Comparative study of lipid phase transitions in vesicles of different sizesusing epr spectroscopy

ALCIDES NICASTRO; DANIEL E. RODRIGUES; ANA M. GENNARO

Buzios, Río de Janeiro, Brasil

Congreso; Biophysics 2009, VII Congreso Iberoamericano de Biofísica; 2009

Sociedad de Biofísica de Brasil y Argentina, IUPAB

Objective: To perform a comparative study of DPPC and DMPC phase transitions detected by EPR (Electron Paramagnetic Resonance spectroscopy) in unilamellar vesicles (LUV) of different sizes and multilamellar liposomes (MLV). Methods: The liposomes were spin labeled using n-SASL (doxyl ring sterified at the nth C position of a stearic acid molecule). X-band spectra of the labeled liposomes were taken as temperature was scanned through the different phases. The characteristics of the local environment of the spin label are revealed by the features of the EPR spectra (hyperfine parameters 2Amin, 2Amax , and a partition coefficient when more than one component appears in the spectra). Results and conclusions: The temperature variation of the hyperfine parameters for 5- and 12-SASL spin labeled liposomes show that the main phase transition is more cooperative in the case of MLV than in LUV. 2Amin in MLV of both lipids show a kink suggesting the onset of the pretransition, but LUVs do not show this feature. The spectra of 12-SASL labeled liposomes show two components: one characteristic of the spin label in a highly mobile environment and the other in a more constraining surrounding. We define a partition coefficient () using the relative weights of both components. By following the changes of (T) we are able to observe the onset of the pretransition in MLV but also in LUV of the larger sizes studied (400 and 200 nm). In the smallest LUVs (100 nm) the changes in (T) through the pretransition zone are smeared out. These effects are discussed in terms of the different lipid packing and surface tension in each vesicle system.