RESUMO
Small sugars are known to stabilize biological membranes under extreme conditions of freezing and desiccation. The proposed mechanisms of stabilization suggest membrane-sugar interactions to be either attractive or repulsive. To obtain new insight into the problem, we use a recently developed low-frequency Raman scattering approach which allows detecting membrane mechanical vibrations. For model membranes of palmitoyl-oleoyl-glycero-phosphocholine (POPC) hydrated in aqueous sucrose and trehalose solutions, we studied the Raman peak between 12 and 15 cm-1 that is attributed to an eigenmode of the normal mechanical vibrations of a lipid monolayer. For both sugars, similar results were obtained. With an increase in sugar concentration in solution, the frequency position of the peak was found to decrease by â¼13% which was interpreted as a consequence of the membrane thickening due sugar monolayer adsorption on the membrane surface. The concentration dependence of the peak frequency position was satisfactorily described by a Langmuir monolayer adsorption model. It is concluded that, at small sugar concentrations (less than 0.2 M), the membrane-sugar interactions are attractive, while at higher concentrations (more than 0.4 M) the attraction disappears. The data obtained show that one sugar molecule on the surface interacts with approximately 3-4 polar lipid heads.
RESUMO
A lipid bilayer containing a ternary mixture of low- and high-melting lipids and cholesterol (Chol) can give rise to domain formation, referred to as lipid rafts. Low-frequency Raman spectroscopy at reduced temperatures allows detection of normal membrane mechanical vibrations. In this work, Raman spectra were obtained in the spectral range between 5 and 90 cm-1 for bilayers prepared from dioleoyl-glycero-phosphocholine (DOPC), dipalmitoyl-glycero-phosphocholine (DPPC) and Chol. A narrow peak detected between 13 and 16 cm-1 was attributed to the vibrational eigenmode of a lipid monolayer (a leaflet). For the equimolar DOPC/DPPC ratio, the Chol concentration dependence for the peak position, width and amplitude may be divided into three distinct ranges: below 9 mol%, the intermediate range between 9 mol% and 38 mol%, and above 38 mol%. In the intermediate range the peak position attains its minimum, and the peak width drops approximately by a factor of two as compared with the Chol-free bilayers. Meanwhile, this range is known for raft formation in a fluid state. The obtained results may be interpreted as evidence that bilayer structures in the raft-containing fluid state may be frozen at low temperatures. The drop of peak width indicates that at the spatial scale of the experiment (â¼2.5 nm) the intermolecular bilayer structure with raft formation becomes more homogeneous and more cohesive.
RESUMO
Sugars can stabilize biological systems under extreme desiccation and freezing conditions. Hypothetical molecular mechanisms suggest that the stabilization effect may be determined either by specific interactions of sugars with biological molecules or by the influence of sugars on the solvating shell of the biomolecule. To explore membrane-sugar interactions, we applied electron spin echo envelope modulation (ESEEM) spectroscopy, a pulsed version of electron paramagnetic resonance (EPR), to phospholipid bilayers with spin-labeled lipids added and solvated by aqueous deuterated sucrose and trehalose solutions. The phospholipids were 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). The spin-labeled lipids were 1,2-dipalmitoyl-sn-glycero-3-phospho(TEMPO)choline (T-PCSL), with spin-label TEMPO at the lipid polar headgroup. The deuterium ESEEM amplitude was calibrated using known concentrations of glassy deuterated sugar solvents. The data obtained indicated that the sugar concentration near the membrane surface obeyed a simple Langmuir model of monolayer adsorption, which assumes direct sugar-molecule bonding to the bilayer surface.
