On the Potential of Silicon as a Building Block for Life

Abstract

Despite more than one hundred years of work on organosilicon chemistry, the basis for the plausibility of silicon-based life has never been systematically addressed nor objectively reviewed. We provide a comprehensive assessment of the possibility of silicon-based biochemistry, based on a review of what is known and what has been modeled, even including speculative work. We assess whether or not silicon chemistry meets the requirements for chemical diversity and reactivity as compared to carbon. To expand the possibility of plausible silicon biochemistry, we explore silicon’s chemical complexity in diverse solvents found in planetary environments, including water, cryosolvents, and sulfuric acid. In no environment is a life based primarily around silicon chemistry a plausible option. We find that in a water-rich environment silicon’s chemical capacity is highly limited due to ubiquitous silica formation; silicon can likely only be used as a rare and specialized heteroatom. Cryosolvents (e.g., liquid N2) provide extremely low solubility of all molecules, including organosilicons. Sulfuric acid, surprisingly, appears to be able to support a much larger diversity of organosilicon chemistry than water.

The Astrobiology of Alien Worlds: Known and Unknown Forms of Life

“…Under Earth conditions, however, silicon reacts both with water and molecular oxygen, and is quickly immobilized as silicates. Petkowski et al [75] explored silicon's chemical complexity in several solvents present in planetary environments such as water, cryosolvents, and sulfuric acid. They concluded that in no environment would life based primarily on silicon chemistry be a plausible option.…”

Evaluating Alternatives to Water as Solvents for Life: The Example of Sulfuric Acid

“…Some aromatic phosphines are highly stable in sulfuric acid, but there are few representatives of these in the datasets used here. More unexpected is the depletion of silicon-containing compounds, as our previous analysis [ 87 ] suggests that a greater diversity of organosilicon compounds should be stable in sulfuric acid than in water. We hypothesize that the relative instability of silicon compounds seen in Figure 7 is the result of the use of specific, selected silicon chemistry in organic synthesis, and is not representative of the more broad qualitative analysis of the stability of organosilicon chemistry done in [ 87 ].…”

“…More unexpected is the depletion of silicon-containing compounds, as our previous analysis [ 87 ] suggests that a greater diversity of organosilicon compounds should be stable in sulfuric acid than in water. We hypothesize that the relative instability of silicon compounds seen in Figure 7 is the result of the use of specific, selected silicon chemistry in organic synthesis, and is not representative of the more broad qualitative analysis of the stability of organosilicon chemistry done in [ 87 ]. Specifically, the large majority of silicon-containing compounds in the chemical dataset are trimethylsilyl ethers or compounds containing phenylsilyl groups, both of which are known to be acid-labile [ 97 , 98 ].…”

“…In both cases, silicon groups are being used as acid-cleavable protecting groups, and hence have been selected to be unstable in sulfuric acid. We tested this hypothesis by generating a set of molecules containing C, N, O, and H, with or without Si atoms, using the combinatorial chemical structure generator Combimol-B [ 87 ]. Molecules up to eight non-H atoms were generated.…”

“…Combimol-B has been described in [ 87 ], and is available for non-commercial use from WB. This program generates a systematic set of all chemicals that can be built from a defined set of atoms, bonds, and molecular topologies.…”