DEPARTMENT OF CHEMICAL SCIENCES,FACULTY OF SCIENCE,

NIGER DELTA UNIVERSITY,WILBERFORCE ISLAND,

BAYELSA STATE.

SEMINAR PRESENTATION ON

PHOTOISOMERIZATION OF 11-CIS RETINAL IN THE RETINA

BY

CHRISTOPHER UNYIME EBONGUG/011/2501

SUPERVISED BY:

PROF. D. A. OKORIE

APRIL, 2015.

INTRODUCTION• Photochemical reactions are initiated by the absorption of energy in the form

of light. The consequence of molecules absorbing light is the generation of transient excited states whose chemical and physical properties differ greatly from the original molecules. These new chemical species can fall apart, change to new structures.

• The ability to see the world starts with a photochemical reaction in the eye, in which retinal, a molecule in the photoreceptor cell rhodopsin, isomerizes (or change shape) about a double bond after absorbing light.

• Light is an important aspect of life due to its sustainable influences alternating from photosynthetic processes in plants, upon which almost all life on earth depends. Vitamin D, essential for normal bone and teeth development and kidney function, is formed in the skin of animals after exposure of the chemical 7-dehydrocholesterol to sunlight.

• Living organisms sense light from the environment by a specialized cells known as photoreceptors which includes the rods and cones embedded in the retina.

The Eye

FIG.1: THE HUMAN EYEThis is a schematic drawing of the human eye. Light enters the front of the eye through the pupil and is focused by the lens onto the retina. Rod cells on the retina respond to the light and send a message through the optic nerve fiber to the brain.

Retina

Rod Cells

Cone Cell

PHOTORECEPTORSThe retina is lined with many millions of photoreceptor cells that consist of two types: 7 million cones and; 120 million rods.

FIG.2: STRUCTURE OF A CONE CELL FIG.3: STRUCTURE OF A ROD CELL

11-CIS RETINAL

IUPAC NAME: 7-Dimethyl-9-(2,6,6-trimethyl-1-cyclohexenyl) 2,4,6,8-nonatetraenalMOLECULAR FORMULAR: C20H28O

Oxidation

RHODOPSIN

The reaction between aldehydes and amines forms imine derivatives, also known as Schiff bases (compounds having a R2C=N) with the elimination of water.

11-cis retinal Opsin Rhodopsin +

MECHANISM OF SCHIFF BASE (IMINE) FORMATIONLysine side chain ends in -NH2

1. Nucleophilic attack2. Proton transfer3. Protonation by an acid4. Removal of water5. Deprotonation

1

2 3

4 5

INTERACTION OF LIGHT WITH RHODOPSINRhodopsin contains 11 cis-retinal (vitamin A), which act as a chromophore which is embedded inside a single peptide transmembrane protein called opsin. When light strikes the retina of your eyes, the 11-cis-retinal portion of rhodopsin absorbs a photon, and a cis-trans photoisomerization occurs:

PHOTOISOMERIZATION Photoisomerization is a molecular behavior in which

structural change between isomers is caused by photoexcitation. The cis-trans isomerization of the chromophore of rhodopsin is the primary reaction in vision, which is likely to occur in the electronically excited state of rhodopsin. In photoisomerization no chemical bonds are broken, but the molecule changes shape.

Photoisomerization reaction occurs as the first step in the chemistry of vision. Cis-trans isomerization is believed to be caused by weakening of the C11–C12 bond of the rhodopsin.

MECHANISM OF PHOTOISOMERIZATION

SOURCES OF 11-cis retinal Sources of 11-cis-retinal include Vitamin A and β –carotene. Animal sources of vitamin A; Liver, Milk, Butter, Cheese, Egg Yolk

and β-Carotene: Carrots, Spinach, Sweet Potatoes, Pumpkin.

A diet lacking in Vitamin A can lead to night blindness, whereas a diet rich in β-carotene (found in carrots) can improve vision.

CONCLUSION

• Photoisomerization reaction occurs as the first step in the chemistry of vision.

cis trans isomerization is believed to be caused by weakening of the C11–C12

bond of the rhodopsin due to the absorption of a photon. The electron

transition loosens the bond, the bond swings around into a new conformation

forming all-trans-retinal. The electron returns to the ground state locking the

molecule in the trans conformation. The new trans-retinal conformation

requires more space than the cis-retinal conformation. The molecules presses

against the opsin stimulating the rods and cones. The time required for this is

approximately 0.25-050 ms.

• A diet lacking in Vitamin A can lead to night blindness, whereas a diet rich in

β-carotene (found in carrots) can improve vision. Therefore it is advisable to

ingest food that are rich in Vitamin A and β-carotene.

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