Spin-labeled Stearic Acid Behavior and Perturbations on the Structure of a Gel-Phase-Lipid Bilayer in Water: 5-, 12- and 16-SASL

MARTÍN R. VARTORELLI; ALBERTO S. GARAY; DANIEL E. RODRIGUES

JOURNAL OF PHYSICAL CHEMISTRY B - (Print)

AMER CHEMICAL SOC

Año: 2008 vol. 112 p. 16830 - 16842

1520-6106

We have studied the effect of the insertion of spin-labeled molecules n-doxyl-stearic acid (n-SASL, n ) 5, 12, 16) on the structure and dynamics of a model lipid bilayer in gel-like phases using molecular dynamics simulations. We have studied the atomic density depth profiles and configurations of the labeled molecules in a host hydrated stearic acid bilayer system. We have found that the 5-SASL label positions its paramagnetic group at the water-lipid interface, and its polar head builds H bonds to neighboring lipids and to the solvent. 16-SASL positions its paramagnetic group at the lipid-lipid interface. The 12-SASL label presents two configurations at high lateral pressure. In one configuration, the doxyl ring lays at the lipid-lipid interface, shifting its polar head toward the bilayer center. The other equilibrium configuration of 12-SASL presents its paramagnetic group laying in the center of the compact hydrophobic region of the layer (erected configuration). It was determined that the coexistence of these two configurations is governed by the polar head-water interaction. We have found that the insertion of the labeled molecules at the concentrations used in the present work (0.36 mol %) do not perturb global properties like area per lipid, tilt angle, or order parameters. Nevertheless, there are local perturbations of the host system that are confined to a 10 Å neighboring shell around the spin label molecule. To study the interactions that determine the position of the labeled molecules in the bilayer, we performed simulations at different lateral pressures, which allowed us to extract important conclusions.