Color, and the Chemistry of Vision

Which of the following compounds would you expect to show ultraviolet absorptions in the 200 to 400 nm range  

Why are some organic compounds colored while others aren t j8-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 /8-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-ds-retinal. 

---

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

A derivative of vitamin A plays a crucial role in vision when it is bound to a protein called opsin. The cone cells in the retina of the eye contain several types of opsin and are responsible for vision in bright light and for color vision. The rod cells in the retina contain only one type of opsin they are responsible for vision in dim light. The chemistry of vision has been more extensively studied in rod cells than in cone cells, and we shall discuss events that take place in rod cells. 

Chemical processes that are triggered by photons are also understood using the kinetics concepts mentioned in this chapter. The chemistry of vision is one example. The currently accepted mechanism for vision involves a compound called rhodopsin, which is composed of a protein molecule (opsin) attached to a colored polyene molecule called c/s-retinal. (C/s-retinal is chemically related to a class of molecules called carotenes, which are highly colored compounds responsible for the colors of carrots and tomatoes. Eating carrots does help vision, specifically in low light.) The vision process begins when the c/s-retinal absorbs a photon and is isomerized about one of its double bonds to make frans-retinal ... 

The term dye is frequently used to describe two major chemical classes colorants that affect the perceived color of other materials, and transducers that accept photons of a specific wavelength and generate a technical response that can take on a number of forms. Dyes as colorants are not total analogs to visual photo-reception, but the chemistry of photography is. This field has nearly all of the same constraints and requirements as does vision chemistry. It can be very instructive to review dye chemistry as it apphes to photography and transition that analog to the natural vision chemistry arena. 

There has been significant effort expended in trying to relate the genetic code to the simpler conditions of color blindness. However, the papers reviewed below have consistently taken a very elementary view of the operation of the visual system. They have assumed that all failures related to abnormal color vision are based on the chemistry of the chromophores. Section 18.1 provides a more comprehensive description of the types and potential sources of such failures. Section 18.1.5.6.4 documents the very important work of Lakowski. Lakowski documents the variety of failures that can occur within the neurological portion of the visual system. 

In the eye, specialized photoreceptor cells of the retina, called rod cells are primarily responsible for low-light vision, with relatively little color detection. Rod cell outer segments contain lamellar protein disks rich in the protein opsin (Figure 19.26). Oxidation and isomerization of all-im/75 -retinol yields an intermediate, W-cis retinal, which is important in photoreception. The chemistry of photoreception is shown in Figure 19.27 and summarized as follows. 

Opalescence o-po- le-s nt adj (ca. 1813) (1) The limited clarity of vision through a sheet of transparent plastic at any angle, because of diffusion within or on the surface of the plastic. (2) Of a plastic material, the quality of having inner, tiny colored lights resembling those of opals. (Solomon DH, Hawthorne DC (1991) Chemistry of pigments and fillers. Krieger, New York)... 

---