14.9 • Conjugation, Color, and the Chemistry of Vision
Why are some organic compounds colored while others are not? β-Carotene, the pigment in carrots, is yellow-orange, for instance, while cholesterol is colorless. The answer involves both the chemical structures of colored molecules and the way we perceive light.
The visible region of the electromagnetic spectrum is adjacent to the ultraviolet region, extending from approximately 400 to 800 nm. Colored compounds have such elaborate systems of conjugation that their “UV” absorptions extend into the visible region. β-Carotene, for instance, has 11 double bonds in conjugation, and its absorption occurs at λmax = 455 nm (Figure 14.14).
“White” light from the sun or from a lamp consists of all wavelengths in the visible region. When white light strikes β-carotene, the wavelengths from 400 to 500 nm (blue) are absorbed while all other wavelengths are transmitted and can reach our eyes. We therefore see the white light with the blue removed, which we perceive as a yellow-orange color for β-carotene.
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 β-carotene, which is converted to vitamin A by enzymes in the liver, oxidized to an aldehyde called 11-trans-retinal, and then isomerized by a change in geometry of the C11–C12 double bond to produce 11-cis-retinal.
There are two main types of light-sensitive receptor cells in the retina of the human eye: rod cells and cone cells. The 3 million or so rod cells are primarily responsible for seeing dim light and shades of gray, whereas the 100 million cone cells are responsible for seeing bright light and colors. In the rod cells of the eye, 11-cis-retinal is converted into rhodopsin, a light-sensitive substance formed from the protein opsin and 11-cis-retinal. When light strikes the rod cells, isomerization of the C11–C12 double bond occurs and trans-rhodopsin, called metarhodopsin II, is produced. In the absence of light, this cis–trans isomerization takes approximately 1100 years, but in the presence of light, it occurs within 2 × 10–13 seconds! Isomerization of rhodopsin is accompanied by a change in molecular geometry, which in turn causes a nerve impulse to be sent through the optic nerve to the brain, where it is perceived as vision.
Metarhodopsin II is then recycled back into rhodopsin by a multistep sequence involving cleavage to all-trans-retinal and cis–trans isomerization back to 11-cis-retinal.