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"Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade. Another opsin found in the mammalian retina, melanopsin, is involved in circadian rhythms and pupillary reflex but not in image-forming.
Rhodopsin (Rh1, OPN2, RHO) – expressed in rod cells, used in night vision
Four cone opsins (also known as photopsins) – expressed in cone cells, used in color vision
Long Wavelength Sensitive (LWS, OPN1LW) Opsin – λ max in the red region of the electromagnetic spectrum
Middle Wavelength Sensitive (RH2 or MWS) Opsin – λ max in the green region of the electromagnetic spectrum
Short Wavelength Sensitive 2 (SWS2) Opsin – λ max in the blue region of the electromagnetic spectrum
Short Wavelength Sensitive 1 (SWS1) Opsin – λ max in the violet/UV region of the electromagnetic spectrum
"Many amino acid residues, termed functionally conserved residues, are highly conserved between all opsin groups, indicative of important functional roles. Certain amino acid residues, termed spectral tuning sites, have a strong effect on λ max values. ... It is important to differentiate spectral tuning sites, residues that affect the wavelength at which the opsin absorbs light, from functionally conserved sites, residues important for the proper functioning of the opsin. They are not mutually exclusive ... The impact of spectral tuning sites on λ max differs between different opsin groups and between opsin groups of different species."
Opsin, Wikipedia
"Opsin proteins are responsible for capturing light during the initial steps of vision. Opsins have a characteristic cofactor, retinal, which harnesses light energy. Color vision is possible because different groups of opsins tune the spectral sensitivity of retinal from 350 nm to greater than 600 nm. The origins of this spectral tuning remain a mystery. The primary event describes the transformation of opsin from its initial state to the first trappable (by temperature) intermediate, bathorhodopsin, after retinal absorbs a quantum of light. During this event, as much as 60% of the light energy is chemically trapped by the protein. This energy is then used in subsequent steps to alter the conformation of the protein to a state which binds and activates a G-protein. I have studied the primary event in two poorly characterized but important classes of opsins: the short-wavelength sensitive and invertebrate opsins. These proteins have not been studied extensively biochemically due to experimental limitations in obtaining significant quantities. Using heterologous expression systems, I have demonstrated that significant quantities of these proteins can be produced for cryogenic UV-Vis spectroscopy. By analyzing the primary event in short-wavelength sensitive opsins, I report that the mechanism used to adjust the spectral sensitivity of the violet opsins is the same as the other visible-absorbing pigments. Computer simulations combined with experimentally derived primary event data, indicate that the primary mechanism responsible for spectral tuning is modulation of the Schiff base--counterion distance. Furthermore, spectral tuning of the UV-sensitive opsins must be different. An investigation of the primary event of Drosophila rhodopsin1 reveals that the retinal binding site is similar to the vertebrate binding sites. The binding site must contain a single counterion stabilizing a protonated Schiff-base. This is contrary to previous resonance Raman experiments which have shown that the vertebrate counterion equivalent position is neutral in a cephalopod opsin."
The primary event of visual opsins
"Rhodopsin (Rh) and bathorhodopsin (bathoRh) quantum-mechanics/molecular-mechanics models based on ab initio multiconfigurational wave functions are employed to look at the light induced pi-bond breaking and reconstitution occurring during the Rh -> bathoRh and bathoRh -> Rh isomerizations. More specifically, semiclassical trajectory computations are used to compare the excited (S1) and ground (S0) state dynamics characterizing the opposite steps of the Rh/bathoRh photochromic cycle during the first 200 fs following photoexcitation. We show that the information contained in these data provide an unprecedented insight into the sub-picosecond pi-bond reconstitution process which is at the basis of the reactivity of the protein embedded 11-cis and all-trans retinal chromophores. More specifically, the data point to the phase and amplitude of the skeletal bond length alternation stretching mode as the key factor switching the chromophore to a bonding state. It is also confirmed/found that the phase and amplitude of the hydrogen-out-of-plane mode controls the stereochemical outcome of the forward and reverse photoisomerizations."
The Ultrafast Photoisomerizations of Rhodopsin and Bathorhodopsin Are Modulated by Bond Length Alternation and HOOP Driven Electronic Effects
"A model for the primary process of vision is proposed, which involves a novel concerted-twist motion. Application of such motions to rhodopsin and bathorhodopsin successfully accounts for the properties of bathorhodopsin and related intermediates, including specific assignment of molecular structures to bathorhodopsin, to lumirhodopsin, and, less specifically, to hypsorhodopsin."
The primary process of vision and the structure of bathorhodopsin: a mechanism for photoisomerization of polyenes
"Mantis shrimp not only have the ability to see colours from the ultraviolet through to the infrared, but have optimal polarisation vision -- a first for any animal
"They see the world in 11 or 12 primary colours as opposed to our humble three, and now we find that this species can see a world invisible to the rest of us.
"Shrimp of the species Gonodactylus smithii have eyes that simultaneously measure four linear and two circular polarisations
"Each eye measures the six polarisation components that are precisely required for optimal polarisation vision. In fact, the physics we used to understand what was going on is the same physics that we use in quantum computing for optimal storage of information"
Weird Shrimp Has Astounding Vision, Science Daily
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