ABSTRACT
Roles for the element silicon (Si) in biochemical processes have been considered in both science fiction and real science. The hypothesis that Si could replace carbon (C) as the core element in so-called silicon-based life fails on a number of grounds, even though Si is just below C in the periodic table and has many similar bonding properties. First, bonds between two Si atoms or between Si and C are weaker than those between two carbon atoms, and bonds between Si and a host of common elements (hydrogen, carbon, halogens, and nitrogen) are chemically and photochemically reactive and subject to decomposition. Second, the chemistry of Si, in general, lacks molecules that contain unsaturation, in particular, double bonds (Si==C, Si==Si, Si==O, Si==N). Molecules containing the analogous unsaturated carbon molecules (C==C, C==O, C==N) are integral parts of almost all biomolecules, including amino acids, proteins, carbohydrates, nucleic acids, fatty acids, and vitamins. Devoid of unsaturated molecules, organosilicon systems could not serve as building blocks for the complex chemistry necessary for life. However, because of the high strength and stability of the Si—O bond, as in silicates, such systems can participate in the biochemistry of carbon-based systems. Silica can be taken up and incorporated into the structure of numerous plant and animal organisms. Silicates can be chiral and possibly serve in the process of chiral resolution of carbon molecules. Silicates can select and sequester unstable carbohydrates in their synthesis via the formose reaction. Thus, Si can participate in life processes but cannot serve as the central element.
