Effects 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)

GARAY, ALBERTO S.; RODRIGUES, DANIEL E.

Montevideo, Uruguay

Congreso; XXXVI Reunión Anual de la Sociedad Argentina de Biofísica, 6th International Conference on Biological Physics y 5th Southern Cone Biophysics Congress; 2007

IUPAP, IUPAB, SAB, SUB

We have studied the effect of the insertion of a spin label molecule (10-doxyl-Stearic Acid) on the structure and dynamic of a model lipid bilayers in gel-like, and in liquid crystalline-like phases using molecular dynamic simulations. The perturbing effects of the labeled molecule on the structure of the bilayers were analyzed. We have also studied the relation between the structural and dynamic properties of the bilayer phase and those of the labeled molecule. We found that the label insertion in the bilayer at the concentration that was analyzed here (1:70) produces local and global perturbations in the gel-like phase. There is an increase of the area associated to the lipid molecules that translates into a larger tilting angle of this condensed phase. We found that the z component of the order parameter of the labeled molecule that is associated with the EPR spectra follows the temperature dependence of the axial correlation times of the lipid molecules in this gellike phase. The mechanism by the which the doxyl reorientation senses the dynamics of the layers is the correlation between the gauche defects of the labeled alkyl chain and its environment. For the liquid crystalline-like phase we found that the cholesterol molecules play the role of wedges that open free spaces in the lipid structure below their ring portion, and order the alkyl chains at the depths of the rings, leading to negligible inclination angles. The doxyl ring of the labeled molecule is located just below the cholesterol ring moiety with fewer gauche defects than in the gel-like system. The change in the depth position of the doxyl ring causes a reorientation of this group that lead to an increase of the order parameter with the temperature rise.