The Structure of the Lipid Membrane

In the `Scientific American` (April 2018) appeared the article by American science journalist Douglas Fox `Brain Cells Communicate with Mechanical Pulses, Not Electric Signals`.

(There is in Russian: «В мире науки», 2018, № 5-6: <SciAmMembr>.)

He tells about experimental research and ideas of Professor Thomas Heimburg and his group in the Niels Bohr Institute Heimburg(University of Copenhagen). One can say, Prof. Heimburg is a heretic: he tries to refute the classical theory of the nerve impulse by Hodgkin and Huxley (1952). He believes that a mechanical perturbation (a wave) propagates along the axon, causing a local phase transition of the membrane to a quasi-crystalline state. Heimburg assumes that in this area a current of sodium ions into the axon occurs and admits they penetrate not through special protein Na+ channels, but directly through the lipid layer. He also considers the puzzle of how general anesthetics act.

In connection with Prof. Heimburg`s research, I`d like to remind about the hypothesis about HONEYCOMB-LIKE MODEL OF THE MEMBRANE.

In 1980s Dr. Raik-Hiio Mikelsaar (Tartu University) suggested a new structure for the biomembrane lipid bilayer. РайкФотоWorking (= playing) with Tartu plastic atomic-molecular space-filling models (made under his leadership) he discovered that the three phospholipid molecules can form a right hexagonal prism. Every prism is closed above by `a hat` of three polar groups (heads of lipids) — they are bound by electrostatic interactions. According to Mikelsaar`s hypothesis, such hexagonal trimeric units cover all the surface of membrane — it looks like the floor of a room with the parquet hexagonal tiles. And it is similar to a honeycomb.

But inside prisms, there are cavities which must be filled with some substance. It turned out that the three molecules of prismcholesterol perfectly fit it (on the photo); however, the quantity of this steroid in the lipid layer can vary and be not enough to fill all prisms. In this case, the prisms can contain tubes of structured (ice-like) water (they are named shafts); thus so-called a hydrophobic lipid membrane may contain significant amounts of water. It is important that in the hydrophobic environment of lipid tails, this water (shafts) will freeze not at zero by Celsius but at a higher temperature. Ice`s melting will cause greater mobility of lipids, and that`s the physical meaning of the membrane phase transition (it is known, the high amount of cholesterol diminishes phase transition, now it becomes clear, why: the absence of water – the absence of transition).

A very interesting opportunity this honeycomb model opens for the molecular mechanism of nerve impulses. It is established that this process is accompanied by the shift of charged atomic groups (gate current). One can imagine such a picture: at the potential jump on the membrane polar heads of lipids will rise, turn at a certain angle and fall into new positions, forming connections with the heads of neighboring prisms. The gates open and each prism will become a channel for sodium ions — the geometry of the holes at the top will allow Na+ (but not K+) to pass through: in this membrane`s domain, the quasi-crystalline state will arise. The model also gives a new understanding of general anesthesia: molecules of anesthetics (it may be even inert gases) form clathrates structures in water shafts.

In my view, Mikelsaar`s hypothesis in the case of its confirmation will make a revolution in membranology: it easily and gracefully explains many complex effects.

Publications:

1) Raik Mikelsaar. `A Hypothesis on the Structure of the Biomembrane Lipid Bilayer`
Molecular Crystals and Liquid Crystals, 1987, Vol. 152,  pp. 229—257

https://www.researchgate.net/publication/232832696_A_Hypothesis_on_the_Structure_of_the_Biomembrane_Lipid_Bilayer

2) Various aspects of the hypothesis are popularly and in more detail covered in the article: P.-Х. Н. Микельсаар. «Мембрана, схожая с торцовой мостовой» (in Russian) published in the Russian popular science magazine «Химия и жизнь» («Chemistry and life», 1990, № 4). It is presented on our website:  <МембранаРайк-90.4.>.