Biochemistries on other planets

[Riemann]

@Ruken

Maybe you’ll figure it out if you give the OP another look. Silicone, not silicon.

Oh, apologies, I missed that part of the OP.

[Riemann]

Nick Lane’s “The Vital Question” has a lot of material that will be useful to you, I think. He doesn’t speculate much on alternative biochemistries, but he has some good thoughts on what we should expect to be universal.

Specific questions about “could X do the job of Y” are really difficult, I don’t have that level of expertise I’m afraid. Unless you can find some literature on the specific question that your concerned with, I think you’d need an expert biochemist in the specific field to give you a cogent answer.

[Riemann]

@RealityChuck

…In science fiction, you only have to talk about what’s important to the story, not the background. If people are nitpicking the science, the story has failed on other levels.

This depends somewhat on the kind of science fiction you want. I’m rather dismayed that the science is incidental to a lot of sci-fi. Many sci-fi stories could be set in Victorian England or Chicago in the 1950s, but just happens to be set on Omicron Persei 8 in the distant future. I’m much more interested in sci-fi where the science and its radical implications are the story.

[septimus]

@Ruken

Maybe you’ll figure it out if you give the OP another look. Silicone, not silicon.

Perhaps I should have written polysiloxane instead of silicone. Anyway, it seems that Si-Si-Si- structures are much less stable than C-C-C- or C-C-N- chains, while Si-O-Si-O- chains are almost too stable. I guess Si-C-Si-C- and Si-N-Si-N- chains are also alternatives. In any case, the side chains (analogs of amino acids or nucleotides) would derive from just C,O,N,H and presumably not Si.

A big difference between carbon- and silicon-based life is that the analog of CO[sub]2[/sub] — a waste gas easily disposed of — is SiO[sub]2[/sub] (silica) which is a solid. Many plants use silica phytoliths or frustules for structural support, e.g. in cell walls.

[Busy_Scissors]

@septimus

Thank you, Riemann; that was very helpful. Especially the notion that the hydrophilic phosphates make it easier for DNA strands to bond.

Does sulfate have similar properties? I note that phosphorus is, very roughly, about three times more common than sulfur in both Earth’s crust and human body.

Would it be out of line if one planet’s life used “replicating molecules very similar to RNA but with sulfate playing the role of phosphate”?

A sulfur diester would have no charge, so quite a big difference in hydrophilicity and character of the proposed SDNA. I think the main problem, though, is that they would be too labile. Not sure how much more, but phosphate diesters are rock-like - phenomenal half lives in water under neutral conditions. Obv enzymes can cleave them, but if you just stick a strand of DNA into water at physiological conditions, it’s extremely stable.

Someone published a very high profile paper in Science a few years ago claiming a strain of bacteria that could grow and metabolise arsenic, and directly inferred arsenate DNA linkages and arsenylation - supermassive discovery if true, even held a press conference. Alas, through either ignorance or malice, it was just a bunch of arse. Paper got torn to shreds in about 5 mins - the primary reason [amongst many] that it sounded like bollox was that AsO4 diesters are known to be extremely labile in water. So there would need to be some profound rationalisation of this fact for the work to be taken seriously, and there was none.

[septimus]

I was slightly disingenuous when I mentioned my science fiction epic. (Oh, I’d like to write that book, but my To Do list is already long and time is running out.) I mainly thought “exo-life” would be an interesting topic to discuss and hope those who’ve responded also find it interesting.

Via a cite on Wikipedia I found Is there a common chemical model for life in the universe? (pdf), a scholarly exploration of this topic. I intend to print this article and study it, but here are a few excerpts based on a first read.

Recognizing that geothermal energy is easily placed at the bottom of a food chain, Stevenson noted that small, Earth-like planets that support life from geothermal energy might be ejected from forming solar systems [26]. They could then travel the galaxy, carrying life completely independent of a star. Depending on the frequency of these ejections, such planets might hold the vast majority of life in our galaxy, all living without a sun on the decay of radioactive nuclei left over from past supernova explosions.
…
The reactivities of silicon and carbon differ in some notable ways. First, its position in the third row of the periodic table means that silicon carries low energy d orbitals. Therefore, the associate mechanism for reaction at silicon, where an incoming nucleophile forms a bond before a leaving group leaves, is available to silicon, but not to carbon. This makes many compounds containing silicon more reactive than their carbon analogs. Whether this difference in reactivity is advantageous or disadvantageous depends on perspective. The greater reactivity of silicon compared with carbon may be an advantage in cold environments.
…
Many Terran metabolic multistep pathways are run close to equilibrium for internal steps (Figure 3), with the first step being energetically downhill, and a site of regulation. Required, however, is that the last step be energetically downhill, thereby pulling the reaction to completion [43]. This feature may be universal in metabolic pathways, simply because it exploits most economically a surrounding chemical disequilibrium. Having large free energy drops at every step in a pathway is wasteful. This characteristic energetic relationship between a set of compounds that are intermediates in an evolved metabolism may be a universal biosignature. If an inventory of the small molecules in a suspected living system (on Saturn’s largest moon, Titan, for example) reveals this characteristic energetic relationship, this may be evidence of Darwinian evolution acting to create an optimal metabolism.
…
[Liquid phase has advantages but gas or solid life is possible, but probably with VERY slow metabolism] … Nevertheless, given cosmic lengths of time, and the input of energy via high energy particles, a biochemistry able to support Darwinian evolution can be conceived [45]. A weird life form might reside in solids of the Oort cloud (a large sphere of cosmic debris surrounding the solar system, and the origin of comets) living in deeply frozen water, obtaining energy occasionally from the trail of free radicals left behind by ionizing radiation, and carrying out only a few metabolic transformations per millennium.
…
[Ammonia, sulfuric acid, formamide and especially hydrocarbons like methane are plausible solvents alternative to water. (Even the “supercritical dihydrogen–helium mixtures found on gas giants” is a possible solvent.)] Water droplets in hydrocarbon solvents are, in addition, convenient cellular compartments for evolution… in many senses, hydrocarbon solvents are better than water for managing complex organic chemical reactivity. … Titan certainly meets all of the stringent criteria outlined above for life. Titan is not at thermodynamic equilibrium. It has abundant carbon containing molecules and heteroatoms. Titan’s temperature is low enough to permit a wide range of bonding, covalent and non-covalent. Titan undoubtedly offers other resources believed to be useful for catalysis necessary for life, including metals and surfaces
…
[The role of phosphate in replicating molecules is discussed, making Riemann’s point and more]
…
[CO[sub]2[/sub] has troublesome properties] … Indeed, if we encounter non-Terran carbonbased life, it will be interesting to see how they have come to manage the unfortunate properties of carbon dioxide

[Ruken]

The reactivities of silicon and carbon differ in some notable ways. First, its position in the third row of the periodic table means that silicon carries low energy d orbitals. Therefore, the associate mechanism for reaction at silicon, where an incoming nucleophile forms a bond before a leaving group leaves, is available to silicon, but not to carbon. This makes many compounds containing silicon more reactive than their carbon analogs. Whether this difference in reactivity is advantageous or disadvantageous depends on perspective. The greater reactivity of silicon compared with carbon may be an advantage in cold environments.

Wait wat? Babby’s first organic chemistry is the S[sub]N[/sub]2 reaction.