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In the reaction involving conversion of 11-cis-retinal to all-transform, why does the isomerization specifically occur at the 11th carbon in the molecule? When reforming the broken pi-bond, why wouldn't it happen at the cis-conformation again? Is this due to the trans-form being more stable in this case and that it has a lower enthalpy? Reaction

MrProgrammer
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  • Yes, probably, all such changes are governed by that. From what I could gather, there's likely a steric issue there. It would help if you add an image of the reaction, including all the conditions. – Martin - マーチン Sep 02 '23 at 11:26
  • @Martin-マーチン Ok, I just added an image! – MrProgrammer Sep 02 '23 at 20:49
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    related https://chemistry.stackexchange.com/questions/73289/isnt-11-trans-retinal-more-stable-than-11-cis-retinal https://chemistry.stackexchange.com/questions/64219/when-is-rotation-around-double-bonds-to-form-cis-trans-isomers-allowed https://chemistry.stackexchange.com/questions/107505/what-is-thermal-isomerization – Mithoron Sep 02 '23 at 22:04

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There are more steps than the net reaction scheme posted by the OP. The trans-isomerization goes along with hydrolysis of a Schiff base, releasing the molecule from opsin. Several enzymes then recycle the 11-cis-retinal before it forms a Schiff base again with opsin, ready for the next light-induced isomerization to the trans form. Here is an overview from a 2010 review (doi: 10.1016/j.tibs.2010.01.005):

enter image description here

The isomerization from the all-trans to the 11-cis form (step iv) is catalyzed by the RPE65 enzyme containing an iron(II) center via a carbocation intermediate. The details are not yet clear (no well-resolved structures of the substrate or product complex), but the presence of an enzyme can help to isomerize a specific double bond rather than obtaining a product mixture. Also, by coupling unfavorable reactions with favorable ones, you can get a molecule that is kinetically trapped in a high-energy state, ready to undergo isomerization (step i) as a way to detect light.

The "why 11-cis" question is addressed in this computational paper: "The results show that the electrostatic interaction between retinal and opsin dominates the natural selection of 11-cis-retinal over other cis isomers in the dark state."

Karsten
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