Saturday, September 15, 2012

45. Chemical Adaptation and Chemical Evolution

Lipid bilayers and micelles are striking examples of chemical self-organization. The amount of information contained in such assemblies is very high. This information is distributed among the shapes of the component molecules, and in the interaction patterns among them.

The build up of such information involves a succession of stages: molecular recognition; self-assembly; self-organization; chemical adaptation; and chemical evolution. The first two were discussed in Part 44. Let us now focus on molecular self-organization, and chemical adaptation and evolution.

Jean-Marie Lehn is a pioneer in the fields of supramolecular chemistry and evolution of chemical complexity. He defined self-organization as the spontaneous but information-directed generation of organized functional structures in equilibrium conditions’. The necessary information (‘coding’) for self-organization is contained in the molecular-recognition and self-assembly proclivities of the component molecules (cf. Part 44). This coding also determines how the self-assembled edifice further self-organizes into a functional structure in equilibrium. For a survey of the various types of coding for self-organization, see, for example, my book Complexity Science.

Self-organization is a far more ubiquitous phenomenon than something at just the molecular level. Here are some examples:

  • A laser is a self-organized system. Under properly engineered conditions, photons spontaneously group themselves into a configuration in which they all move in phase, resulting in a powerful laser beam.
  • A hurricane is a self-organized system. The steady influx of energy from the Sun draws water from the oceans, as well as drives the winds. And mild tropical winds may grow into an organized configuration of a hurricane when some critical threshold is crossed.
  • A living cell is a self-organized system, which organizes itself all the time, depending on the environment.
  • An economy is a self-organizing system. The demand for goods and services, as also the demand for labour, constantly reorganizes the economy in a spontaneous way, without any central controlling authority.
Given a set of conditions, molecules in an open system tend to self-organize so as to minimize the overall free energy. This is chemical adaptation. Now suppose this set of conditions changes. This is very likely, in fact inevitable, because we are dealing with an open system. A further round of self-organization must occur, governed as always by the second law of thermodynamics for open systems. This is chemical evolution.

Moreover, the set of changing conditions, i.e. the changing environment experienced by the molecules, need not necessarily be that external to the set of molecules. Even internal changes in the molecular system present a changed environment to every member of the set. And molecular configurations are changing all the time. Thus, chemical adaptation and evolution occur in an open system of molecules (including our ecosystem) all the time.

One can draw analogies with Darwinian evolution to see if ‘natural selection’ (i.e. molecular selection) and ‘survival of the fittest’ also occur in chemical evolution. The answer is ‘yes’ because, when the resources are limited, there is competition among the alternative molecular-reaction pathways, and only the fittest pathways can survive so far as consumption of precursor molecules and energy-rich molecules is concerned.

Such considerations aroused special interest for explaining the origin of life-sustaining molecules. Some pioneering work in this direction was done by Melvin Calvin (1969), who introduced the idea of autocatalysis as a mechanism for molecular selection. I shall consider autocatalysis in the next post.

To sum up, the lock-and-key idea explained in Part 44 is crucial for explaining the evolution of molecules and molecular assemblies of increasing complexity in Nature. Two molecules may ordinarily interact only weakly, but a snug fitting of portions of the two molecules can lead to a much stronger degree of cohesion between them, because they ‘touch’ or attract each other at many points.

Another crucial factor for the chemical evolution of complexity is the reversibility of the non-covalent bonding between molecules; reversibility is the key to self-assembly. And once a stable self-assembly has got created (through molecular trial and error), generally there is no ‘looking back'. The overall large cohesive energy has a stabilizing effect.

But there can still be a looking forward: If the environment changes, the self-assembled system can respond (i.e. adapt) by again exploiting the reversible nature of its non-covalent interactions. This is chemical evolution. As we shall see in due course, chemical evolution has led to biological evolution. And evolution and emergence are the hallmarks of complexity.

I conclude this post by mentioning vesicles, which provide a good, even dramatic, example of self-organization in nonliving complex systems. Vesicles are spherical supramolecular assemblies separating an aqueous interior volume from an external solvent by means of one or more lipid bilayers. Vesicles can form naturally, or they can be prepared artificially. The later are known as liposomes.

Given the right conditions, lipids can self-assemble into giant vesicles the size of biological cells. The basic driving force for their self-assembly is the hydrophobic interaction. As vividly described by Menger & Gabrielson (1995), vesicles can mimic the living cell in many ways, even though they are not living entities:
When a giant vesicle, which happens to have a smaller vesicle inside it, is exposed to octyl glucoside, the smaller vesicle can pass through the outer membrane into the external medium (‘birthing’). The resulting injury to the membrane of the host vesicle heals immediately. Addition of cholic acid, on the other hand, induces a feeding frenzy in which a vesicle grows rapidly as it consumes its smaller neighbours. After the food is gone, the giant vesicle then self-destructs (a case of ‘birth, growth, and death’). Such lifelike morphological changes were obtained by using commercially available chemicals; thus these processes should be assigned to organic chemistry, and not to biology or even biochemistry.