Seminars Archive

Application of ATR-FTIR spectroscopy to the study of membrane heterogeneity

Abstract
One of the major current issues in membrane biophysics is the incidence and stabilization of lateral heterogeneous structure of biological membranes. Interesting model systems are lipid mixtures containing cholesterol, while they exhibit different lipid phases and phase coexistence regions at appropriate temperature and amount of cholesterol. For example, lipids can be in the so called liquid-ordered phase, which in some respect resembles the cholesterol-rich lipid domains, sometimes called lipid rafts, in biological membranes. Since vibrational states of lipid molecular groups depend on molecular conformations and interactions, infrared (IR) spectroscopy can be used to detect heterogeneous structure of a lipid membrane. Beside the molecular spatial scale, also the time scale in the picosecond range is appropriate. The advantage of IR spectroscopy is that spectroscopic probes are not required. By using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy the temperature and composition dependence of the shape of the carbonyl (amide I) band in phosphatidylcholine/cholesterol (sphingomyelin/cholesterol) model membranes was examined. It will be shown that ATR-FTIR spectroscopy is very suitable for experiments with hydrated samples. The results confirm that cholesterol has a significant indirect influence on the band shapes through conformational and hydration effects. In addition, the presence of direct interaction with cholesterol through hydrogen bond was evaluated.