Vision, photochemistry

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... 

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 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... 

Vision is a process in which light is absorbed by a pigment in a photoreceptor cell (by a dye in the eye) and the photochemistry that ensues ultimately produces a transient electrical signal that is transmitted to the brain and interpreted as a visual image. There is much that is not fully understood about this process, but we shall discuss briefly the chemistry involved. 

Vibrationally Coherent Photochemistry in the Femtosecond Primary Event of Vision. 

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 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 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 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. 

For a comprehensive treatise, see "Handbook of Sensory Physiology—Photochemistry of Vision" (H. J. A. Dartnall, Ed.), Springer-Verlag, New York, 1972. 

BonaCiC-Kouteck, V., Kouteck, J., Michl, J. (1987), Neutral and Charged Biradicals, Zwitterions, Funnels in S, and Proton Translocation Their Role in Photochemistry, Photophysics and Vision, Angew. Chem. Int. Ed. Engl. 26, 170. 

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. 

This chapter has gathered together the current understanding of retinal photoisomerization in visual and archaeal rhodopsins mainly from the experimental point of view. Extensive studies by means of ultrafast spectroscopy of visual and archaeal rhodopsins have provided an answer to the question, What is the primary reaction in vision We now know that it is isomerization from 11-cis to all-trans form in visual rhodopsins and from all-trans to 13-cis form in archaeal rho-dopsin. Femtosecond spectroscopy of visual and archaeal rhodopsins eventually captured their excited states and, as a consequence, we now know that this unique photochemistry takes place in our eyes and in archaea. Such unique reactions are facilitated in the protein environment, and recent structural determinations have further improved our understanding on the basis of structure. In parallel, vibrational analysis of primary intermediates, such as resonance Raman and infrared spectroscopies, have provided insight into the isomerization mechanism. 

Photochemical cis-trans isomerization is a major area of interest in modem photochemical research and is also studied as part of organic photochemistry. Photochemical cis-trans isomerization has a major role in many photobiological phenomena, such as vision (rhodopsin) [1], ATP synthesis (bacteriorhodopsin) [2], phototaxis (Chlamydomonas) [3], and other allied processes. It has practical application in industry [4-6], i.e., vitamin A and D processes. Furthermore, it is a likely candidate for many optoelectrical and optomechanical switching and storage devices [7]. In this chapter, mainly various aspects of cis-trans isomerization originating from the singlet excited state will be discussed. 

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