You’ve seen it on Star Trek or the X-Files, maybe in a sci-fi movie or book. Silicon-based life forms have resided in the human imagination for decades. Why? On the surface, silicon seems like an ideal substitute for carbon in another living system. Theoretically, silicon has bonding chemistry identical to that of carbon, and like carbon, can combine with four other elements to construct an incredible range of different macromolecules. So why not silicon-based life?

First, let’s look at the competition. Carbon, the MVP in all known biological molecules from sugar to DNA and even squid ink, is unique in that its bonding versatility allows it take on many forms: long side chains that make up fatty acids and cell membranes, ring structures that compose hormones and sugars, and even simple gaseous molecules like methane (CH4) or carbon dioxide (CO2). Can silicon compete?

The short answer is probably not. Silicon simply doesn’t have the moves. While carbon is perfectly comfortable in a variety of different structures (rings, long chains, multi-ring chains, and double-bonded carbon catenations), silicon’s analogous structures are comparatively unstable and sometimes highly reactive. Additionally, such analogous silicon compounds may never occur in nature; the largest silicon molecule ever observed had only six silicon atoms. In contrast, some carbon-based molecules can have tens of thousands!

Silicon also has the formidable disadvantage of being less abundant in the universe. The birthplace of all heavier elements—older stars—tend to produce far more carbon than silicon. Thus the likelihood of a living system to evolve based on silicon is lower based on the sheer rarity of naturally produced silicon compared to carbon. In fact, astronomical observations of the spectra of various stars and nebulae reveal that organic carbon ring structures (also known as polycylic aromatic hydrocarbons, or PAH’s) exist even in the far reaches of space. In a laboratory at NASA Ames Research Center, NAI astrochemist Lou Allamandola simulates the conditions under which it is believed these PAH’s are produced in space. His experiments have yielded a variety of carbon-based, biologically interesting molecules. Click here to view a seminar given by Dr. Allamandola originally webcasted live in January 2002.

Another chemical property unique to carbon chemistry that silicon lacks is chirality, or “handedness.” All organic carbon molecules may be found naturally in left or right-handed conformations. However, life as we know it utilizes only the right-hand form of sugars, integral components in DNA structure, and the left-hand form of amino acids, the building blocks of proteins. Very few silicon compounds have handedness at all. The biochemical reactions of life are incredibly specific, and in fact, many larger biomolecules are so precise that a single conformational change (right to left) around one carbon atom would block the reaction. Without chirality, the ability of biomolecules to recognize specific substrates would be crippled, ultimately limiting the number of different reactions available and achievable by a silicon-based system.

So, while the chances for silicon-based life may be slim, silicon may have played a role in emergence of life on Earth. One of the unsolved mysteries in the origin of life is why life came to employ one chiral version of a molecule (left vs. right) in its reactions and not the other. Some chemists believe that the chiral selection process in the pre-biotic “soup” might have been aided by a “handed” silica (SO2) surface. Both left- and right-handed molecules could have interacted with the chiral surface, and were aligned according to handedness. In this manner chiral molecules were separated and sorted in preparation for pre-biological selection. So even if silicon is an unlikely participant in the biological reactions of life, it could have certainly lent a helping hand to the origin of life.