Photochemistry 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. 

Eye is a system of lens and retina. There are two types of light sensitive cells are found in retina, known as RODS and CONES. Both the cells act as receptor of photon. The rods have been found highly sensitive and functioning at low light intensity. Hence rods are responsible for vision in dim light. While cones are low sensitive and work as colour selector. The cones are more is number than rods and responsible for detail colour vision in good light. Central part of visual field in retina are made up only of cones while sides by rods. 

The photosensitive compounds present in Human and most other mammals are two proteins-opsin and retinal. Out of these two proteins retinal act as receptor of photon and use to give geometrical isomerisation. 

The photosensitive pigment found in rods is called rhodopsin or visual purple. Its retinal is 11-cis-retinal and its opsin is scotopsin. Rhodopsin shows absorption maximum at - 500 nm. Human rhodopsin has molecular weight 41000 and 348 amino acid residues. The 90% of the total protein in the membrane of red disk are made up of rhodopsin. It is coupled with G-protein as  

Retinal is parallel to the surface of membrane and is attached to a lysin residue at position 296 in the seventh trans membrane domain. 

---

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... 

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

Biochemistry/medicine Protein structure and folding, dynamics and structure of membranes. Donor-acceptor distances, photochemistry of vision Limitations in sensitivity due to natural prompt fluorescence are reduced... 

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. 

---