Spin labelling study of interfacial properties of egg-phosphatidylcholine liposomes as a function of cholesterol concentrations

In order to gain insight into interfacial properties of liposomes composed of egg-phosphatidylcholine (egg-PC) and dihexadecyl-phosphate (DHP) as a function of 0, 8, 15, 29, 38, 45 mol% of cholesterol, dynamic properties of two long-chain spin labels: TEMPO-stearate (2,2,6,6-tetramethylpiperidine-1-oxyl-4-yl)-octa-decanoate) and TEMPO-stearamide (2,2,6,6-tetramethylpiperidine-1-oxyl-4-yl)-octa-decanamide) were studied by CW-ESR spectroscopy. These spin labels reflect motional properties in the region of phospholipid head-groups.Two different environments of TEMPO-stearate were determined at 29, 38 and 45 mol% of cholesterol. In the newly formed domain above 29 mol%, N–O moiety of the spin label was surrounded by larger amount of bound water and experienced slower motion than in the cholesterol poor domain. The fraction of the second more hydrophilic environment of the spin label increased with cholesterol concentration. TEMPO-stearamide, a hydrogen-bond donor, reported more polar environment and slower motion than TEMPO-stearate even in the absence of cholesterol. Only one spin label environment was determined for all cholesterol concentrations. Slowing down of the TEMPO-stearamide motion was obtained even at 8 mol% of cholesterol.