Conjugation, Color, and the Chemistry of Vision

Why are some organic compounds colored while others aren t jS-Carotene, the pigment in carrots, is purple-orange, for instance, while cholesterol is colorless. The answer involves both the chemical stmctures of colored molecules and the way we perceive light. 

Conjugation is crucial not only for the colors we see in organic molecules but also for the light-sensitive molecules on which our visual system is based. The key substance for vision is dietary /3-carotene, which is eonverted to vitamin A by enzymes in the liver, oxidized to an aldehyde called ll-frans-retinal, and then isomerized by a change in geometry of the Cl J-C12 double bond to produce 11-d.s-retinal. 

There are two main types of light-sensitive receptor cells in the retina of the human ey e, rod cells and cone cells. The 3 million or so rod cells are 

Metarhodopsin 11 is then recycled back into rhodopsin by a multistep sequence involving cleavage to all-tra/is-retinal and cis-trans isomerization back to 11-ds-retinal. 

Manufacturing the ultrathin circuitry on this computer chip depends on the organic chemical reactions of special polymers. 

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Conjugation, Color, and the Chemistry of Vision 503 Table 14.2 1 Ultraviolet Absorptions of Some Conjugated Molecules... 

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