Astrobiology. Charles S. Cockell

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       Figure 4.16 The wood frog (Lithobates sylvatica). The frog can tolerate freezing temperatures by using glucose as an antifreeze protectant in its tissues.

      Source: Reproduced with permission of Brian Gratwicke, https://en.wikipedia.org/wiki/Wood_frog#/media/File:Lithobates_sylvaticus_(Woodfrog).jpg.

      The property of floating ice and its tendency to trap warmer water with its benefits for life raises a more fundamental question. Are the properties of water fine-tuned for life? Some people take this point of view, but it invites the problematic idea that someone or something has produced a solvent ideal for producing biology. A more evolutionary point of view is to see this from the opposite standpoint: Is life fine-tuned to use the properties of water? For example, the use of water as a proton wire and as part of the active site of enzymes can be understood as life evolving to use the beneficial properties of the water molecule that confer selective advantage, rather than water having an uncanny set of properties “just right” for life.

It is also worth pointing out that water does have some properties that are not always conducive to life. In certain situations, it can be a reactive solvent, causing the dissociation of molecules. For example, it can be disruptive to the hydrogen bonding between amino acids in proteins, making it sometimes unconducive to protein folding. Water provides a medium for the reactive deamination of nucleobases in nucleic acids, meaning that organisms must constantly repair their DNA against this chemical damage.

      Furthermore, as we have seen, the formation of some biological molecules, such as proteins, involves condensation reactions where water is removed from a bond, eliminating it from the chemistry. This is not a deleterious property of water as such, but it does show that chemistry conducive to forming the macromolecules of life involves the removal of water from molecules. We should be careful not to view water as a perfect solvent. However, of all possibilities, and despite some biochemically harmful properties in certain situations, water might be the best solvent for carrying out a large variety of chemical reactions in carbon-based life. We can revisit this question when we discuss alternative solvents later in the chapter.

4.11 Alternative Chemistries

      We have looked at the key requirements for building the macromolecules in life on Earth and the solvent, water, in which these macromolecules operate, but what about other possible chemistries? We have no empirical evidence to suggest that alternative chemistries can be used by life or that other life forms on some distant planet are using other chemistries, but to put the previous discussions into context, it is worthwhile to briefly consider what alternatives have been discussed.

      As the book progresses, you can consider for yourself whether our view that carbon as the ideal backbone element for life and water as the ideal solvent is a sensibly considered scientific conclusion or merely an Earth-centered prejudice.

      4.11.1 Alternative Core Elements

      What about alternative core elements other than carbon? One popular suggestion is silicon. As an element of group 14, below carbon in the Periodic Table, it shares many common chemical characteristics with carbon. The silicon atom has 14 electrons (1s²2s²2p⁶3s²3p²), compared to carbon's 6 (1s²2s²2p²). In both atoms, there are four electrons available in the outer shell to form bonds. The major difference is that the silicon atom has a larger atomic radius (111 pm) than carbon (70 pm).

      Silicon can be more reactive than carbon, which is attributed to three characteristics, all of which are related to its larger atomic radius. First, silicon, like carbon, typically forms four bonds, but unlike carbon it can more easily accept additional electrons and form five or six bonds (it can have a higher number of near neighbors than carbon). This allows some reactions to occur at lower energies. Second, many silicon bonds with other elements are weaker than in carbon, requiring less energy to break them. The SiH bond strength (318 kJ mol⁻¹) is lower than the CH bond strength (411 kJ mol⁻¹). For example, silane (SiH₄) combusts spontaneously in air even at 0 °C, whereas its carbon analog, methane (CH₄), remains completely stable, even in pure oxygen, unless energy is imparted to it. Finally, silicon is more electropositive (a greater tendency to donate electrons) than carbon, leading to strongly polarized bonds with other non-metals that are much more susceptible to chemical reactions. Although the more reactive nature of silicon (at least in the absence of oxygen) may at first appear to be a disadvantage in any potential biochemistry, this high reactivity might make it more conducive to life in low-temperature environments, where the reaction rates would be slower.

      Silicon has some impressive properties. It forms stable covalent bonds with N, P, S, and many other elements. It can also form stable tetra-, penta-, and hexa-coordinated compounds with N, C, and O bonds, analogous to the generation of molecular diversity in carbon chemistry. Oligosilane structures are known to have many consecutive SiSi bonds that resemble fatty acids in carbon-based membranes. Remarkably, amphiphilic silicon compounds with a charged end attached to an uncharged tail can be made that assemble in water to form vesicles like phospholipids. In Chapter 5, we explore the structure of cellular membranes in more detail. For now, you might like to note the potential to form similar structures using silicon as the core atom.

Although silicon cannot easily form a six-membered ring structure like benzene, it can form a ring structure (siloxene) in which oxygen atoms hold together the silicon atoms. Cage-like molecular systems such as silsesquioxanes can be linked with a wide diversity of side groups to allow for a remarkable diversity of molecules (Figure 4.17) that have industrial uses from chemical catalysis to making light-emitting diodes.