Order parameters in membranes: Following Joachim Seelig's path

Following the publication of biological membrane models in the 1970s, Joachim Seelig was the first to experimentally demonstrate the dynamic nature of these membranes. He conducted the first ssNMR experiments to measure the order parameters of the CD (²H) bond of lipids deuterium-labelled, showing a fairly fluid membrane interior. Since then, the order parameters of the CD, CH and CC bonds have been measured. They can be used to describe the dynamics of membranes on several space and time scales: intramolecular (Å/ns-ps), molecular (nm/ns–100 ns) and collective (membrane deformations, μm/μs). The profile of CD, CH, CC order parameters across the membrane bilayer allows us to describe the lipid membrane as being very rigid at the glycerol and chain levels and very fluid at its center and surface. This is true for lipid chains carrying double bonds, rings or branched methyl groups. Bipolar lipids that span the entire membrane do not have a very fluid membrane interior. Sterols modulate membrane dynamics, increasing order parameters in the fluid phase and decreasing them in the gel phases. They can be described as regulators of membrane dynamics, as they maintain the membrane in a dynamic state that varies very little when environmental factors change (temperature, pH, etc.). The description of order parameters by statistical mechanics allows the length of the chains, the thickness of the bilayer and the membrane elastic constants to be calculated accurately. The surface area of each lipid in the membrane can also be calculated from the plateau of order parameters (positions C3-C10): $\left(A_L\right)=83\left(1-\left(S_{\mathrm{CD}}^{\mathrm{plat}}\right)\right)$.