Interpretation of Molecular Dynamics in Unilamellar Vesicles Using Field-Cycling NMR Relaxometry. Cholesterol Effect

C. FRAENZA; C. J. MELEDANDRI; DERMOT F. BROUGHAM; E. ANOARDO

Conferencia; XLI Reunion Anual de la Sociedad Argentina de Biofísica; 2012

 It was previously demonstrated that fast field cycling (FFC) proton NMR spin-lattice relaxation rate dispersions of liposomes can be explained through a physical model that accounts for the molecular and collective dynamics of the lipids[1,2]. The model was successfully compared with experimental measurements of liposomes prepared with different lipids (DMPC and DOPC), sizes (100-200nm) and temperatures (within the fluid liquid crystalline phase), using values for the different physical constants and parameters available in the literature. The FFC NMR method turned to be a useful tool for the study of the molecular dynamics of lipids and the viscoelastic properties of biological membranes. It was claimed that, up to a limited concentration of cholesterol, the membrane remains in the disordered liquid crystalline phase (ld)[3,4]. However, other authors define a region around each molecule of cholesterol where lipids become strongly affected (much ordered). In consequence, the lipids population divides in unaffected and affected, depending on the proximity to a cholesterol molecule[5,6,7,8,9]. The unaffected lipids are considered to be in the ld phase, while the affected lipids are described in an ordered liquid crystalline phase (lo). In this work we confront both approaches with new experimental data as a further application of our model. We analyze experimental FFC relaxation rate dispersion curves obtained at 298 K for liposomes of radius between 68 and 80 nm composed of DOPC and cholesterol at 10 and 25mol%. The consistence obtained in our analysis suggests that the model previously used to explain the relaxation-rate dispersion in free-cholesterol DOPC liposomes[1,2], can be extended to the study of liposomes containing cholesterol in the membrane.   Referencias: [1] Meledandri C. J., Perlo J., Farrher E., Brougham D. F., Anoardo E., J. Phys. Chem. B, 2009, 11, 15532. [2] Perlo J., Meledandri C. J., Anoardo E., Brougham D. F. J. Phys. Chem. B, 2011, 115, 3444. [3] Filippov A., Orädd G., Lindblom G. Biophys. J. 2003, 84, 3079. [4] Filippov A., Orädd G., Lidblom G., Langmuir 2003, 19, 6397. [5] Edholm O., Nyberg A. M., Biophys J. 1992, 63, 1081. [6] Robinson A. J., Richards W. G., Thomas P. J., Hann M. M. Biophys J. 1995, 68, 164. [7] Chiu S. W., Jakobsson E., Mashl R. J., Scott H. L. Biophys J. 2002, 83, 1842. [8] Jedlovszky P., Mezei M., J. Phys. Chem. B, 2003, 107, 5311. [9] Dai J., Alwarawrah M., Huang J. J. Phys. Chem. B, 2010, 114, 840.