Facilitation of Ion Permeability of Bilayer Membranes and their Phase Transition

Abstract

The rate of relase of Tl⁺ from phospholipid vesicles of different composition was measured by pulse polarography as a function of temperature or in the presence of valinomycin, tetraphenyl boron (TPB^‒) or dipicylamine (DPA^‒) as transport facilitators. The release from pure dipalmitoyl phosphatidylcholine (DPPC) vesicles increased abruptly around the pretransition temp-earture. The steepness of the increase decreased with the width of the transition peak. Valinomycin TPB^‒ (tetraphenyl boron) and DPA- (dipicryl amine) facilitate release of Tl⁺ from unilamellar vesicles above their phase transition temperature with a first order release rate constant. They do not facilitate release below the phase transition. Bursts of release were observed upon their addition to the vesicles but after annealing, which was completed within less than a minute, the vesicles were re-sealed. The rate of phase transitions, their reversibility or irreversibility and hysteresis with concomitant existence of metastable states, will be discussed.