Effect of increased lipid packing on the surface charge of micelles and membranes

Abstract

We have investigated the responsiveness of micelle and bilayer surfaces to changes in bulk pH through titrations, and to changes in lipid packing through the application of high hydrostatic pressure using two fluorescent, pH-sensitive surface probes. In micelles, the surface is very sensitive to bulk pH while in phosphatidylcholine and phosphatidic acid bilayers the surface charge changed little through a large pH region. Application of pressure on micelles causes proton dissociation due to the volume reduction achieved from the contraction of water around the charges (electrostriction). However, in bilayers, the effect of electrostriction is greatly reduced, most likely due to the energy needed to expand and hydrate the surface. The sign and amount of change in dissociation the probe undergoes with pressure depend on the initial degree of probe dissociation, which is in turn dependent on the particular surface rather than the charge of the lipid head groups comprising the bilayer. This finding may limit the use of fluorescent probes to determine the exact surface potential. By assuming the change in .DELTA.V for proton dissociation from the probe is constant for a given pH, we can calculate the changes in local pH that occur under pressure relative to a neutral or uncharged system. In doing so, we find that the local pH around the probe in bilayers changes very little (.apprx. 0.1 pH unit or less) in the first 2000 bars. Also, if pressure data are coupled with titration curves, we can determine the change that the bulk pH must undergo to produce the observed change in dissociation seen under pressure. Here, we find that raising the pressure from 1 to 2000 bar has no effect on dioleoylphosphatidylcholine bilayers whereas raising the pressure of dioleoylphosphatidic acid bilayers is equivalent to decreasing the bulk pH from 7 to 3. Although this appears to be a large effect, the actual change in surface charge in this pH region is quite low.