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Recently I was surfing the internet and I found thisWikipedia article. It states that

The element silicon has been much discussed as a hypothetical alternative to carbon. Silicon is in the same group as carbon on the periodic table and, like carbon, it is tetravalent. Hypothetical alternatives to water include ammonia, which, like water, is a polar molecule, and cosmically abundant; and non-polar hydrocarbon solvents such as methane and ethane, which are known to exist in liquid form on the surface of Titan.

This leads me to think if there is any hypothetical alternative for Nitrogen as well?

I want to know in the context of amino acids and protein, as in is there any element/molecule that is eligible to replace the amino group of amino acid and yet be able to form proteins.

Please consider, I'm expecting hypothetical views. I'm from a biology background and well aware of the basic nature of amino acids.

Thanx for reading :)

classy_BLINK
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    There are no serious hypothetical alternatives to the second period elements like $\ce{C},$ $\ce{N},$ $\ce{O}$, unless for very specific cases. They differ too much from elements of other periods as well as mutually between each other. Another issue would be the relative abundance in the Universe. E.g. there is hard to replace the basic aminogroup $\ce{-NH2}$ or RNA/DNA coding. – Poutnik Oct 06 '21 at 09:40
  • @Poutnik, going by your words, it is hard to replace the basic amino group because it's highly abundant. I'm assuming that it is not impossible then. If it is just hard to replace and not impossible.., then what element/molecule has minor chances or nano chances to replace aminogroup in that case. – classy_BLINK Oct 06 '21 at 10:40
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    While looking a superficial properties of elements like silicon as possible alternatives to carbon (eg tetravalency) suggests they might be feasible alternatives for life, the detailed chemistry does not. Life needs chemistry that can sustain very highly complex molecules; no combinations of other elements (like second-row congeners of Si, N, O etc.) have ever been shown to achieve that. – matt_black Oct 06 '21 at 11:29
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    @classy_BLINK Those are not my words, I have just added the last sentence at wrong place so they may have looked so. Abundance of N is just additional factor. Hypothetical life founded on an alternative element is even less probable if it is relatively rare. – Poutnik Oct 06 '21 at 12:14
  • I've read that among binary compounds of elements with hydrogen, only those with nitrogen are basic in water without reducing it. That makes nitrogen hard to replace where it provides a basic functional group, such as in amino acids, unless you find nonaqueous life forms. – Oscar Lanzi Oct 06 '21 at 12:43
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    Does this answer your question? Why did life end up with the big 4 elements, and specifically nitrogen? – Tyberius Oct 07 '21 at 15:12

1 Answers1

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If you want to make exact analogs of biological molecules, you need an element with a similar valence to nitrogen, so you're pretty much looking at phosphorus as your best option. But phosphorus behaves very differently from nitrogen in ways that will negatively affect the stability and function of those molecules. Specifically,

$\ce{-PH2}$ groups are not basic like $\ce{-NH2}$ groups are, so the cation functionality and potential for ionic interactions and even strong H-bonding is lost

$\pi$ bonds to phosphorus are generally less favored than sigma bonds, so the many iminium-type compounds (especially heterocycles) would not be as stable with P.

Reduced phosphorus compounds oxidize much more easily than corresponding nitrogen compounds

I think it would be much more productive to speculate about a completely different set of macromolecules in a completely different environment rather than trying to reproduce known molecules with elemental substitutions. The macromolecules we have evolved the way they have precisely because of their current elemental compositions and the existing Earth environment. There is no reason to think they are good templates for other elemental compositions or planetary conditions. Maybe start with the constraints of the environmental conditions and abundant elements of another specific planet and work from there.

Andrew
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