Can retinal emit as well as absorb photons?

Yamayama

Retinal: the molecules which absorb photons of light in the photoreceptors at the back of our eyes; they combine with an opsin molecule to form photopigments.

It seems to me that: just as isomerization of retinal from 11-cis-retinal to all-trans-retinal corresponds to the absorption of a photon, changing back from all-trans-retinol to 11-cis-retinol - which is a necessary step for rhodopsin regeneration - surely corresponds to the emission of a photon.

If my suspicion is correct, I guess the reason this isn't observed is because rhodopsin regeneration occurs in the RPE (retinal pigmented epithelium, which is high in melanin - fantastic for absorbing light).

Can anyone affirm/disaffirm my suspicions?

Loren Booda

Subjectively speaking, yes. Looking closely into one woman's eyes I once felt that way. It was the most intimate communication I had ever experienced in public. Her eyes were dilated, and I could see the heartbeat in her irises. But photons? More like epiphany.

Njorl

While there are generally 2 ways any reaction can go, implying that anything that absorbs light can emit it, I bet the non-emitting relaxation processes are dominant.

Also, everything emits blackbody radiation, but that is trivial.

Njorl

Yamayama

hmmmm...nice anecdote...well, I was hoping more for an objective response, but hey...lucky you!

Njorl:
You're quite possibly right.

The events involved in rhodopsin regeneration are quite complex I expect, and I'm not sure they're even fully understood yet.

Also, it's just been pointed out to me that if a photon is emitted, its energy is not necessarily in the visible range, i.e. it could be infrared.

Monique

You might be interested in the following papers

Nature. 1976 Apr 22;260(5553):675-8.
Observation of light emission from a rhodopsin.
Lewis A, Spoonhower JP, Perreault GJ.
PMID: 1264238 [PubMed]

Photochem Photobiol. 1982 Mar;35(3):385-90.
Light-induced change in rhodopsin emission: phosphorescence and fluorescence.
Andley UP, Chakrabarti B.
PMID: 7063554 [PubMed]

I'm not sure how common light emission from rhodopsin is, since the conformational change from cis to trans is biochemically coupled to downstream signalling cascades. It would depend how soon the energy transfer to an electrical signal can happen when it it goes into trans, and how readily it is able to go back into cis with the emission of a photon.

You are right, the color absorped most by rhodopsin is purple, so it is to be expected that the emitted photon would be in the infrared (longer wavelength).

Yamayama

Thanks very much Monique,

I should have thought of searching PubMed myself of course. Anyway, unfortunately I can't seem to get those articles online, as issues that old aren't available in their online catalogs, but maybe I can figure out another way to get them.
Appreciate the help

Monique

Ask a librarian to fetch a copy for you, those issues are probably still standing in some library and they'll know where :)