Silicon biochemistry

Silicon biochemistry The structure of silane, the silicon-based analogue of methane.See also: organosilicon The most commonly  basis for an alternative biochemical system is the silicon atom, since silicon has many chemical properties similar to carbon and is in the same periodic table group, the carbon group. Like carbon, silicon can create molecules that are sufficiently large to carry biological information.

However, silicon has several drawbacks as a carbon alternative. Silicon, unlike carbon, lacks the ability to form chemical bonds with diverse types of atoms necessary for the chemical versatility required for metabolism. Elements creating organic functional groups with carbon include hydrogen, oxygen, nitrogen, phosphorus, sulfur, and metals such as iron, magnesium, and zinc. Silicon, on the other hand, interacts with very few other types of atoms. Moreover, where it does interact with other atoms, silicon creates molecules that have been described as "monotonous compared with the combinatorial universe of organic macromolecules".This is because silicon atoms are much bigger, having a larger mass and atomic radius, and so have difficulty forming double bonds (the double bonded carbon is part of the carbonyl group, a fundamental motif of bio-organic chemistry).

Silanes, which are chemical compounds of hydrogen and silicon that are analogous to the alkane hydrocarbons, are highly reactive with water, and long-chain silanes spontaneously decompose. Molecules incorporating polymers of alternating silicon and oxygen atoms instead of direct bonds between silicon, known collectively as silicones, are much more stable. It has been suggested that silicone-based chemicals would be more stable than equivalent hydrocarbons in a sulfuric-acid-rich environment, as is found in some extraterrestrial locations. Complex long-chain silicone molecules are still less stable than their carbon counterparts, though.

Another obstacle is that silicon dioxide (a common ingredient of many sands), the analog of carbon dioxide, is a non-soluble solid at the temperature range where water is liquid, making it difficult for silicon to be introduced into water-based biochemical systems even if the necessary range of biochemical molecules could be constructed out of it. Another problem with silicon dioxide is that it would be the product of aerobic respiration. If a silicon-based life form were to breathe using oxygen, as life on Earth does, it would possibly produce silicon dioxide as a by-product of this, assuming that the only difference between the two types of life is silicon in place of carbon. This implies that the exhaled product, silicon dioxide, would be a solid, thus filling the respiratory organs of the organism with sand. This however would be solved if the organism lives in temperatures of several hundred to thousand degrees, where the silicon dioxide becomes a liquid. Oxygen-breathing silicon life, if it exists, is therefore most likely to exist in environments with very high temperatures or pressure.

Finally, of the varieties of molecules identified in the interstellar medium as of 1998[update], 84 are based on carbon while only 8 are based on silicon. Moreover, of those 8 compounds, four also include carbon within them. The cosmic abundance of carbon to silicon is roughly 10 to 1. This may suggest a greater variety of complex carbon compounds throughout the cosmos, providing less of a foundation upon which to build silicon-based biologies, at least under the conditions prevalent on the surface of planets. Somewhat in support, in September 2012, NASA scientists reported that polycyclic aromatic hydrocarbons (PAHs), subjected to interstellar medium (ISM) conditions, are transformed, through hydrogenation, oxygenation and hydroxylation, to more complex organics - "a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively". (Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks.")

Also, even though Earth and other terrestrial planets are exceptionally silicon-rich and carbon-poor (the relative abundance of silicon to carbon in the Earth's crust is roughly 925:1), terrestrial life is carbon-based. The fact that carbon, though rare, has proven to be much more successful as a life base than the much more abundant silicon, may be evidence that silicon is poorly suited for biochemistry on Earth-like planets. For example: silicon is less versatile than carbon in forming compounds; the compounds formed by silicon are unstable and it blocks the flow of heat. Even so, biogenic silica is used by some Earth life, such as the silicate skeletal structure of diatoms. This suggests that extraterrestrial life forms may have silicon-based structure molecules and carbon-based proteins for metabolic purposes, therefore enabling the ability to feed on a common resource on a terrestrial planet like Earth for building up the silicon-based part of their body.

Silicon compounds may possibly be biologically useful under temperatures or pressures different from the surface of a terrestrial planet, either in conjunction with or in a role less directly analogous to carbon.