The Self-Made Tapestry Phần 2 pptx

Soap bubbles and foams do not last for ever, and I suppose that is part of their appeal: fragile beauty, gone in a moment. The collapse of foams is brought about partly by the drainage of the films, under the influence of gravity and capillary forces, until they become too thin to resist the slightest disturbancea vibration or a breath of air. But in their passing, soap films can treat us to a wonderful display. Held vertically on a wire frame, a thinning soap film becomes striated with bands of rainbow colours that pass from top to bottom (Plate 2). Finally the top becomes silvery and then black; and the blackness, like a premonition of the film's demise, Page 28 moves over the entire surface. Once it is black, the film is doomed to burst at the merest perturbation. Fig. 2.17 Cell membranes are made from double layers of surfactants called phospholipids. These bilayers are studded with other membrane components, such as protein molecules, and are sometimes strengthened with a protein web called the cytoskeleton. These colours are the result of interference between light reflected from the front and the back of the film. Interference takes place when the distance between the front and back becomes comparable to the wavelength of light (a few hundred millionths of a millimetre), and as this distance changes, so too does the wavelength (that is, the colour) of the light affected by interference. When the film is black, all reflected visible light cancels itself out by interference. The film is by that stage only about four-and-a- half millionths of a millimetre thickabout the same thickness as a double layer of soap molecules. The two films at the surfaces have almost met back to back. This back-to-back arrangement of surfactant molecules has some similarities to the wall of a living cell. Cell membranes (Fig. 2.17) are composed of amphiphilic moleculesbiological surfactants, if you likecalled phospholipids, or just lipids. A double layer of lipids, called a bilayer, is one of the fundamental architectural features of living organisms, providing the housing in which nature's chemistry takes place. Lipid bilayers also divide up cells of multicelled organisms (like ourselves) into several compartments, each of which acts as the location for specific biological processes. One critical difference between a black soap film and a lipid bilayer, however, is that in the former the surfactants meet head to head and in the latter they meet tail to tail. Thus lipid bilayers present a horde of water- soluble head groups at their surface, and the water-insoluble tails are buried within, where they are shielded from water. In a loose sense, cell membranes can be considered to be microscopic, inside-out bubbles, afloat in a watery sea. Of course, real cells are anything but 'hollow'their insides are filled with biological hardware, including the DNA that allows the cells to generate copies of themselves. But in the 1960s, researchers at Cambridge University found that phospholipids would come together spontaneously in solution to form empty cell-like structures called vesicles, when the solution was jiggled by sound waves. This self-assembly of vesicles is driven by the tendency of lipids to form bilayers_in order to bury their insoluble tails. A lot of work has been devoted to studying the shapes that lipid bilayer vesicles can adopt, because these can provide clues about the factors that control the shapes of real cells. The range of shapes is far more varied and interesting than those of soap bubbles: vesicles can be spherical, but they can also take on other stable shapes too. In the broadest sense, these shapes are determined by the same driving force that dictates the shapes of soap films: the tendency to minimize the total (free) energy. The principal contribution to the energy of a soap film comes from the surface tension, so the film adopts a shape that minimizes this by finding the smallest surface area. But for a vesicle, the surface area is essentially fixed: once a vesicle is formed, the number of surfactant molecules in its wall stays pretty much the same, and each molecule occupies a fixed area on the bilayer surface. This means that another factor is able to exercise a dominant influence on the energy: the surface curvature. The way that shape affects the curvature energy is rather subtle, and it may turn out that the lowest-energy shape is not that with constant mean curvaturea spherebut some other, more complex shape. This balance can be shifted by changing the nature of the vesicle's environmentfor example, by warming it upand so the vesicle may undergo changes in shape as the temperature is changed. The German biophysicist Erich Sackmann and co-workers have shown that under certain conditions, the most stable shape of a vesicle is that of a disk with dim- Page 29 ples in the top and bottom (Fig. 2.18a), which is precisely the shape that a red blood cell adopts. They saw these vesicles change shape to become bowl-like entities as the temperature was increased (Fig. 2.18a), and were able to show theoretically that these shape changes are to be expected because of the changing balance in energies. The bowl-like shape, called a stomatocyte, may eventually curl up on itself to generate a small, spherical vesicle inside a larger one, connected via a narrow neck which eventually became pinched off. Under different conditions, a vesicle can become elongated from an egglike shape into a pear shape, ultimately pinching off a little bud at the thin end (Fig. 2.18b). Both of these processesthe budding and expulsion of a small vesicle from the outside of a cell membrane and the engulfing and budding off of a small interior vesiclehappen in real cells, where they are called exocytosis and endocytosis. The former process allows cells or interior sub-compartments of cells called organelles to send out little chemical messagesa package of protein molecules, perhapsin soft wrappers, while the latter enables a cell to ingest material. In cells these processes are controlled by protein molecules embedded in the cell membranes, but we can see that they can also come about through nothing more than the 'blind' physical forces that determine a membrane's geometry. Fig. 2.18 Vesicles are closed, cell-like bilayer membranes. They adopt a range of different shapes at different temperatures, which are determined by the subtle influences of elastic and curvature energy. In (a) a flattened vesicle with a shape like a red blood cell develops a concavity which becomes a separate internal vesicle. In (b) an elongated vesicle develops a bud, which eventually separates from the main body. Both of these sequences, seen experimentally under a microscope (top frames), can be reproduced by calculations of the equilibrium shape that minimizes the total energy (lower frames). (Images: Reinhard Lipowsky, Max Planck Institute for Colloid Science, Teltow-Seehof, Germany.) Fig. 2.19 Starfish vesicles (a), and the corresponding shapes calculated with an energy-minimization model (b). (Images: Udo Seifert, Max Planck Institute for Colloid Science, Teltow-Seehof, Germany.) Udo Seifert and co-workers at the Max-Planck Institute for Colloid Science in Teltow-Seehof, Germany, have found that under some conditions the driving force to minimize curvature energy can push vesicles through extremely bizarre contortions. Under the microscope they saw multi-armed vesicles that looked like starfish or ink blots (Fig. 2.19a). If these were living amoeba dragging themselves around by extending pseudopodia, we might not consider the shapes surprising; but they are merely empty sacs whose shapes are the product of a mathematically well-defined minimization principle! Seifert and colleagues showed that they could reproduce the shapes theoretically by minimizing the curvature energy of the bilayers subject to the constraints of fixed surface area and enclosed volume (Fig. 2.19b). This 'mathematics of blobs' appears to hold some symmetry principles: the researchers could Page 30 not, for example, find starfish vesicles with four symmetrical arms either experimentally or theoretically. A part of the curvature energy of a vesicle arises not from the size or shape of the surface but from its topologythe overall 'connectedness' between different parts of the membrane. Two shapes are topologically equivalent if one can be converted into the other without any tearing or puncturing. For example, a spherical vesicle is topologically equivalent to all of the disk- and bowl-like vesicles in Fig. 2.18a, because they can be made just by flattening and bending the sphere. However, the shape on the far right of Fig. 2.18b is topologically non-equivalent to a sphere: when the small vesicle is pinched off at the neck, so that it can float free from the larger one, the topology is altered because the membrane has to be ruptured to create this arrangement. Another shape that is topologically different from the spherical vesicle is the doughnut, technically called a torus. Vesicles with this shape have been seen by David Bensimon and co-workers at the Ecole Normale Supérieure in Paris (Fig. 2.20a). Bensimon's team showed that these shapes can become the most energetically favourable under some circumstances. You might notice that they can be generated from an extreme version of the disk-like shapes on the left of Fig. 2.18a, when the two dimples touch each other in the middle. At that stage the upper and lower membranes may merge and a hole open up in the middlethe topology is then abruptly transformed. Bensimon's group have reported even more topologically complex vesicle shapes, such as double toruses (Fig. 2.20b), which are topologically distinct from the single toruses. The point to bear in mind here is that even these apparently complicated shapes are selected according to relatively simple physical principles that minimize the vesicle's energy. Bubbles in flatland Fig. 2.20 Vesicles with holes: a doughnut or torus (a, showing top and side views and a double torus (b). Even these topologically complex shapes correspond to equilibrium structures that represent energetic minima. The scale bar indicates 10 micrometres in all frames. (Photos: Xavier Michalet and David Bensimon, Ecole Normale Supérieure, Paris.) Vesicles are rarely formed in solutions of surfactants or lipids unless given some encouragement, in the form of sonic vibration for instance. Left to their own devices, surfactants display a gallery of other aggregate structures with their own propensity for pattern formation. Imagine gradually adding soap molecules to water. The first thing they'll do is gather at the water surface, where the insoluble, hydrophobic tails can poke out into the air. The water surface becomes gradually covered with a molecular film just one molecule thick. Benjamin Franklin was captivated by these thin films in the Page 31 eighteenth century, which he observed by gently pouring an oil onto the surface of a pond. He took to carrying oil in a little vial in his walking stick, and would merrily create a miniature oil slick on every pond he encountered, particularly that on London's Clapham Common. What amused him was that just the tiny volume of oil that he carried would spread across the entire pond, and as it did so it would lower the surface tension of the water surface and leave it smooth as a mirror. I don't recommend trying this, however, unless you fancy you can explain to a park attendant that you are reproducing a historical experiment by Ben Franklin. Fig. 2.21 Surfactants at the water surface will form a variety of different states when the surface layer is compressed. (a) A dense, disordered liquid-like state (called the LC state, dark patches) grows within a less-dense state (LE) that contains a fluorescent dye (light regions). (b) As the LC domains grow, they become ordered in a hexagonal pattern. (c) Eventually the LC domains become squeezed into worm-like shapes by their mutual repulsion. (d) The stripe phase of surfactant films is analogous to the striped arrangement of magnetic domains in thin films of garnet. Here too the stripes