Surface potential and energy-coupling in bioenergy-conserving membrane systems

Abstract

In order to understand the localization of dyes and the nature of their responses in membranes and particularly in those involved in energy-conservation processes, the influence of micelles of neutral and ionic surfactants on the pK _a of solubilized fluorophoric (umbelliferone) and chromophoric (bromthymol blue and methyl red) indicator dyes is studied. It is shown that the pK _a of the indicator adsorbed onto micelles shifted towards the acid extreme with cationic micelles, to the alkaline side with anionic micelles while it was not significantly modified by the neutral ones. Maximal displacements were observed with Methyl Red where the difference in pK _a between anionic and cationic micelles was as large as 3 pH units. Phospholipid liquid crystals (Liposomes) of phosphatidylcholine with and without adsorbed long-chain ions introduced in order to confer to it a net surface charge induced displacements of the pK _a of UBF analogous to those detected in the presence of detergent micelles. It was demonstrated that UBF can monitor reversal of charge phenomena such as that obtained by the interaction of phosphatidylcholine + dicetyl phosphate liposomes (anionic colloid) with poly-L-lysine (cationic colloid). The partition of the indicator dyes between micellar and aqueous phases was determined by gel filtration revealing thequasi exclusive presence of the dyes in the micellar phase. Fluorescence polarization measurement of solubilized UBF in either ionic micelles or submitochondrial particles indicate that the dye tumbling rate is as rapid as in pure water suggesting that the dye is mobile in an interfacial environment where it can experience modifications due to changes in surface potential. The use of UBF as a probe of respiration-dependent energy-linked reactions in submitochondrial particles is presented. The available data on the use of indicator dyes in mitochondrial, chloroplast and bacterial chromatophore membranes is reevaluated, on the basis of the evidence of the extreme sensitivity of these probes to surface charge. The implications of these results and considerations are discussed in terms of the importance of the surface potential in the primary event of the energy-coupling process in oxidative and photosynthetic phosphorylation.