Retinal is a pigment in your eyes that is necessary for vision. This is why vitamin A is so essential for vision - retinal is just one oxidation state up from retinol or one down from retinoic acid.
Upon absorbing a packet of light, retinal assumes the trans conformation you see above. Normally, it's found in the cis form (as seen here). This conformational change essentially amounts to a finger pushing a lever; some proteins translate it into a nerve impulse, off it goes to your brain, and hey! look! you've got sight!
The whole system is a little more complicated, especially when you add in color, but retinal is at the heart of it.
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Retinal I believe is derived from beta-carotene, a carotene carotenoid (as cf. with an oxidized xanthophyll). The carotenoids are truely a fascinating group of molecules. Lutein is now known to be essential in the folding of Photosystem II's light harvesting proteins (it lies at the center of these proteins literally wrapped into the protein complex), and zeaxanthin resides along the outer wall of the light harvesting chlorophylls quenching radical oxygen, triplet states (releasing energy as heat), etc. Because lutein and zeaxanthin are ubiquitous in plants they are considered dietary (as well as beta-cryptoxanthin and beta-carotene).
With over 900+ known carotenoids their evolution is remarkable. p450, diiron iron centers are active in their metabolism (which ultimately goes back to sterol and fatty acid chemistry). Carotenoids are certainly ancient molecules.
I thought I'd make a note as well that in June I will be publishing a web page on carotenoids and songbirds (they use them for red/orange/yellow coloration - sexual attractant - indicative of health, etc). The page will be at the Molecule of the Month (MOTM) site at the University of Bristol so do drop by in June and catch up on your songbird carotenoid biochemistry (will include an evolutionary discussion as well).
Just caught your post and it happened to be on subject with a bit of recent research so thought I'd go ahead and share a few facts.
Keep up the good work here -
Do you know of any source that would detail the absorption efficiency vs wavelength figures for various biological photochemicals?
Craig - haven't looked into that aspect of the carotenoids - in biological systems absorption is very sensitive to the pigment's environment - everything from nearby phospholipids and proteins to ionic strength, etc. A carotenoid may be orange in a songbird but bound to a protein in a crab,e.g., the same carotenoid may be blue, etc.
Since the spectrum of organic pigments sensitive to light varies significantly (with no commmon biochemical origin - except for classes - such as terpenes, etc) you would have to get lucky I think to find the data you are searching for - especially collectively; but you might - wish I could help.
W.r.t. to carotenoids if I were interested in absorption phenomenom I'd probably google up a dissertation or two along with a string of publications on the behavior of zeaxanthin and lutein in absorbing and transferring photons to the PSII light harvesting proteins - much work has been done in this area. My initial guess would be that you would have to follow up with each group of pigments individually (you might do a search on textbooks and find one closely related to your topic).
One dissertation which I have that goes into quite a bit of detail on photon absorption and transfer in carotenoids near PSII is entitled:
Plant Carotenoids: functional genomic of xanthophylls biosynthesis and role in Arabidopsis thaliana
You might take a look at it.
Thanks, I'll try to check it out.