4
$\begingroup$

The aliens in my story have biochemistry that uses amino acids and sugars with chirality opposite to that of terrestrial life.

The aliens also have nanites that are capable of changing the chirality of amino acids and sugars found on Earth to that of their own biochemistry and back again, so that the aliens can digest terrestrial food and not poison terrestrial life-forms that might encounter the aliens' bodily wastes.

That then begs the question: what is the minimum net energy cost to perform this conversion of amino acids and sugars to molecules of the opposite chirality?

Edit:

The nanites will be working on the things that the aliens eat before they are digested, converting food with our chirality to their chirality. Anything that remains undigested has its chirality changed back before it is defecated, so that opposite chirality alien feces won't poison local terrestrial life-forms. The nanites are not attempting to change the chirality of anything that is part of anything alive.

Improve this question
$\endgroup$
5
  • $\begingroup$ @VogonPoet What genome? I'm not talking about nucleic acids here, just amino acids and sugars in the things that the alien eats before they get digested, and back again before they are defecated. $\endgroup$
    – Monty Wild ♦
    Feb 26, 2022 at 5:07
  • 2
    $\begingroup$ OK got it. The answer is more than Pun Intended draws out tho. It's breaking bonds, then assembling bonds; that is a net zero from a chemical balancing equation but not from the perspective of the nanobot's paycheck... Let me add: reversing chirality on each protein literally reverses most of the molecular bonds. It's a calculation done per-protein. $\endgroup$
    – Vogon Poet
    Feb 26, 2022 at 5:08
  • $\begingroup$ @VogonPoet If you have an answer that isn't a net zero, I'd like to see it. $\endgroup$
    – Monty Wild ♦
    Feb 26, 2022 at 5:13
  • $\begingroup$ Consider feces isn't just food, it is us. It contains entire dead cells. These cells have DNA and RNA in them. That means you actually ARE changing the chirality of something that is part of something living if you are processing feces or urine to make them not poisonous. But I digress. If I wrote an answer I would do it for exactly one protein, and one alone, because each will be different. So... $\endgroup$
    – Vogon Poet
    Feb 26, 2022 at 5:18
  • $\begingroup$ If I chose to reverse an RNA sugar, for example, it would be the sum of the Gibbs free energy of every single right-handed nucleic acid and its components. Billions of them, is it? I am going to assume with much confidence that if you wished to detox a right-handed alien turd, the organ responsible will be lit up like a thermonuclear reactor. $\endgroup$
    – Vogon Poet
    Feb 26, 2022 at 5:19

2 Answers 2

Reset to default
6
$\begingroup$

No (or trivial) energy cost

Delta H of reaction

In chemistry, deltaHf is the standard enthalpy of formation - in other words, the amount of energy contained in a molecule's bonds.

The image above shows the delta H of a reaction, which is calculated by the difference in bond potential energy of products and reactants. Since your chiral amino acid pairs contain the exact same bonds (mirrored, but in the same quantities), there is no difference in the deltaHf of products and reactants. Regardless of the activation energy required for this reaction, that energy is a threshold, rather than an amount consumed.

As an aside, enzymes work to lower the activation energy of a reaction (see below in dashed red). If your aliens are capable of engineering nanobots to modify their macromolecular chiralities at will, you can bet they'll have figured out how to lower that activation energy to nearly zero, effectively making this reaction take place at whatever rate they like.

enter image description here

Improve this answer
$\endgroup$
1
  • $\begingroup$ Since the beginning and ending molecules would need to be almost perfectly identical (just mirror images), there would always need to be SOME energy invested, but with the right enzymes, the cost could, indeed, be quite low. Still +1 $\endgroup$
    – DWKraus
    Feb 26, 2022 at 16:24
6
$\begingroup$

Not zero, apart from that it will really depend on the thermal efficiency/energy storage capacity of your nanites, you have to break and reestablish a notable percentage of the chemical bonds in every compound you want to treat, there will be carbon chain sections that can be reused as is but the amino acids will have to be unfolded, then all the interesting bits have to be broken down and rebuilt and then the amino acid allowed to rekink with it's newly reversed chiral twist. Untwisting the protein chains to start the process, and keeping them that way during the process, is going to take energy that you aren't getting back. The breaking and reforming of the bonds in the active areas of the amino acids could potentially be very nearly net zero in terms of energy use, the liberated energy of breaking or forming the bond being stored and used for the reverse reaction. The proteins will refold themselves, that has been shown repeatedly in laboratory experiments made on many different scales, that is a kinetic process and is going to liberate a little waste heat to the material in question; that will give you some energy for the unfold but there's still a net cost to holding the proteins unwound while you work on them.

"Fun" consequence, if your nanites get in the way of the refold or fail to untwist a section of protein properly for treatment then you will get strains of protein that aren't folded quite right.

Improve this answer
$\endgroup$
3
  • $\begingroup$ Did the OP specify 100% efficient nanites? A net energy calculation isn't really worth anything. Is this answer assuming the nanites are powered by energy of formation/dissociation for each bond it is working somehow? Because if that is the case, then how do nanites avoid getting frozen by the endothermic reactions? This is a really messy business that can just as well say"perpetual motion nanites $\therefore$ I win." And what material do you make nanite "claws" of that can hold one atom? What are the wires made of? The CPU? I can't even enter a science-based conversation in this. $\endgroup$
    – Vogon Poet
    Feb 26, 2022 at 13:41
  • $\begingroup$ @VogonPoet A "nanite" at this level is simply a molecular machine, like a protein, but not limited to the structural components of biochemistry. Proteins can already perform these tasks with machines built of amino acids, cofactors,, sugars, etc. A nanite could be more powerful, since they would not be so limited in engineering. Of course, the nanites need to be made, delivered to the appropriate places, not result in an immune response, etc. so there is no PERFECT solution. $\endgroup$
    – DWKraus
    Feb 26, 2022 at 16:22
  • $\begingroup$ Yes, like a kinesin. I know what they are, except things that size don't follow programs, they follow the laws of thermodynamics. $\endgroup$
    – Vogon Poet
    Feb 26, 2022 at 19:45

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .