Some basic examples of molecular self-organization

The simplest examples of molecular self-organization are based on molecules, such as those of soaps and the fatty and oily substances known as lipids, that tend to associate with each other. Soaps and lipids are typical “amphiphilic” substances – that is, their molecules contain a hydrophilic (water-loving) head group, and one or more long hydrophobic (water-hating) chains (seeFigure 8.1a). Because of this “schizophrenic” character, amphiphilic molecules, when put in water, do not make normal solutions, but instead tend to associate with each other, spontaneously forming ordered structures which, depending upon conditions, can take different forms and names, such as bilayers, micelles, reverse micelles, vesicles, and so on (seeFigure 8.1b). The forces which hold them together are usually the hydrophobic forces. These structures are well known in the literature and are important both in basic science and in applied technology. Generally, they change the properties of water surfaces and are therefore known as “surfactants” (seeBox 8.1).

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Figure 8.1 (a) Schematic picture of a surfactant with hydrophilic head and two hydrophobic tails. (b) At a critical concentration, called critical micelle concentration (cmc) or generally critical aggre- gate concentration (cac), soap molecules form spherical micelles or the much larger vesicles (in cross section, and not drawn in scale), giving rise also to a series of emergent properties – e.g., the formation of distinct compartments and collective dynamic movements.

Why do these substances aggregate? The simple sketch ofFigure 8.2 explains why. In the upper panel, we see two oil droplets floating on a surface of water. After a while, spontaneously, the droplets come together and fuse to make a single, larger droplet of oil. The driving force is the decrease of the total surface exposed to water. This decrease of surface “liberates” water molecules from an energetically unfavored contact with oil. As a consequence, there is an overall increase of entropy (or disorder) due to the “liberation” of water molecules, which makes the process thermodynamically favorable. There are two remarkable aspects of this process: first, the spontaneous formation of local order, attended by an overall increase of entropy (or disorder), and second, the formation of spherical compartments (seeFigure 8.1), which, as we shall see later on, are very important as a model for biological cells.

Now, the same happens with lipids. Lipids, in particular, phospholipids (seeBox 8.1), are the main components of all our biological membranes. The oily, hydrophobic parts tend to assemble and form an oily microenvironment, known as a “microphase,” by expelling water from their surroundings. Therefore, the formation of membranes, just like the formation of micelles and vesicles (seeFigure 8.3), is a thermodynamically favored process. Again,

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Figure 8.2 In the upper panel, two oil droplets spontaneously come together to form a single droplet having a smaller total surface than the sum of the two, a process which is thermodynamically favored. In the lower panel, the same situation is shown in the case of three (or more) surfactant molecules. Their assembly proceeds with an increase of entropy caused by the expulsion of water molecules from the formed hydrophobic assembly.

Figure 8.3 Schematic structure of micelle and liposome (not in scale) showing the structural analogy between liposomes and the membrane bilayers. In all cases, water molecules are excluded from the internal hydrophobic microphase.

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Box 8.1

Surfactants, lipids, and liposomes

Surfactants are organic compounds that are amphiphilic, meaning they contain both hydrophobic groups (their “tails”) and hydrophilic groups (their “heads”). The name is a contraction of “surface active agents.” Because of their property of reducing the surface tension of water, surfactants play an important role as soaps and as wetting, dispersing, emulsifying, foaming, and antifoaming agents in many practical applications (annual global production 13 million metric tons in 2008).

Surfactants can be classified as cationic (positive charge in the hydrophilic head), anionic (negative charge), zwitterionic (both a positive and a negative charge), and nonionic (no charges in head). When dispersed in water, surfactants associate to form ordered assemblies, such as bilayers, micelles, or vesicles).

Figure 8.4 A typical surfactant with the indication of the hydrophilic and hydrophobic parts; POPC is the abbreviation of the best-known phospholipid, palmytoyl-oleoyl-phosphatydil choline. The middle panel shows the structure of a simpler surfactant, oleic acid/oleate, of the class of fatty acids, very diffuse in the natural world, and important in prebiotic chemistry (see later on the section on autocatalysis, where fatty acid plays a most important role).

Lipids (from the Greek lipos – “fat” or “grease”) constitute a broad group of naturally occurring molecules, some of which are surfactants. They include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E, and K), and glycerolipids (seeFigure 8.5). The latter are composed of fatty acids and glycerol, joined by ester bonds. The best known are the so-called triglycerides shown in the figure. The main biological functions of lipids include energy storage as structural components of cell membranes.

Although the term “lipid” is sometimes used as a synonym for fats, fats are a subgroup of lipids that should be called triglycerides. An important class of glycerolipids are

glycerophospholipids, also referred to simply as phospholipids (seeFigure 8.5), which are ubiquitous in nature and are key components of the lipid bilayers of cells, as well as being involved in cellular metabolism and cell signaling.

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Figure 8.5 Some typical lipids, including the triesters of glycerol with fatty acids (carboxylic acid with a long chain) and a typical phospholipid.

there is an increase of order in spite of an overall increase of entropy. As we shall see in our discussion of the dynamic aspects of self-organization inSection 8.3, such “islands of order in a sea of disorder” are characteristic of the “dissipative structures” of living systems. In our examples of lipids, the whole structure remains compatible with water, because its surface is made by hydrophilic compounds, as shown schematically inFigure 8.3.

Notice in this regard the apparent contradiction between our water-based life, and the existence of so many hydrophobic (water-hating) compounds and substances in our biological structures, like the lipids, steroids, alkaloids, fats, and waxes. Nature solves this problem by the self-organization of these substances, with the formation of microphases that exclude water from their interior, but are made compatible with water by an external layer of hydrophilic head groups, as shown inFigure 8.3.