Phospholipid Vesicle Aggregation: Effect of Monovalent and Divalent Ions
Nejat Düzgüneş: 0000-0001-6159-1391
A study on cation-induced aggregation of unilamellar phospholipid vesicles was made by measuring the turbidity of vesicle suspensions. As the cation concentration was increased, the degree of aggregation of acidic phospholipid vesicles increased. The concentration at which the sharpest increase in turbidity was observed was defined as the “threshold” concentration. The threshold concentrations of various divalent cations for aggregation of phosphatidylserine vesicles in 0.1 M NaCl, pH 7.0, were 0.65 mM Mn2+, 0.8 mM Ba2+, 1.0 mM Ca2+, 1.2 mM Sr2+, and 4.5 mM Mg2+. This order of threshold concentrations was the same for vesicles composed of mixtures of phosphatidylserine and phosphatidylcholine. A decrease in the monovalent ionic strength of the vesicle suspension solutions raised the threshold concentrations for Mn2+ and Ca2+. The order of effectiveness for monovalent cations to cause phosphatidylserine vesicle aggregation was H+ > Na+ > Li+ > K+ > TMA+ (tetramethylammonium). The effect of pH on phosphatidylserine vesicle aggregation was studied in the presence of Na+. The threshold pH for vesicle aggregation was about 2.6 in 100 mM NaCl. An attempt was made to explain vesicle aggregation in terms of surface potential, surface charge densities of the membranes, and the electrostatic repulsive interaction energy between two vesicles. It was proposed that the slope of the total interaction energy (repulsive interaction) at the Debye distance is related to the degree of massive aggregation occurred among phosphatidylserine vesicles. Knowing the intrinsic binding constants of Ca2+ (30 M−1) and Na+ (0.6 M−1) to a phosphatidylserine membrane and assuming that the slope of the interaction energy of two interacting membranes is the same at the Debye distance at the observed threshold concentrations of ions for vesicle aggregation, we calculated the binding constants of other metal ions: Mn2+ = 49 M−1, Ba2+ = 37 M−1, Sr2+ = 25 M−1, Mg2+ = 10 M−1, Li+ = 0.4 M−1, K+ = 0.2 M−1, and H+ = 100 M−1. © 1982, American Chemical Society. All rights reserved.
Phospholipid Vesicle Aggregation: Effect of Monovalent and Divalent Ions.
Biochemistry, 21(9), 2127–2133.