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An ESR study of cholesterol effect on interfacial properties of egg-phosphatidylcholine vesicles

One of the important properties of biological membranes is that they act as a barrier to the permeation of polar molecules (solutes and solvents). This effect is largely due to the hydrophobic property of the membrane interior and is related to the water penetration into the membranes1, 2. The influence of different cholesterol concentrations in liposomes (composed of egg-phosphatidylcholine and dicetilphosphate) on the polarity profile across the lipid bilayer was studied by FT-ESEEM method3, 4 at 35 K. The effect of D2O on ESR spectra of • moiety of n-doxyl-stearic acids (n = 5, 7, 10, 16) was used to resolve the extent of water penetration into liposomes from effects of the nearby hydrogens from methylene groups. An increase of D2O penetration into liposomes was determined above the 15 mol% of cholesterol. In order to obtain information on how cholesterol influences interactions between the phospholipids head-group motional properties of the spin label TEMPO-stearate in liposomes was studied at 312 K. This spin label incorporated into liposomes reports on the structure of the double layer of the surface water containing the counter-ions5. Rotational correlation times ( ) of the spin label were calculated from the CW– ESR spectra by Chili program as a part of the EasySpin program6. A change of motional properties of the label was determined at the cholesterol concentration of 15 mol%. Above that concentration spin label reported not only slowing down of the motion but as well different motional averaging. The results obtained by both methods suggest that above 15 mol% of cholesterol interhead-group interactions between phosphatidylcholine molecules are broken and this enables the increase in the penetration of water into the liposome bilayer. [1] Simon S.A. and McIntosh T.J. ; Methods in Enzymology, 1986, 127, 511. [2] Marsh, D. ; Proc. Natl. Acad. Sci. USA, 2001, 98, 7777. [3] Bartucci, R., Guzzi, R., Marsh, D. And Sportelli, L., Biophys. J. 2003, 84, 1025. [4] Noethig-Laslo, V., Cevc, P., Arcon, D., and Sentjurc, M. ; Orig. Life Evol. Bios. 2004, 34, 237. [5] Alves, M. and Peric, M. ; Biophys. Chem. 2006, 122, 66. [6] S. Stoll and A. Schweiger, EasySpin, a comprehensive software package for spectral simulation and analysis in EPR, J. Magn. Reson. 178 (2006) 42-55.

ESR; egg-phosphatidylcholine; cholesterol

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