Ratio Of Membrane Thickness Research Articles

Cholesterol inhibits oxygen permeation through biological membranes: mechanism against double-bond peroxidation.

The presence of oxygen molecules (O2) in biological membranes promotes lipid peroxidation of phospholipids with unsaturated acyl chains. On the other hand, cholesterol is considered to be an antioxidant molecule as it has a significant barrier effect on the permeation of O2 across membranes. However, a comprehensive explanation of how cholesterol affects the distribution and diffusion of O2 within lipid bilayers is yet to be established. In this study, we investigated the interaction of oxygen molecules with polyunsaturated lipid bilayers using molecular dynamics (MD) simulations. The degree of lipid unsaturation and the concentration of cholesterol were varied to study the permeation of O2. The free energy profile of O2 diffusing from the water phase to the lipid bilayer was calculated using biased umbrella MD simulations. The results show that O2 passively translocates into the membrane without changing the physical properties of the bilayer. Interestingly, in the unsaturated lipid bilayers the presence of cholesterol led to a significantly decreased permeation of O2 and an increase in the lipid chain order. Our results indicate that the hydroxyl groups of cholesterol strongly interact with the O2 molecules effectively inhibiting interactions between the oxygens and the double bonds in unsaturated lipid tails. In addition, a linear relationship between permeation and the ratio of membrane thickness and area per lipid was found. These insights can help our understanding of how the degree of unsaturation in a lipid tail and cholesterol affect lipid peroxidation at the molecular level.

Analysis of transmembrane electrical potential across nanofiltration membranes based on electrostatic and steric-hindrance model

Transmembrane electrical potential (TMEP) across nanofiltration (NF) membranes was calculated analytically in single electrolyte solution -– NF membranes systems with electrostatic steric-hindrance (ES) model in this study. Moreover, a simplified expression with average membrane parameters was obtained to give explicit explanations by combining ES model and irreversible thermodynamics. The effects of electrolyte species with common co-ion Cl_ (KCl and MgCl2), electrolytes concentration c, diffusion coefficient ratio of co-ion over counterion D2/D1 , pore radius rp , ratio of membrane thickness over porosity ?x/Ak , effective volume charge density Xw on TMEP were investigated. The results showed that with the existence of membrane potential, dependencies of TMEP on solution flux were nonlinear. When ξf -1 (z1v1cf/Xw ) was larger than 50 for 1-1 electrolytes and 100 for 2-1 electrolytes, TMEP tended to be constant and three potentials (TMEP, membrane potential and convection potential) crossed at D2/D1Di = 1.0, which implied that the electrostatic effect could be neglected. When the isoelectric point of membranes is judged in different pH based on the zero point of TMEP, solutes with D2/D1 = 1.0 is recommendatory. Because when D2/D1 > 1.0, the zero point will locate on where Xw is negative, and when D2/D1 < 1.0, zero points will appear when Xw is positive, and only when D2/D1 = 1.0, the zero point of TMEP appears when membrane is neutral (Xw is zero). Moreover, a sufficient condition t1m/z1+t2m/ z2 = 0 was proposed to explain the coincidence of zero point of membrane potential and minimum of reflection coefficient.