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The Physics of Non-Specific Interactions between Biomembranes

Subject Area Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Term from 2014 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 256222543
 
Final Report Year 2023

Final Report Abstract

Biomembranes are vital components of all living organisms. Their surfaces are formed by the headgroups of lipids and by membrane-bound saccharides, polypeptides, and polymers. The interaction between membranes in their congested physiological surroundings is sensitive to this composition and affects structural organization and functions of membrane systems like organelles. In order to elucidate the influence of the surface composition on membrane interactions, we have designed well-defined planar experimental models of biomembrane surfaces, composed of lipids and lipid-anchored macromolecules of known chemical structures, and have been studying them under non-interacting conditions and under controlled interaction conditions. Thermodynamic aspects of the interaction, such as pressure-distance curves, have been investigated alongside with structural aspects such as molecular conformations and elemental distributions. The latter have been determined with the help of x-ray and neutron scattering techniques. By using atomistic computer simulations, we have been interpreting the experimental results on a mechanistic level. In the course of the project we have established solid-supported double-monolayer architectures that allow for structural investigation of interacting biomembrane model surfaces and other soft interfaces as a function of their separation and under conditions of full hydration. With the help of neutron reflectometry and standing-wave x-ray fluorescence we have structurally characterized interacting lipid surfaces displaying hydrophilic polymers and interacting monolayers of wild-type bacterial lipopolysaccharides. These surfaces realistically mimic the outer surfaces of Gram-negative bacteria. The results provide unprecedented insights into the structure aspects of contacts between bacteria in colonies or biofilms. For membrane surfaces displaying charge-neutral glycolipids we observed, by neutron diffraction and molecular dynamics simulation, considerable membrane attraction, which can even withstand electrostatic repulsion. Interestingly, this qualitative behavior was observed for most saccharide motifs investigated, demonstrating that attractive sugar-sugar interactions do not only occur for few specific sugar motifs, as usually assumed. Our findings suggest that this phenomenon plays important roles in biology, for example to stabilize naturally stacked membrane systems like photosynthetic membranes. From the 6th year on (prolongation period), we have also systematically investigated the in-plane organization of glycolipids with grazing-incidence x-ray diffraction, which revealed a rich phase behavior including the formation of highly ordered mixed lipid domains. Overall, we have established a powerful suite of experimental and theoretical methods for the study of membrane surfaces and utilized them to comprehensively investigate the effects of membranebound polymers, saccharides, and other motifs on membrane structure and interactions.

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