Individual Electrophoretic Mobilities of Liposomes and Acidic Organelles Displaying pH Gradients Across Their Membranes

Abstract

This report focuses on measuring the individual electrophoretic mobilities of liposomes with different pH gradients across their membrane using capillary electrophoresis with laser-induced fluorescence detection (CE-LIF). The results from the individual analysis of liposomes show that, using surface electrostatic theories and the electrokinetic theory as the first approximation, zeta potential contributes more significantly to the electrophoretic mobility of liposomes than liposomal size. For liposomes with an outer pH 7.4 (pH(o) 7.4) and a net negative outer surface charge, the most negative electrophoretic mobilities occur when the inner pH (pH(i)) is 6.8; at higher or lower pH(i), the electrophoretic mobilities are less negative. The theories mentioned above cannot explain these pH-induced electrophoretic mobility shifts. The capacity theory, predicting an induced electrical charge on the surface of liposomes, can only explain the results at pH(i) > 6.8. In this report, we hypothesize that there is a flip-flop process of phospholipids, which refers to the exchange of phospholipids between the outer and inner layers of the membrane. This flip-flop is caused by the pH gradient and membrane instability and results in the observed electrophoretic mobility changes when pH(i) is <6.8. Furthermore, it is found that the mobilities of acidic organelles are consistent with the predictions of liposome models we used here.

Recent advances in the analysis of biological particles by capillary electrophoresis

“…Subsequently, they demonstrated that different surface charge distributions and polarizabilities help predict the electrophoretic behavior of liposomes [9]. The effects of the pH gradients across the membrane of liposomes and acidic organelles on their electrophoretic mobilities were also investigated [7]. When the internal pH is higher than the external pH, the liposomes have more negative electrophoretic mobilities, which is consistent with a capacitive model.…”

“…There has been some progress in developing theories that describe the electrophoretic mobility of bioparticles using liposomes as models [7][8][9]. Other work focusing on the use of liposomes as stationary phases, or earlier work on the analysis of liposomes by CE, has already been covered in two recent reviews [10,11] and is outside our scope.…”

“…However, when the internal pH is lower than the external pH, such as in the case of acidic organelles, other theories may be required (e.g. flip-flop of lipids) to explain the electrophoretic behaviors observed [7].…”

Nanocarriers for Cytosolic Drug and Gene Delivery in Cancer Therapy

“…Using capillary electrophoresis with laser induced fluorescence detection it was shown that pH gradient across liposomal membrane induce electrophoretic mobility shifts based on capacity theory. Moreover, it was also proved that mobilities of acidic organelles are in congruency with predictions based on liposomal models (Chen et al, 2007). The major determinants of cytoplasmic rheology are fluid-phase viscosity and translational diffusion coefficient.…”

Liposomes in chromatography

“…The results indicated that zeta potential contributes more significantly to the electrophoretic mobility of liposomes than liposomal size (Chen and Arriaga, 2007).…”