The Photochemistry of Vision

The chemical changes that occur when light impinges on the retina of the eye involve several of the phenomena that we have studied. Central to an understanding of the visual process at the molecular level are two phenomena in particular the absorption of light by conjugated polyenes and the interconversion of cis-trans isomers. The conjugated polyene, derived from a compound called retinal, is a part of a molecule called rhodopsin. 

When rhodopsin absorbs a photon of light, the 11-c/s-retinal chromophore isomerizes to the all-transform, causing 

Lycopene, a compound partly responsible for the red color of tomatoes, also has 11 conjugated double bonds. Lycopene has an absoplitxi maximum at 505 nm whoe it absorbs intensely. (Approximately 0.02 g of lycopene can be isolated from 1 kg of fiesh, ripe tomatoes.) 

Long-Wavelength Absorption Maxima of Unsaturated Hydrocarbons 

Compounds with carbon-oxygen double bonds also absorb hght in the UV region. Acetone, for example, has a broad absorption peak at 280 nm that corresponds to the excitation of an electron from one of the unshared pairs (a nonbonding or n electron) to the TT orbital of the carbon-oxygen double bond  

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The photochemistry of vision provides us with an example of host-guest supramolecular photochemistry where the smaller 11-cis-retinal guest molecule is held within the internal cavity of the much larger protein host molecule (opsin) as a result of noncovalent bonding. 

Before considering the photochemistry of vision in higher animals, it will be interesting to look briefly at the process known as phototaxis which is sometimes considered to be a very early form of vision . It has been observed that some photosynthetic bacteria are able to swim selectively towards illuminated areas, and to avoid dark places this light-controlled motion has been named phototaxis (Figure 5.9). This type of behaviour is... 

This Chapter begins with a comprehensive review of the quantum-mechanical properties of organic molecules and how this affects their photon excitation. A series of detailed definitions and concepts are presented that are not normally found in biological treatises. These concepts are vital to an understanding of the mechanisms involved in the photochemistry of vision. 

The photochemistry of vision depends on a quantum-mechanical foundation and involves states of matter that may not be familiar to the average investigator. These must be carefully defined. In the absence of careful definition, it is impossible to account for the spectral characteristics of vision. Recently, science has defined a variety of states of matter beyond the conventional gas, liquid and solid. These states may each be observed in several forms... 

The "Free Rotor" or "Loose Bolt" Effect on Quantum Yields 953 Single-Molecule FRET 961 Tni/js-Cyclohexene 967 Retinal and Rhodopsin— The Photochemistry of Vision 968 Photochromism 969... 

Kuhne WF (1879) The photochemistry of vision and retinal purple. McMillan, London... 

Imine formation occurs in many biochemical reactions because euzymes ofteu use an —NH2 group to react with an aldehyde or ketone. An imine linkage is important in the biochemistry of pyridoxal phosphate (which is related to vitamin B see The Chemistry of. .. box on the next page), and in one step of the reactions that take place during the visual process (see The Photochemistry of Vision, Section 13.9). 

The photochemistry of vision is triggered by absorption of a photon and induces cis-trans isomerisation. The conjugated polyenal, ll-c/s-retinal and the protein opsin combine in retina to give the red-purple 11-cis- amine, rhodopsin. 

Carotenoids absorb visible light (Section 13 21) and dissipate its energy as heat thereby protecting the organism from any potentially harmful effects associated with sunlight induced photochemistry They are also indirectly involved m the chemistry of vision owing to the fact that p carotene is the biosynthetic precursor of vitamin A also known as retinol a key substance m the visual process... 

Outline the importance the photochemistry of C=C bonds in the phototherapy of infants with neonatal jaundice, the process of vision and the effects of ultraviolet radiation on DNA. 

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... 

The example of vision demonstrates the profound influence of a protein matrix on the photochemistry of its constituent cofactor (guest molecule). This occurs by stabilization of unstable conformers and strained geometries and by fixation of the relative arrangements of systems of co-factors and generation of contacts between co-factors. Although the complexity of the structure of the protein precludes their use in everyday laboratory control of photoreactions, the lessons learned from the example of vision (and photosynthesis) are useful in designing media that provide better control of photoreactions than that obtained in isotropic solution. Let us compare the site (termed the reaction cavity) at which the reaction occurs in a protein and an isotropic solution medium. 

Retinal as Visual Pigment Model Spectroscopy and Physical Chemistry. As in previous years, several theoretical, spectroscopic, and photochemical studies of retinal (136) and related compounds, especially Schiffs bases, have been reported,and in many cases the main aim was to obtain information relevant to the functioning of rhodopsin and related visual pigments. Particularly valuable are surveys of the year s literature on the photochemistry of polyenes, excited states of biomolecules,and recent developments in the molecular biology of vision. 

A lot of research has been done in recent time targeting photochemistry of vision. The eye is an extraordinary instrument, sensitive within Visible region [400 nm to 800 nm]. The sensitivity can be understood by the fact that a fully darken eye can clearly detect object in light so dim that only 10,000 quanta absorbed per second by retina, that is one quanta per three minutes to receptor cell on the retina. 

Because the high quantum yield originates from the high-rate isomerization, which competes with other relaxation processes in the excited state of rhodopsin, ultrafast laser spectroscopies were applied to investigate the isomerization process of the retinal chromophore. Picosecond time-resolved spectroscopy was appHed to the photochemistry of rhodopsin, and the formation of the primary intermediates was reported, such as photorhodopsin and bathorhodopsin at room temperature. - - However, the time resolution needed to be improved in order to detect the cis-tram isomerization process in the excited state of rhodopsin. The direct observation of the rhodopsin excited state was reported in 1991, in which the primary intermediate photorhodopsin formed from the excited state of rhodopsin within 200 fs. Later, the effects of oscillatory features with a period of 550 fs (60 cm ) on the formation kinetics of photorhodopsin, were observed, suggesting that the primary step in vision is a vibrationally coherent process. 

Wang Q, Schoenlein R W, Peteanu L A, Mathies R A and Shank C V 1994 Vibrationally coherent photochemistry in the femtosecond primary event of vision Science 266 422... 

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