Evaluation of a phase transition of stearic acid bilayers labelled with 12-doxyl stearic acid by molecular dynamics

VARTORELLI, MARTIN R.; GARAY, ALBERTO S.; RODRIGUES, DANIEL E.

Rosario, Argentina

Congreso; XXXV Reunión Anual de la Sociedad Argentina de Biofísica; 2006

SAB

Electronic Paramagnetic Resonance of spin-labeled fatty acids inserted in membranes has been a useful experimental technique in the study of soft matter (e.g. biological membranes, model bilayers).The perturbation originated by this spin-labeled-molecule on the native system under study has been a subject of debate. We performed Molecular Dynamics (MD) simulations of bilayers of stearic acid in water with and without the spin label inserted into. The system is studied under several conditions of lateral pressure and temperature. The system without the spin label was studied at temperatures of 27°C (and 42°C), with surface tensions in the range 50 to 2000 bar.nm (50 to 1850 bar.nm). In both cases, increasing the surface tension (lowering the lateral pressure) caused a moderate increase of the area per lipid and tilt angle, and a the decrease of the order parameter (OP). At 27°C a phase transition was observed at a surface tension of 1900 bar.nm. At this point a transition from a regular hexagonal bidimensional lattice (a=4.81Å, b=4.50 Å, φ=70.57º) to a oblique one (a=7.06Å, b=5.90 Å, φ=47.59º) takes place. At 42°C the transition occurs at 1850 bar.nm between a symetric hexagonal lattice (with parameters a=5.14Å, b=4.77 Å, φ=60.16º) and a oblique lattice (with parameters a=4.19Å, b=4.45 Å, φ=85.95º). The MD simulation of the system with the spin-labeled molecule inserted at 27°C showed that the perturbation affects the OP of the stearic acids molecules closest to the spin marker, specifically in the polar head and tail zones.