EPR spectra of spin labels in liposomes formed with erythrocyte lipids

C. J. BONIN; P. RODI; A. M. GENNARO

Águas de Lindóia, Sao Pablo

Congreso; XXXVI ENCONTRO NACIONAL DE FISICA DA MATÉRIA CONDENSADA: XXXVI Brazilian Meeting on Condensed Matter Physics; 2013

Sociedade Brasileira de Física

Cell membranes are composed essentially by a lipid bilayer and great variety of proteins, many of them traversing the bilayer (integral proteins). The degree of order of the acyl chains of the lipids forming the bilayer is thus influenced by the presence of these proteins. In order to discriminate this effect, we performed a lipid extraction from human erythrocyte (red blood cells) membranes, prepared unilamellar liposomes (LUVs) which are expected to be similar to the original membranes, but lacking the integral proteins. LUVs were obtained by extrusion of multilamellar liposomal suspensions using membrane filters with a pore size of 0.2um. The spin labels derived from stearic acid were incorporated to the LUVs to sense the bilayer at different depths using Electron Paramagnetic Resonance (EPR) spectroscopy: 5-SASL (near lipid polar headgroups), and 12-SASL (in the mid portion of the carbon chains). X-band EPR spectra were characteristic of the slow motional regime, and thus they were fitted with the chili function of the EasySpin package (www.easyspin.org) using the Microscopic Order Macroscopic Disorder (MOMD) model, to obtain parameters characterizing molecular dynamics and orientational ordering in lipid bilayer. Quite good fits were obtained with axially symmetric tensors besides limited spectral resolution, characteristic of this dynamics regime. The magnetic parameters (g-tensor and hiperfine A-tensor) had to be varied around their more common values g=[2.0088 2.0061 2.0027] and A(mT)=[0.59 0.54 3.29], to adapt the tensor elements to the lipid membrane environment in each case. The parameter related to orientational order decreases towards bilayer center. The same happens with tranversal rotational correlation time indicative of lipid dynamics. These parameters are compared with those obtained in spin labeled intact erythrocytes, in order to inquire how do membrane proteins affect the lipidic order.