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Background

I am currently building a planet that weighs about 4 earth masses and is 2.3 earth radii long. It is an ammonia planet (as the title suggests), and has ammonia oceans with some dissolved water ice, methanol and salts. The average temperature is about 260ºK on the planet and the average pressure at sea level is about 2.5 atm.

All life on earth uses nucleic acids (either DNA or RNA) which contains the instructions on what organisms should do and develop. I would like my organisms to be able to have a set of instructions which naturally leads me to nucleic acids.

DNA would probably work quite well as nitrogen-fixing bacteria use it on Earth (thanks @Pica for telling me about this), but I would like to understand if any alternative xeno nucleic acids would also work and develop with DNA.

Requirements

This acid should meet all of these requirements:

  • similar to DNA/RNA and use pretty much the same CHON elements (although I'm ok with chlorine being added in significant amounts also, which is used in a number of important reactions for many organisms).
  • able to evolve and replicate like earth's DNA and RNA
  • able to dissolve in ammonia
  • work in cold temperatures (~260ºK) though I don't know if this is an issue
  • I'm fine with stuff that only exists synthetically on earth as long as it could plausibly form on an ammonia planet like mine

The issue

I honestly don't have the time for lots of research and I'm unable to find evaluate these alternatives to regular DNA or RNA and don't even know if they would work in the conditions of this world. That's why I would like your help to find or make a suitable nucleic acid that I can use.

Thanks in advance.

Potential Options

All of these still could work, but these are options that I have in mind. I like 2′FANA the most, then PNA, but I'm still fine with GNA if it is the best option.

PNA

Peptide Nucleic Acid (PNA) is one option I've been thinking of. It replaces DNA and RNA's sugar and phosphate backbone with N-(2-Aminoethyl)glycine linked by peptide bonds. It may have been used by very early earth organisms in the reducing atmosphere we used to have, and my atmosphere is reducing also.

Peptides also dissolve in dilute ammonia, so I don't think it's a stretch to say that PNA can dissolve in the ammonia in my organisms.

GNA

Glycol Nucleic Acid (GNA) is another potential option. It is quite simple and propylene glycol, its dissolves in many different liquids including water and acetone. Because of its simplicity, I'm considering using it.

2′FANA

Fluoro-arabinose (2′FANA) forms stable bonds and is also considered a favored alternative to DNA. It replaces the hydroxide radicals in arabinonucleic acid (ANA) with fluorine as far as I can tell.

Fluoride salts are also known to dissolve pretty well in liquid ammonia, but fluorine is pretty rare in the universe. It does sound like an interesting alternative though, but I'm not sure if it can form naturally.

My biochemistry is very different from earth though, and some organisms on this world are already able to take out the halogen from halogen-alkali metal salts, so maybe some early organisms could have done that with fluorine too the fluorine for the compounds necessary?

I actually quite like the idea of this compound, and it seems to have its upsides, but I'm not sure if it can actually form naturally.

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    $\begingroup$ You are basically trying to do something that is highly speculative and possibly impossible. I would say that you probably won't get a very specific answer on this. Though as this is highly speculative you probably should get away with making it semi realistic. Though generally i don't think you can make DNA as we know it in this way. $\endgroup$ Mar 25 at 0:39
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    $\begingroup$ I know it's speculative and unrealistic, and I am also not expecting super specific answers. However, I'd love some analogue to use. $\endgroup$
    – Neil Iyer
    Mar 25 at 1:07
  • $\begingroup$ As far as i am concerned life is possible if you have a good liquid solvent. Though another matter is if carbon or silicon can form live under such conditions as really they are the only options realistically available. That is if you don't want to go down an extra speculative path. $\endgroup$ Mar 25 at 18:39
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    $\begingroup$ Note it is now believed that Earth started with a carbon dioxide / nitrogen atmosphere rather than one based on methane: astrobiology.nasa.gov/news/earths-early-atmosphere-an-update $\endgroup$
    – Slarty
    Mar 27 at 8:10
  • $\begingroup$ Note that interstellar matter contains much lesser amount of N as C/O. That is because nitrogen has a much lesser stable nucleus (with its odd number of protons). $\endgroup$
    – Gray Sheep
    Mar 27 at 22:55

2 Answers 2

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PNA lacks the charged phosphate groups that repel the complementary backbones of DNA strands, which causes it to bind more tightly to DNA and RNA than either do to themselves. It also makes it hydrophobic in isolation-in other words, no, it doesn't easily dissolve in water or ammonia. It could operate in oil-filled organelles, and glycopeptides would probaly form fairly esily in the nitrogen-rich environment, but being more strongly bound isn't really something you want in a lower-temperature environment. It might work well with modified bases that have fewer hydrogen bonds between strands.

