinacrine in beef heart submitochondrial membranes

1. The absorption spectrum of quinacrine in aqueous solution, in the visible region, changes with the pH of the medium in the pH range from 6.0 to 9.0 with an isosbestic point at 353 nm. This indicates that the monoprotonated (quinacrine - H+) and the diprotonated (quinacrine - 2H+) forms of quinacrine at equilibrium in this pH range have a 1 to 1 stoichiometry. 2. The monoprotonated and the dipronated forms to quinacrine exhibit similar fluorescence emission spectra, but distinctive fluorescence excitation spectra. 3. The relative fluorescence quantum yields of quinacrine in aqueous media of various pH values are estimated. The relative fluorescence quantum yield of quinacrine at pH 9.0 is more than 3 fold of that at pH 6.0. 4. The fluorescence excitation and emission spectra, as well as the relative fluorescence quantum yield of quinacrine associated with non-energized submitochondrial membranes, are similar to those of quinacrine alone. 5. Analyses of the absorption spectra, the fluorescence excitation spectra and the relative fluorescence quantum yield indicate that the energy-linked fluorescence decrease of quinacrine associated with the energized submitochondrial membranes results from the protonation of quinacrine - H+ to form quinacrine - 2H+. 6. Quantitative data are provided indicating that the maximal efficiency of protonation of quinacrine - H+ to form quinacrine - 2H+ depends on the concentration of H+ in the membranes generated through energy coupling, and the concentration of quinacrine - H+ initially present in the reaction medium. Under optimal conditions virtually complete conversion of quinacrine - H+ into quinacrine - 2H+ is observed. 7. The fluorescence intensity of quinacrine, either alone or associated with non-energized submitochondrial membranes, decreases with increasing temperature. When quinacrine is associated with the energized membranes, however, its fluorescence intensity increases slightly with increasing temperature. This unusual fluorescence behavior towards temperature, together with the fact that under optimal conditions virtually all the quinacrine molecules associated with the energized membranes are in the diprotonated form, further substantiate our earlier conclusion that the diprotonated quinacrine molecules are tightly bound to the energized membranes in a fashion which does not permit ready equilibration with the external medium.