Polyenes and Vision

When we hear or read the word spectroscopy, we tend to imag e rooms full of expensive machines measuring away. It s quite an inhuman vision. Yet we are spectrometers ourselves. All of our sensory experiences, most obviously vision, are examples of spectroscopic measurements. Those machines in our imaginations are mere extensions of our biological, in-house equipment. When you see a red apple among a basket of green ones, and decide that it is the ripe red one you will eat, you are acting on a spectroscopic measurement. Let s examine vision a litde bit here, because it depends on polyene chemistry. 

Many polyenes are important in biological processes, and as we might imagine, polyenes that absorb in the dsible spectra are important in human dsion. Vitamin A is trans-xtX no, for example (Fig. 12.33). We have two en mes, one of which, retinol dehydrogenase, oxidizes /rawr-retinol to /rawr-retinal and another, retinal isomerase, which isomerizes one double bond of /rawr-retinal to produce m-retinal. 

FIGURE 12.33 Vitamin A trans-retinol) is oxidized to /rawt-retinal and then enzymatically isomerized to cij-retinal. 

FIGURE 12.34 ch-Retinal has the correct shape to react with opsin to form rhodopsin. The squiggly lines indicate the rest of the molecule. 

CHAPTER 12 Dienes and the Allyl System 2p Orbitals in Conjugation 

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