Membrane-damaging action of ricin on DPPC and DPPC-cerebrosides assemblies. A Raman and FTIR analysis

Perturbations induced by a toxic lectin (ricin) on lipid organisation of model membranes prepared with DPPC and DPPC-cerebrosides mixtures have been analysed by Raman and infrared spectroscopy, two powerful and non-invasive methods. Our approach involves the observation of changes in the vibrational spectra of liquid multilayers in the PO2-, C = O and CH2 spectral regions for two lipid:ricin molar ratios (225:1, 75:1). The interfacial and polar regions of the multilayers, analysed by FTIR, appear to be perturbed by the protein. With both kinds of membranes, ricin mainly perturbs the C = O ester groups of the sn-2 acylchain of DPPC. In the PO2- stretching region, the frequency shifts are correlated with changes in polar group hydration. In the hydrophobic core of the multilayer membrane studied by Raman spectroscopy, the interaction of ricin is associated with changes in lipid packing. These perturbations depend upon the lipid composition of the membrane. With DPPC membranes, an affect is detected at temperatures lower than Tm. It corresponds to a decrease of the lipid ordering. With DPPC-cer membranes, the protein increases the acyl-chain packing order regardless of the temperature of the experiments (10 degrees C less than T less than 75 degrees C). No perturbation of Tm is observed after addition of ricin to either DPPC or DPPC-cer membranes. The different perturbations detected by Raman and FTIR suggest that ricin mainly interacts with the interfacial domains of the membranes.