Vision chemical reaction

Control of Exothermic Chemical Reactions", HSE Video, available from CFL Vision, PO Box 35, Wetherby, Yorks LS23 7EX... 

When this molecule absorbs light, it rapidly isomerizes (changes its structure) to the all-trans form. This specific chemical reaction is the central event in human vision. The molecule is bound within a protein (the combination is called rhodopsin), the isomerization triggers a series of later processes, ultimately leading to an electrical signal which is sent to the brain. 

The global field of photochemistry has been defined in Coxon Halton as the study of chemical reactions initiated by light. 13 Although succinct, it may be too restrictive for the purposes of vision. The field might be better described by replacing the expression chemical reactions by chemical processes or chemical interactions. The revised definition would allow mechanisms, such as sensitization of one species by another without any change in species to occur. This... 

A primary reason for the above disparities is the critically important structural organization of the chromophores when found in-vivo. These relationships make a major (several orders of magnitude) difference in the absorbance of the material and also lead to anisotropic absorption. These relationships have not been maintained by the chemists. A second reason relates particularly to the L-channel. The chromophore of that channel exhibits a more intimate relationship with the electronic portion of the photoreceptor neuron than do the S- and M-channel chromophores. As a result, the L-channel exhibits an effective absorption characteristic very different from that observed by the chemist. This characteristic also accounts for the loss of red response in the mesopic and scotopic regions. These relationships have not been emulated in the environment of the chemist. Failure to emulate these conditions leads to extraneous absorption spectra for the L-channel chromophore. A third reason is due to the frequent chemical reactions occurring in the chemists solutions that he may not be aware of. It has been rare in the past for the chemist to document the pH of the solutions he has measured. This is a particular problem as mentioned in a later section [Section 5.5.12], The chromophores of vision are members of the "indicator class of chemicals. Their spectral characteristics are intimately related to the pH of their environment. They are also complex organics. Their spectral characteristics are a function of the organic solvent used. They are also subject to chemical attack. This mechanism has been documented by Wald, et. al. and more recently by Ma, et. al. 

The primary chemical reaction in vision, the one responsible for generating an impulse in the optic nerve, involves ds-trans isomerization around one of the double bonds in the retinal portion of rhodopsin. When rhodopsin is active (that is, when it can respond to visible light), the double bond between carbon atoms 11 and 12 of the retinal (ll- >retinal) has the cis orientation. Under the influence of light, an isomerization reaction occurs at this double bond, producing M-trans-Tetinal. Because the all-traw5 form of retinal cannot bind to opsin, zA-lrans-retinal and free opsin are released. As a result of this reaction, an electrical impulse is generated in the optic nerve and transmitted... 

Quantum yield, in photochemical reactions, is the ratio of the number of chemical reactions caused by light to the number of photons absorbed by the chemical species. With the exception of some rare photochemical processes in bio-inorganic chemistry, in which chain reactions initiated by absorption of a single photon result in multiple catalytic events and hence quantum yield greater than unity, in the vast majority of photochemical reactions and in all known photobiological reactions such as photosynthesis, vision, and phototropism the quantum yield is less than 1.0. 

Antonietti. M. Landfester. K. Mastai, Y. The vision of Nanochemistry, or is there a promise for speeific chemical reactions in nano-restricted environments Israel J. Chem. 2001. 41. 1. 

The femtosecond laser technique has been applied to unravel the mechanisms of many chemical reactions and biological processes such as photosynthesis and vision. It has created a new area in chemical kinetics that has become known as femtochemistry. 

The primary chemical reaction of vision in rod cells is the absorption of light by rhodopsin, followed by the isomerization of a carbon-carbon double bond from a cis configuration to a trans configuration. 

Those of us who worked with Martin Fleischmann likely remember him for different aspects of his multifaceted personality and extreme scientific diversity. For the authors of this chapter, his major accomplishment was the discovery of anomalous heat effects in the electrochemical palladium deuterium system (Pd/D). Few would have had the vision to see such a possibility, the courage to pursue it, and the skill to test it. These anomalous effects were and are consistent in magnitude with excess enthalpy production by nuclear reactions. These are several orders of magnitude larger than can be explained by chemical reactions or lattice storage energy. 

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