Critical analysis of the 12-SASL EPR spectra through the phase transitions of DMPC and DPPC liposomes

A. NICASTRO; A.S. GARAY; J.F. SILVERA; D.E. RODRIGUES; A.M GENNARO

Salta

Congreso; 3rd.Latinamerican Protein Society Meeting-XXXIX Biophysical Soc. Meeting; 2010

SAB y Sociedad Latinoamericana de Proteinas

An important tool to study the dynamic order in lipid bilayers is EPR spectroscopy of n-SASL spin labels (SL), where n stands for the position of the doxyl ring that hosts the nitroxide radical in the acyl chain of stearic acid. It is usually assumed that these SL's sense the dynamics of the unperturbed bilayer around the position of the nth acyl C of its component phospholipids. The interpretation of spectra of SL's in compact phases (e.g. Gel and Ripple) presents additional difficulties related to the identification of the local environment of the SL. In this work we performed EPR experiments on liposomes (MLV and LUV) of DMPC and DPPC labeled with 12-SASL to characterize the local order on their phase transitions with temperature. The experiments are conducted at pH 5.5 / 9 at which it is recognized that the polar head of the SL is protonated/deprotonated respectively. We performed a critical examination of the experimental data and use the results of Molecular Dynamics (MD) simulations to extract the information about the local environments in various phases. The spectra show different behavior of the SL depending on the length of the phospholipid acyl chains, the pH of the buffer, and the respective phase. The fitting of the spectra using the theory of rapid motion along with the results of MD allow us to draw conclusions on the local environment of the SL. Single and multiple component spectra that reveal the simultaneous coexistence of different environments of the SL are analyzed.