RESUMO
Rates of pH-dependent micellization of multilamellar vesicles by the hydrophobic polyelectrolyte poly(2-ethylacrylic acid) (PEAA) have been measured turbidometrically. This polymer shows a strong ph-dependence in its affinity for phospholipid membranes, binding in increasing amounts as pH is lowered and ultimately solubilizing membranes to form mixed micelles (Tirrell, Takigawa and Seki (1985) Ann. N.Y. Acad. Sci. 446, 237). The rate of solubilization of dipalmitoylphosphatidylcholine (DPPC) vesicle suspensions by PEAA increases approximately linearly with reductions in pH below a threshold at pH 6.55. Interestingly, negatively-charged dipalmitoylphosphatidylglycerol membranes showed qualitatively similar behavior in the presence of PEAA, and incorporation of 10% or 20% dipalmitoylphosphatidic acid in DPPC membranes did not affect solubilization rates, demonstrating that membrane charge is not an important factor in determining micellization kinetics. Micellization of DPPC and dimyristoylphosphatidylcholine membranes occurs most rapidly at their respective gel-liquid crystalline transition temperatures (Tm); the rate enhancement is correlated with a peak in the temperature-dependent binding of a fluorescently-modified PEAA in slightly alkaline solutions in which no micellization is observed. The lateral compressibility of the membrane, which has a similar peak at Tm, is proposed to be an important determinant of the rate and extent of polymer adsorption, and consequently of the rate of micellization.