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5.7: Conjugation, Color, and the Chemistry of Vision

  • Page ID
    227575
  • Objectives

    After completing this section, you should be able to

    1. explain why some organic compounds have different colors based on compound structure and our perception of light.
    2. state the relationship between frequency of light absorbed and the extent of conjugation in an extended pi electron system.

    Introduction

    White light, like that from the sun or a light bulb includes all frequencies of visible light (approximately 400 to 800 nm). There are photoreceptors in eyes that take these specific wavelengths and turn them into information that the brain perceives as color.

    Physical Characteristics of Light

    Visible light comprises a very small band of the entire electromagnetic spectrum with wavelengths from about 400 nm to 800 nm. The energy of visible light is greater with shorter wavelength, so violet is the highest energy while red is the lowest energy.

    Energy converting chemicals

    Beta carotene, found in carrots and other natural products is cleaved into the liver and converted into Vitamin A, also known as retinol. Vitamin A is critical for vision because it is needed by the retina of eye. Retinol can be convert to retinal, and retinal is a chemical necessary for rhodopsin. As light enters the eye, the 11-cis-retinal is isomerized to the all-"trans" form.

    Bond line drawing of beta-carotene

    Bond line drawing of cis vitamin A (retinol)

    Bond line drawing of retinal.

    Mechanism of Vision

    Rhodopsin, is made up of a protein (opsin) and retinal. Opsin does not absorb visible light, but when it is bonded with 11-cis-retinal by its lysine side-chain to from rhodopsin, the new molecule has a very broad absorption band in the visible region of the spectrum.[2]

    1,1-cis-retinal reacts with opsin to produce rhodopsin.

    The reaction above shows Lysine side-chain from the opsin react with 11-cis-retinal when stimulated. By removing the oxygen atom form the retinal and two hydrogen atom form the free amino group of the lysine, rhodopsin is formed.

    Signal transduction pathway

    In human eyes, rods and cones react to light stimulation, and a series of chemical reactions happen in cells. These cells receive light, and pass on signals to other receiver cells. This chain process is a signal transduction pathway, which is a mechanism that describe the ways cells react and respond to stimulation.

    The molecule cis-retinal can absorb light at a specific wavelength. When visible light hits the cis-retinal, the cis-retinal undergoes an isomerization, or change in molecular arrangement, to all-trans-retinal. The new form of trans-retinal does not fit as well into the protein, and so a series of geometry changes in the protein begins. The resulting complex is referred to a bathrhodopsin (there are other intermediates in this process, but we'll ignore them for now).

    Bond line drawing of cis-retinal and trans retinal.

    As the protein changes its geometry, it initiates a cascade of biochemical reactions that result in changes in charge so that a large potential difference builds up across the plasma membrane. This potential difference is passed along to an adjoining nerve cell as an electrical impulse. The nerve cell carries this impulse to the brain, where the visual information is interpreted.

    References

    1. Biochemistry, L. Stryer (W.H. Freeman and Co, San Francisco, 1975).
    2. The Cambridge Guide to the Material World, Rodney Cotterill (Cambridge University Press, Cambridge, 1985)
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