Supported lipid bilayers offer a unique configuration where a single bilayer, accessible to other molecules like, for example, proteins, peptides or DNA, is supported on a solid substrate. Beyond their interest for biosensor technology, the access they give to a flat immobilized membrane makes them highly relevant for fundamental studies in biophysics and membrane biology. In particular, they provide a unique way to finely characterize the interactions between membranes and their environment, which are not only crucial for membrane fusion and trafficking, endo- and exocytosis, but also fascinating from the physical point of view.
Membranes indeed exhibit extremely complex interactions with their environment, where molecular scale enthalpic and fluctuation related entropic contributions are inextricably involved. In particular, the effect of confinement has been now discussed for 40 years without finding a definitive answer. Helfrich (1973) first realized that, in addition to the “direct'' electrostatic, van der Waals and hydration forces, the long-range “effective'' steric interaction generated by the thermal fluctuations of confined flexible membranes is an essential contribution to the total free energy of interaction. Pure hard wall interaction (hard confinement) was first considered but is not a realistic description of real systems, and especially of living ones. Confinement by a “soft'' potential was treated either by using self-consistent methods leading to effective exponentially decaying potentials, or by estimating average values within a full statistical mechanics approach. Which functional form should be used to describe entropic repulsion in real experimental situations remains, however, an open question.
We have determined, by grazing incidence x-ray scattering, the interaction potential between two lipid bilayers, one adsorbed on a solid surface and the other floating close by. We find that interactions in this highly hydrated model system are two orders of magnitude softer than in previously reported work on multilayer stacks. This is attributed to the weak electrostatic repulsion due to the small fraction of ionized lipids in supported bilayers with a lower number of defects. Our data are consistent with the Poisson-Boltzmann theory, in the regime where repulsion is dominated by the entropy of counter-ions. We also have a unique access to very weak entropic repulsion potentials which allowed us to discriminate between the various models proposed in the literature. We show that our data are best described using the soft potential of Podgornik and Parsegian (1992). We further demonstrate that the interaction potential between supported bilayers can be tuned at will by applying osmotic pressure, providing a way to manipulate these model membranes, thus considerably enlarging the range of biological or physical problems which can be addressed.
We are currently extending these measurements to the fluctuations and destabilization of membranes in an AC electric field.
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