Northern Kentucky University
Cation Permeation Mechanisms in Artificial Lipid Bilayers
Institution
Northern Kentucky University
Faculty Advisor/ Mentor
Stefan Paula
Abstract
Two distinct mechanisms are most frequently discussed to describe the permeation of ions across the lipid bilayers of cells: the solubility-diffusion mechanism and the pore mechanism. In the solubility-diffusion mechanism, ions dissolve into the hydrophobic phase of the bilayer, diffuse across, and then leave the membrane at the opposite side. According to the pore mechanism, ions cross the bilayer through momentary hydrophilic defects, temporary pores caused by thermal fluctuations within the membrane. We were able to discern which model better explains the passive permeation of sodium ions across the lipid bilayer. This was accomplished by comparing the theoretical predictions of both mechanisms with experimentally determined permeability coefficients as a function of bilayer thickness and ionic radius. Phosphatidylcholines with chain lengths between 12 and 20 carbon atoms were used to systematically vary the bilayer thickness. A sodium selective electrode was used to measure the escape of sodium ions from the liposomes over time. The permeability coefficients ranged between 10-12 and 10-14 and tended to decrease as bilayer thickness increased.
Cation Permeation Mechanisms in Artificial Lipid Bilayers
Two distinct mechanisms are most frequently discussed to describe the permeation of ions across the lipid bilayers of cells: the solubility-diffusion mechanism and the pore mechanism. In the solubility-diffusion mechanism, ions dissolve into the hydrophobic phase of the bilayer, diffuse across, and then leave the membrane at the opposite side. According to the pore mechanism, ions cross the bilayer through momentary hydrophilic defects, temporary pores caused by thermal fluctuations within the membrane. We were able to discern which model better explains the passive permeation of sodium ions across the lipid bilayer. This was accomplished by comparing the theoretical predictions of both mechanisms with experimentally determined permeability coefficients as a function of bilayer thickness and ionic radius. Phosphatidylcholines with chain lengths between 12 and 20 carbon atoms were used to systematically vary the bilayer thickness. A sodium selective electrode was used to measure the escape of sodium ions from the liposomes over time. The permeability coefficients ranged between 10-12 and 10-14 and tended to decrease as bilayer thickness increased.