GNA is simpler and easier to synthesize than RNA or DNA... but it is also far more stable and binds more strongly, making replication harder. Again, better for a higher temperature world, not a low-temperature one.

Something like TNA (Threose Nucleic Acid) may be viable. I can't find a structural diagram for threose nucleotides, but with one less OH group it would probably be more stable against hydrolysis than RNA, which is good in the highly basic environment you've proposed. (Well, actually, it would be mildly acidic in the ammonious solvent system, but the important bit is that you've got dissolved water, which would be strongly dissociated and provide a lot of hydroxide ions to attack RNA.) You would probably have some replacement of hydroxide groups with amide groups, though, which would make it "not-actually-threose".

Arabinose, or a variant with amide substitution, seems highly plausible. It's basically ribose with a slightly different bond structure, which might make it less susceptible to hydrolysis. For natural biological purposes, the halogen substitution is entirely unnecessary; but if you want it for exoticness's sake, go with chlorine, not fluorine. It is possible to engineer organisms that manipulate halogen bonds, but nothing in nature on Earth does so. It's another thing that we would expect in higher-energy biospheres, not cold ones. However, Earthlings at least do ion substitution reactions with chlorine, and chlorine is much more common and bioavailable, so it's much more plausible that something would evolve to build chlorinated nucleotides than fluorinated ones.

Locked Nucleid Acid (LNA) is totally pointless from a natural perspective; it's not simple, and there's no reason to evolve into it. It exists solely to make artificial genetic polymers less susceptible to biodegradation.

I can't find much on Hexitol Nucleic Acids. They have been synthesized, and they work, but it's just another type of sugar. Probably fine if you want to use it.

CeNAs (Cyclohexene Nucleic Acids) seem to be most useful for forming hybrid pairs with RNA. It might expect them on a more hydrocarbon-rich world. It's hard to see why anything would bother synthesizing cyclohexenes in your environment.

And... that pretty much exhausts the list of XNAs that have actually been synthesized, and which we know the properties of.

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  • $\begingroup$ For the halogen chemistry thing, my hypothetical organisms use chlorine a lot, and it is used in many heterotrophic metabolisms (C₂H₆ + Cl₂ → C₂H₄ + 2 HCl + Energy and C₂H₄ + HCl → C₂H₅Cl + Energy). However, I like the idea of a plain arabinose nucleic acid system, and TNA also seems pretty good. I think I'll use some sort of system with arabinose, TNA, and DNA instead of our DNA/RNA system. Thanks for the comprehensive answer. $\endgroup$
    – Neil Iyer
    Mar 30 at 2:41
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I don't understand the question, why not use the bacteria we have on earth and build on that chemistry.

https://en.wikipedia.org/wiki/Nitrifying_bacteria

Those little nodes at the roots of soybeans and other legumes are filled with them. This is nothing exotic and is heavily used in non-harber-bosch fertilizing cycles.

Or does it have to be alien chemistry?

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  • $\begingroup$ No it doesn't, but I didn't know if such chemistry would be able to survive or form in this extreme environment. $\endgroup$
    – Neil Iyer
    Mar 27 at 22:34
  • $\begingroup$ Ah, so we need a full evolutionary history and phyla? That is going to be some work. $\endgroup$
    – Pica
    Mar 27 at 22:49
  • $\begingroup$ Yeah it is just something to keep me occupied and interested. $\endgroup$
    – Neil Iyer
    Mar 27 at 23:28
  • $\begingroup$ Another idea I have is to use both DNA and another nucleic acid (like how we use RNA on earth) like PNA or 2’FANA. $\endgroup$
    – Neil Iyer
    Mar 28 at 3:51

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