Conjugation in organic compounds creates a fascinating interplay between molecular structure and color. The alternating single and double bonds allow electrons to move freely, affecting how molecules absorb light. This phenomenon explains why some substances appear colorful to our eyes.

Our ability to perceive color stems from the intricate chemistry of vision. Rod and cone cells in our eyes contain light-sensitive pigments that undergo rapid changes when struck by photons. This triggers a cascade of reactions, ultimately sending signals to our brain to interpret as visual information.

Conjugation and Color

Color and conjugation in organic compounds

• Conjugation alternation of single and double bonds in a molecule allows delocalized electrons to move freely across the conjugated region
• Lowers the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)
  • Smaller energy gap allows the molecule to absorb longer wavelengths of light resulting in a bathochromic shift (red shift)
  • Longer conjugated systems have smaller energy gaps and absorb longer wavelengths of light (beta-carotene)
• Absorption of specific wavelengths of light by a molecule determines its color
  • Color observed is the complementary color of the wavelengths absorbed (chlorophyll absorbs blue and red light, appears green)
• Factors affecting the extent of conjugation and color:
  • Length of the conjugated system: longer conjugation results in a more pronounced red shift (lycopene in tomatoes)
  • Presence of electron-donating or electron-withdrawing groups can extend or disrupt conjugation, affecting the absorption and color (indigo dye)
  • Planarity of the molecule allows for better orbital overlap and more effective conjugation (anthocyanins in red cabbage)

Electromagnetic Spectrum and Vision

• The electromagnetic spectrum encompasses all types of electromagnetic radiation, including visible light
• Visible light is a small portion of the spectrum that the human eye can detect
• Different wavelengths of visible light correspond to different colors
• Spectral sensitivity of the human eye varies across the visible spectrum, with peak sensitivity in the green-yellow region

The Chemistry of Vision

Light absorption process in rod cells

1. Rod cells contain the photopigment rhodopsin, responsible for low-light vision

   • Rhodopsin consists of the protein opsin and the chromophore 11-cis-retinal, a derivative of vitamin A

2. When a photon of light is absorbed by 11-cis-retinal, it isomerizes to all-trans-retinal within picoseconds, the primary photochemical event in vision

   • This process is known as photoisomerization and is crucial for initiating the visual response

3. Isomerization of 11-cis-retinal to all-trans-retinal causes a conformational change in the opsin protein

   • Triggers a series of intermediate states: bathorhodopsin, lumirhodopsin, and metarhodopsin I

4. Metarhodopsin I equilibrates with metarhodopsin II, the active form of rhodopsin

   • Metarhodopsin II activates the G-protein transducin, initiating the visual transduction cascade

5. Visual transduction cascade amplifies the signal and ultimately leads to the hyperpolarization of the rod cell and the generation of a neural signal

   • This process, known as visual phototransduction, converts light energy into electrical signals that can be interpreted by the brain

Rod vs cone cells in vision

• Rod cells responsible for low-light (scotopic) vision, more sensitive to light than cone cells
  • Contain the photopigment rhodopsin with a peak absorption at 498 nm
  • Not involved in color perception, only contain one type of photopigment
  • More abundant in the peripheral regions of the retina
• Cone cells responsible for high-light (photopic) vision and color perception
  • Three types of cone cells, each containing a different photopigment (opsin) with distinct peak absorptions:
    • L-cones (long-wavelength, red): peak absorption at 564 nm
    • M-cones (medium-wavelength, green): peak absorption at 533 nm
    • S-cones (short-wavelength, blue): peak absorption at 437 nm
  • Combination of signals from the three types of cone cells allows for color perception
  • More concentrated in the central region of the retina (fovea), providing high visual acuity
• Distribution and function of rod and cone cells in the retina:
  • Fovea contains a high density of cone cells, enabling high visual acuity and color perception
  • Peripheral regions of the retina have a higher density of rod cells, facilitating low-light and peripheral vision (night vision)