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What sort of xenobiochemistry would support information-carrying macromolecules like DNA if only trace amounts of phosphorus were present on an alien planet? Since the phosphoribosyl backbone of DNA would not be possible in such an environment, what might be a viable substitute?

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  • $\begingroup$ The answer to the question as asked is obviously arsenic, which is the closest to phosphorus in terms of chemical behavior. However, the question is nonsensical. In a world where phosphorus was exceedingly rare, biochemistry would have no reason to develop along the same paths as on Earth, where phosphorus is merely scarce. So, the energy carrying molecule would not be a homologue of ATP. $\endgroup$ – AlexP Sep 1 '18 at 11:45
  • $\begingroup$ You might want to take a look at this: biology.stackexchange.com/questions/11286/… $\endgroup$ – John Locke Sep 1 '18 at 13:27
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There is a rather controversial example of bacteria here on earth that used Arsenic instead of Phosphorus

http://en.wikipedia.org/wiki/GFAJ-1

Subsequent studies refuted the claim, but it's a starting point. It's plausible that a DNA like molecule could form with Arsenic instead of Phosphorus as Arsenic is directly under Phosphorus on the periodic table. This means it has the same number of valence electons, which is important in molecular bonding, and similar physical properties.

Arsenic is a semi-metal, you could probably use Nitrogen in the same way as both Phosphorous and Nitrogen are non-metals.

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  • $\begingroup$ I've discounted N, but probably shouldn't. Nitrate doesn't seem to behave like phosphate. I'd have to look into why, and whether my gut feeling is accurate. The candidate has to do two things: 1) permit conformations necessary to stabilize a double helix (assuming like DNA), and 2) release free energy necessary to polymerize the building blocks. dATP + DNA(n) -> PPi + DNA(n+1) -> 2 Pi + DNA(n+1). The last step is important because it makes the reverse reaction almost impossible. Of course, the temperature and other factors could profoundly affect behavior. $\endgroup$ – Paul Magnussen Nov 6 '14 at 20:26
  • $\begingroup$ TBH, that's getting past my understanding of organic chem. $\endgroup$ – evandentremont Nov 6 '14 at 20:53
  • $\begingroup$ OK, found a ref for N being limited: chemed.chem.purdue.edu/genchem/topicreview/bp/ch10/…. The reason is it doesn't have any d orbitals, so the bonding complexity is reduced. $\endgroup$ – Paul Magnussen Nov 6 '14 at 21:03
  • $\begingroup$ How about Arsenate? en.wikipedia.org/wiki/Arsenate#Arsenate_poisoning It appears arsenate and phosphate are relatively interchangeable. Also depends how real you want to get, if the world requires a high level understanding of chemistry to notice a potential flaw, it's probably good enough for everyone else. $\endgroup$ – evandentremont Nov 6 '14 at 21:08
  • $\begingroup$ Here is a reference containing one answer to the arsenic loving bacterium. normalesup.org/~adanchin/origine/arsenic.html The author claims arsenic is removed by making insoluble As-S compounds, and scavenging traces of P. $\endgroup$ – Paul Magnussen Nov 6 '14 at 21:28
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Arsenic, bismuth and antimony should be able to replace phosphorus because they're all in the same group. I haven't tested the chemistry with these yet, though.

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