Effect of the Inclusion of the Spin Label 10-doxyl-Stearic Acid on the Structure and Dynamics of Model Bilayers in water: Stearic Acid and Stearic Acid/Cholesterol (50:20)

Abstract

The effects of the insertion of a spin-labeled molecule (10-doxyl-stearic acid) on the structure and dynamics of model lipid bilayers in gel-like as well as in liquid-ordered-like phases are studied using molecular dynamic simulations. The perturbing effects of the labeled molecule on the structure of the bilayers are analyzed. We have also studied the relationship between the structural and dynamic properties of the bilayer phase and those of the labeled molecule. We found that the insertion of the labeled molecule in the bilayer at the concentration considered here (1:70) produces local and global perturbations in the gel-like phase. There is an increase of the area associated with the lipid molecules that produces a larger tilting angle of this condensed phase. In this gel-like phase, we also found that the z component of the order parameter of the labeled molecule associated with the electron paramagnetic resonance (EPR) spectra has the same temperature dependence as the axial correlation times of the lipid molecules. The mechanism by which the doxyl reorientation senses the dynamics of the layers is determined by the correlation between the gauche defect transitions of the labeled alkyl chain and its environment. For the liquid-ordered-like phase, we found that cholesterol molecules play the role of wedges that open free spaces in the lipid structure below the ring position and order the alkyl chains at the depths of the rings, leading to small inclination angles. The doxyl ring of the labeled molecule is located just below the cholesterol ring moiety, having fewer gauche defects than in the case of the gel-like phase. The change in depth of the doxyl ring causes a reorientation of this group that leads to an increase of the order parameter as the temperature rises.