Historical Developments In Artificialmembrane Permeability Measurement

7.2.1 Lipid Bilayer Concept

The history of the development of the bilayer membrane model is fascinating, and spans at least 300 years, beginning with studies of soap bubbles and oil layers on water [517-519].

In 1672 Robert Hooke observed under a microscope the growth of "black" spots on soap bubbles [520]. Three years later Isaac Newton [521], studying the ''images of the Sun very faintly reflected [off the black patched on the surface of soap bubbles],'' calculated the thickness of the black patches to be equivalent to 95 A. (Anders Jonas Angstrom, 'father of spectroscopy,' who taught at the University of Uppsala, after whom the A unit is named, did not appear until about 150 years later.)

Ben Franklin, a self-trained scientist of eclectic interests, but better known for his role in American political history, was visiting England in the early 1770s. He published in the Philosophical Transactions of the Royal Society in 1774 [552]:

At length being at Clapham where there is, on the common, a large pond, which I observed to be one day very rough with the wind, I fetched out a cruet of oil, and dropt a little of it on the water ... and there the oil, though not more than a tea spoonful, ... spread amazingly, and extended itself gradually till it reached the lee side, making all that quarter of the pond, perhaps half an acre, as smooth as a looking glass ... so thin as to produce prismatic colors ... and beyond them so much thinner as to be invisible.

Franklin mentioned Pliny's account of fisherman pouring oil on troubled waters in ancient times, a practice that survives to the present. (Franklin's experiment was reenacted by the author at the pond on Clapham Common with a teaspoon of olive oil. The spreading oil covered a surface not larger than that of a beach towel-it appears that technique and/or choice of oil is important. The olive oil quickly spread out in circular patterns of brilliant prismatic colors, but then dissolved from sight. Indeed, the pond itself has shrunken considerably over the intervening 230 years.)

More than 100 years later, in 1890, Lord Rayleigh, a professor of natural philosophy at the Royal Institution of London, was conducting a series of quantitative experiments with water and oil, where he carefully measured the area to which a volume of oil would expand. This led him to calculate the thickness of the oil film [517,518]. A year after publishing his work, he was contacted by a German woman named Agnes Pockels, who had done extensive experiments in oil films in her kitchen sink. She developed a device for carefully measuring the exact area of an oil film. Lord Rayleigh helped Agnes Pockels in publishing her results in scientific journals (1891-1894) [517,518].

Franklin's teaspoon of oil (assuming a density 0.9 g/mL and average fatty-acid molecular weight 280 g/mol) would contain 10+22 fatty-acid tails. The half- acre pond surface covered by the oil, ^2000 m2, is about 2 x 10+23 A2. So, each tail would be expected to occupy about 20 A2, assuming that a single monolayer (25 A calculated thickness) of oil formed on the surface of the pond.

Pfeffer in 1877 [523] subjected plant cell suspensions to different amounts of salt and observed the cells to shrink under hypertonic conditions and swell in hypotonic conditions. He concluded there was a semipermeable membrane separating the cell interior from the external solution, an invisible (under light microscope) plasma membrane.

Overton in the 1890s at the University of Zurich carried out some 10,000 experiments with more than 500 different chemical compounds [518,524]. He measured the rate of absorption of the compounds into cells. Also, he measured their olive oil-water partition coefficients, and found that lipophilic compounds readily entered the cell, whereas hydrophilic compounds did not. This lead him to conclude that the cell membrane must be oil-like. The correlation that the greater the lipid solubility of a compound, the greater is the rate of penetration of the plasma membrane became known as Overton's rule. Collander confirmed these observations but noted that some small hydrophilic molecules, such as urea and glycerol, could also pass into cells. This could be explained if the plasma membrane contained water-filled pores. Collander and Barlund concluded that molecular size and lipophilicity are two important properties for membrane uptake [525].

Fricke measured resistance of solutions containing suspensions of red blood cells (RBCs) using a Wheatstone bridge [518]. At low frequencies the impedance of the suspensions of RBC was very high. But at high frequencies, the impedance decreased to a low value. If cells were surrounded by a thin membrane of low dielectric material, of an effective resistance and a capacitance in parallel to the resistor, then current would flow around the cells at low frequencies, and ''through'' the cells (shunting through the capacitor) at high frequencies. Hober in 1910 evaluated the equivalent electrical circuit model and calculated the thickness of the RBC membrane to be 33 A if the effective dielectric constant were 3 and 110 A if the effective dielectric constant were 10 [518].

In 1917 Langmuir [526], working in the laboratories of General Electric, devised improved versions of apparatus (now called the Langmuir trough) originally used by Agnes Pockels, to study properties of monolayers of amphiphilic molecules at the air-water interface. The technique allowed him to deduce the dimensions of fatty acids in the monolayer. He proposed that fatty acid molecules form a mono-layer on the surface of water by orienting themselves vertically with the hydropho-bic hydrocarbon chains pointing away from the water and the lipophilic carboxyl groups in contact with the water.

Gorter and Grendel in 1925 [527], drawing on the work of Langmuir, extracted lipids from RBC ghosts and formed monolayers. They discovered that the area of the monolayer was twice that of the calculated membrane surface of intact RBC, indicating the presence of a ''bilayer.'' This was the birth of the concept of a lipid bilayer as the fundamental structure of cell membranes (Fig. 7.1).

Figure 7.2 Danielli-Davson membrane model. A layer of protein was thought to sandwich a lipid bilayer.

The first membrane model to be widely accepted was that proposed by Danielli and Davson in 1935 [528]. On the basis of the observation that proteins could be adsorbed to oil droplets obtained from mackerel eggs and other research, the two scientists at University College in London proposed the ''sandwich'' of lipids model (Fig. 7.2), where a bilayer is covered on both sides by a layer of protein. The model underwent revisions over the years, as more was learned from electron microscopic and X-ray diffraction studies. It was eventually replaced in the 1970s by the current model of the membrane, known as the fluid mosaic model, proposed by Singer and Nicolson [529,530]. In the new model (Fig. 7.3), the lipid bilayer was retained, but the proteins were proposed to be globular and to freely float within the lipid bilayer, some spanning the entire bilayer.

Mueller, Rudin, Tien, and Wescott in 1961, at the Symposium of the Plasma Membrane [531] described for the first time how to reconstitute a lipid bilayer


Figure 7.3 Fluid mosaic modern model of a bilayer.

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