This compound derived from vitamin A, is contained in two kinds of cells are cone cells and rod cells. This is located in the retina of the eye, rod cells are responsible for vision in dim light, and they gives the sensation of light or dark but no sensation of color.
And cone cells responsible for color vision, but they are require bright light. Color vision is possible because they are three types of cone cells exist. The one types of absorbs light in the red region of the spectrum, and another absorbs in the green region, and a third absorbs in the blue region. In each of this types of different cells, 11-cis - retinal is attached to a different protien molecule, which affects the region of the light that is absorbed by retinal.
When they photon of light (or) waves absorbed by 11-Cis retinal it is transformed to 11-trans-retinal. This little changes at one carbon - carbon double bond and large movement of one end of the molecule with respect to the other. In another words the absorption of a single photon results in significant changes in molecular geometry,. shape of the retina also affect the shape of the protein molecule to which it's attached and this change results in a sequence of events, not completely understood in which electrical signal generated.
The understanding of the cis-trans conversion when a photon is absorbed, consider the simple molecule cis-1,2 dichloroethene and the double bonds consist σ and π bond. The pi (π) bond results from two electrons in a π type molecular orbital. The corresponding antibonding orbital, π*, has higher energy level than the π orbitals and is unoccupied in the ground state. When the photon of light is ultraviolet region near 180 nano meters is absorbed by cis-1,2-dichloroethene, and electron in the π orbitals undergoes a transition to the π* orbital. Whereas there previously two electrons in the bonding π orbitals contributed one bond there is now one electron in the bonding orbital antibonding orbital with zero net bonding as a result of the absorption of a photon, the double bonds becomes a single bond and rotation about the bond is now possible. The cis isomers tend to rotate to the trans isomers which are more stable. The energy is lost as a heat and the and electron in the π* orbitals undergoes a transition back to the π orbitals, the single bond becames a double bond again.
The net results is the conversion of the cis isomer to the trans isomer.
Retinal differs from the dichloroethene in have six double bonds alternately with single bonds instead of only one double bond. The alternative double bond and single bonds gives the double bond and gives rigid structure and when the middle double bond undergoes the cis- to- trans change in the wavelength of light absorbed and large movement of atoms occurs. The another one is long chain of alternating double bond and single bonds changes in the wavelength of light absorbed in the π to π* transition. The dichloroethene absorbed in the uv light region. The attracted protiens also in visible region. But retinal attached to it's protien also alters the wavelength of absorbed by light. Many biological compounds that are also coloured consists of long chains of alternating double bonds and single bonds. β- carotene is a yellow pigment in carrots and tomato has this structure. It's broken down in the body to gives vitamin A.
11-cis-Retinal 11-trans - Retinal
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