pH fluctuations in unilamellar vesicles

pH fluctuations in small unilamellar vesicles (SUV) are theoretically estim ated. We determine that these fluctuations are dependent on macroscopic var iables such as pH, pK(a), buffer group concentration and surface electrical potential. Based on a previously reported definition of buffer electrical capacitance (Procopio and Fornes, Phys. Rev. E, 1995, 51, 829) an equation is derived which relates the pH fluctuation, the buffering power and the SU V size. We also derived an equation for the mean square charge thermal fluc tuation using the fluctuation-dissipation theorem which coincides with that independently obtained by Kirkwood and Shumaker (Proc. Natl. Acad. Sci., 1 952, 38, 863) using mechanical statistical methods. From our results it is inferred that measurement of pH in small systems has to be performed near t he pK of the buffer groups in order that the fluctuational errors be minimi zed. We show that pH fluctuations diminish with increasing the size of the SUV and the predicted pH fluctuations decrease as the surface potential bec omes less negative as a consequence of decreasing density of charged groups in the inner vesicular surface. It is predicted that measurable effects wi ll appear on the fluorescence detection due to protonic fluctuations close to the pH sensing region of the probes.