Residence time of singlet oxygen in membranes.

Photodynamic therapy uses photosensitizers (PS) to kill cancer cells by generating reactive oxygen species - like singlet oxygen (SO) - upon illumination with visible light. PS membrane anchoring augments local SO concentration, which in turn increases photodynamic efficiency. The latter may suffer from SO's escape into the aqueous solution or premature quenching. Here we determined the time constants of SO escape and quenching by target molecules to be in the nanosecond range, the former being threefold longer. We confined PS and dipolar target molecules either to different membrane monolayers or to the same leaflet and assessed their abundance by fluorescence correlation spectroscopy or membrane surface potential measurements. The rate at which the contribution of the dipolar target molecules to membrane dipole potential vanished, served as a measure of the photo-oxidation rate. The solution of the reaction-diffusion equations did not indicate diffusional rate limitations. Nevertheless, reducing the PS-target distance increased photodynamic efficiency by preventing other SO susceptible moieties from protecting the target. Importantly, our analytical model revealed a fourfold difference between SO generation rates per molecule of the two used PSs. Such analysis of PS quantum yield in a membrane environment may help in designing better PSs.

Line Activity of Ganglioside GM1 Regulates the Raft Size Distribution in a Cholesterol-Dependent Manner.

Liquid-ordered lipid domains, also called rafts, are assumed to be important players in different cellular processes, mainly signal transduction and membrane trafficking. They are thicker than the disordered part of the membrane and are thought to form to compensate for the hydrophobic mismatch between transmembrane proteins and the lipid environment. Despite the existence of such structures in vivo still being an open question, they are observed in model systems of multicomponent lipid bilayers. Moreover, the predictions obtained from model experiments allow the explanation of different physiological processes possibly involving rafts. Here we present the results of the study of the regulation of raft size distribution by ganglioside GM1. Combining atomic force microscopy with theoretical considerations based on the theory of membrane elasticity, we predict that this glycolipid should change the line tension of raft boundaries in two different ways, mainly depending on the cholesterol content. These results explain the shedding of gangliosides from the surface of tumor cells and the following ganglioside-induced apoptosis of T-lymphocytes in a raft-dependent manner. Moreover, the generality of the model allows the prediction of the line activity of different membrane components based on their molecular geometry.

[Stabilization of a complex of fusion proteins by membrane deformations].

The initial stage of membrane fusion under the action of fusion proteins transmitting force to the membrane is considered in the work. Protein inclusions in the membrane create a highly curved bulge, which facilitates fusion of contacting membrane monolayers. Membrane is considered as a liquid-crystal medium subjected to elastic deformations. Deformations of splay and tilt are taken into account and energy is calculated to the second order on these deformations. Protein complexes are modeled as rigid tilted rings embedded into the fusing membranes. The energy needed to locally bring membranes together under the action of proteins is calculated. The dependence of the membrane energy on a protein ring radius is shown to be minimum. It means that membrane deformations stabilize the radius of the protein cluster. The main characteristics of the system, such as equilibrium radius of the protein complex and the minimal energy needed to accomplish the first stage of the fusion, are calculated.