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; Reunión Conjunta XXXIX Annual Meeting of the Argentinean Biophysical Society and 3rd. Latin American Protein Society Meeting; 2010

An important tool to study the dynamic order in lipid bilayers is EPR spectroscopyof n-SASL spin labels (SL), where n stands for the position of the doxyl ring that hoststhe nitroxide radical in the acyl chain of stearic acid. It is usually assumed that theseSL's sense the dynamics of the unperturbed bilayer around the position of the n acyl Cof its component phospholipids. The interpretation of spectra of SL's in compactphases (e.g. Gel and Ripple) presents additional difficulties related to the identificationof the local environment of the SL. In this work we performed EPR experiments onliposomes (MLVand LUV) ofDMPCand DPPC labeled with 12-SASLto characterizethe local order on their phase transitions with temperature. The experiments areconducted at pH 5.5 / 9 at which it is recognized that the polar head of the SL isprotonated/deprotonated respectively. We performed a critical examination of theexperimental data and use the results of Molecular Dynamics (MD) simulations toextract the information about the local environments in various phases. The spectrashow different behavior of the SL depending on the length of the phospholipid acylchains, thepHof the buffer, and the respective phase. The fitting of the spectra using thetheory of rapid motion along with the results ofMDallow us to draw conclusions on thelocal environment of the SL. Single and multiple component spectra that reveal thesimultaneous coexistence of different environments of the SLare analyzed.