arise from mutual repulsion of the domains. (Photos: (a) S. Akamatsu and E. To, University of Paris IV; (b) Helmut Möhwald, Max Planck Institute for Colloid and Interface Science, Berlin; (c) Charles Knobler, University of California at Los Angeles; (d) Michael Seul, BioArray Solutions, Fanwood, New Jersey.) The study of surfactant films (particularly those of the soap-like molecules called fatty acids) on the surface of water was pioneered by Lord Rayleigh at the end of the nineteenth century and by the American chemist Irving Langmuir and his students at the beginning of the twentieth century. These films now bear the name Langmuir films, and they exhibit an astonishing range of pattern-forming behaviour. Langmuir created them in a shallow trough in which a movable barrier skimming the water surface allowed him to marshal the surfactant molecules into an ever smaller area of water surface and so control their densitywhich is to say, the average surface area commanded by each. As this density increases, a Langmuir film can undergo abrupt changes that are two-dimensional 'flatland' versions of the transformations from gas to liquid to solid that a material in three dimensions will undergo as it is compressed. But these films have an extra state: there are two kinds of 'flat' liquid, in both of which the molecules are mobile and disordered but which have distinctly differ- [...]...Page 32 ent densities By adding to the film a fluorescent dye that dissolves more readily in the less dense liquid (the liquid-expanded (LE) phase) than in the more dense liquid (the liquid-condensed (LC) phase), we can 'light up' the LE phase and watch darker 'droplets' of the LC phase coalesce and grow within it (Fig 2. 21a) Fig 2. 22 A gallery of patterns in surfactant films, formed by the growth... which the size of the characteristic repeating unit is of the same order as the size of the constituent molecules, here the scale of the pattern bears no direct relationship to the scale of the component parts from which it is made As these domains are squeezed ever more closely together, something even more dramatic can happen: the strength of the electrostatic repulsion between domains makes them... molecules These blobs, called micelles, have an internal logic: all the surfactants are arranged with their head groups pointing outwards onto the micelle surface, while the tails are buried in the interior (Fig 2. 23a) In this way, the molecules hide their tails from the water, and show only their water-soluble heads G.S Hartley proposed in the 1930s thatmicelles are roughly spherical, and experiments in the. .. can accumulate at the surface; after the surface is full, they are forced to remain in solution There is then the unfortunate fact that most of the molecule is a fatty, water-insoluble tail, and the surfactants have to do something about it What they do is to aggregate together into a bewildering number of different structures, which can impose a regular pattern on the whole system The simplest aggregates... each of the bubble-like domains of an LC phase condensing within an LE phase acts like an electrically charged bubble that repels the other domains The result is that the domains tend to organize themselves so as to keep roughly the same distance between each, and the film becomes organized into a peculiar kind of 'crystal' in which the domains are packed together in a regular manner (Fig 2. 21b) Unlike... showed this to be the case They are formed when the concentration of surfactant in solution is increased above a certain critical level, called the critical micelle concentration Fig 2. 23 (a) A micelle (b) Cylindrical micelles packed together in the hexagonal phase Page 34 In the early 1950s, before the shape of micelles was finally established, the physicist Paul Debye proposed that they might be rod-shaped... catalysts Notice that the characteristic length scale of the pattern in these materials is much larger than the characteristic size of the component partsthe silicate ions or surfactant molecules So you would never guess that the system has the potential for forming such a pattern by looking at these components individually The hexagonal pattern is the result of a self-organizing process Another structure... is made up of flat bilayer sheets, like the walls of vesicles, stacked on top of each other This is called the lamellar phase (Fig 2. 25) These assemblies too can be 'fossilized' by precipitating silica around them; shortly after the Mobil discovery, researchers at the University of California in Santa Barbara made a layered silica material this way Fig 2. 25 The lamellar phase contains stacks of bilayer... alternative in which the pores are positioned in a regular, orderly manner Why should the pores be ordered? Because they have a tendency to repel one another: if two pores get too close together, they create very pronounced curvature of the bilayers in their vicinity, and this costs energy So when there are many pores, they tend to sit at an optimal distance from each other on a regular lattice The surfactant... lattice The surfactant structure then becomes a kind of 'tubular crystal' The sponge phase is really a 'melted', disordered version of this curious crystal The most common of the ordered bicontinuous surfactant phases are the cubic phases (Fig 2. 28), socalled because the symmetry properties of the labyrinth are the same as those of a cube They are examples of what mathematicians call a periodic minimal . a selective gallery (Fig. 2. 22) . Let me just point out, however, the similarity between one of these shapes (Fig. 2. 22d) and those discussed in Chapter 5 (see p. 123 )this is a generic pattern. arranged with their head groups pointing outwards onto the micelle surface, while the tails are buried in the interior (Fig. 2. 23a). In this way, the molecules hide their tails from the water,. crystal. The most common of the ordered bicontinuous surfactant phases are the cubic phases (Fig. 2. 28), so- called because the symmetry properties of the labyrinth are the same as those of a cube. They