Passive permeation in model biological membranes

Abstract

The subject of this thesis is the study of mass transport using experimental and theoretical techniques, specifically the investigation of transport through phospholipid barriers which serve as a model for biological systems. To this end, experimental monolayer and bilayer membranes are produced, and the permeation of gas and weak acid molecules is quantitatively measured. Potentiostatic measurement of oxygen permeation in monolayers formed using two varieties of phospholipid at the air/water interface, under varying levels of compression (and hence surface pressure), was performed using ultramicroelectrodes, followed by finite element simulations to parameterise the approach curves produced and hence determine the first-order rate constant for the permeation process. As expected, the rate of permeation decreases significantly as the monolayers are compressed, with a simple surface pressure model proving insufficient to describe these trends. Molecular dynamics simulations are employed to investigate the excess free energy barriers for permeating oxygen molecules using umbrella sampling and the weighted histogram analysis method. The results are shown to be unreliable in their description of the permeation process. Experimental bilayers are formed from lecithin, pure POPC, and a mixture of POPC and cholesterol in a supporting electrolyte solution. The permeation of a homologous series of protonated weak acids is studied using laser scanning confocal microscopy to selectively excite and detect the state of fluorescein, a dye with pH-sensitive fluorescence intensity. These experimental results are again parameterised with finite element models, and the trend of decreasing permeation coefficient as the weak acid molecules increase in size is reported. This is in direct contradiction to the established Overton’s Rule. Molecular dynamics simulations of the permeation of three of the weak acids in a POPC bilayer is performed to determine excess free energy profiles using umbrella sampling, combined with the weighted histogram analysis method. Serious flaws are found in the method and execution of this aspect of the work