Effect of the concentration and the molecular structure of phosphonic acid dialkyl esters on the kinetics of Pr 3+ permeation through egg phosphatidylcholine small unilamellar vesicles

Abstract

The dependence of the Pr 3+ permeation kinetics in unilamellar vesicles of egg phosphatidylcholine on the incorporation of phosphonic acid dialkyl esters in the membrane was studied by 31P- and 1H-NMR. The phosphonic acid dialkyl esters cause drastic changes of the permeation half-time t 1 2 of the Pr 3+ ions into the inner vesicle compartment. The ion permeation induced by the esters was interpreted as due to the formation of metastable pores. The permeation half-time t 1 2 depends on the ester concentration in the membrane and on temperature. The threshold concentration of the ester necessary for a drastic decrease in permeation half-time t 1 2 reflects certain structural differences of the esters. The results suggest a correlation between the effective molecular shape of the phosphonic acid esters and the ion permeation. The esters are mainly incorporated in the outer vesicle monolayer. This could be understood because of the positive molecular asymmetry of the phosphonic acid dialkyl esters. The permeation half-time t 1 2 of Pr 3+ ions out of the vesicles is much larger than the half-time t 1 2 for the opposite process.