Study of the behavior of a N-doxyl spin label in a model lipid bilayer in gel-like phases by Molecular Dynamics: Structural and dynamic properties and EPR spectra calculations

VARTORELLI, MARTIN R.; 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

<!-- @page { size: 8.5in 11in; margin: 0.79in } P { margin-bottom: 0.08in } --> We have performed Molecular Dynamics simulations of Stearic Acid (SA) bilayers in water to study how the insertion of a N-doxyl labeled SA affects the structure of the lipid assembly and how it senses the dynamic of the host system. The simulations were carried out at the NPZ<gama>T ensemble and several values of <gama> are scanned. The host system present a phase transition between two ordered phases at <gama>=1900 bar.nm for T=27°C (and <gama>=1850 for T=42°C). The analysis of the Structure Factor of the surface shows that the transition occurs between a 2D hexagonal phase with tilt-angle toward NN, and an oblique lattice phase with larger tilting toward the NN neighbors and areas. The axial autocorrelation functions of the acyl chains have correlation times typical of a free rotator phase in both cases. To evaluate the doxyl-labeled effects, the 5D-SA, 12D-SA and 16D-SA, the most popular labels used in EPR experiments are inserted in the host systems. At the spin-label concentration used in this work (1:280) the lipid host systems are only perturbed in the first neighbor shell closest to the doxyl. An important result of these simulations is the average positions where spin-labeled at different sites lie. Our results show a picture very different from the naive vision of a doxyl ring positioned at the depth of the Cn of the alkyl chains of the host system. Only 12D-SA presents configurations where the doxyl ring remains in the middel of each layer. Conversely 5D-SA migrates to the water-lipid interface, and 16D-SA toward the center of the bilayer. These facts lead to different mechanism by the which the spin-label senses the dynamics of the host system. Also there are other variables that affect the properties surveyed by the EPR spectra.