14.9 Conjugation, Color, and the Chemistry of Vision
3 min read•may 7, 2024
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
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- 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 () and the ###Lowest_unoccupied_molecular_orbital_()_0###
- Smaller energy gap allows the molecule to absorb longer wavelengths of light resulting in a (red shift)
- Longer conjugated systems have smaller energy gaps and absorb longer wavelengths of light ()
- Absorption of specific wavelengths of light by a molecule determines its color
- Color observed is the complementary color of the wavelengths absorbed ( 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 ( in tomatoes)
- Presence of electron-donating or electron-withdrawing groups can extend or disrupt conjugation, affecting the absorption and color ()
- Planarity of the molecule allows for better orbital overlap and more effective conjugation ( in red cabbage)
Electromagnetic Spectrum and Vision
- The 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
- 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
- Rod cells contain the photopigment , responsible for low-light vision
- Rhodopsin consists of the protein and the , a derivative of vitamin A
- When a photon of light is absorbed by 11-cis-retinal, it isomerizes to within picoseconds, the primary photochemical event in vision
- This process is known as and is crucial for initiating the visual response
- Isomerization of 11-cis-retinal to all-trans-retinal causes a conformational change in the opsin protein
- Triggers a series of intermediate states: , , and I
- Metarhodopsin I equilibrates with metarhodopsin II, the active form of rhodopsin
- Metarhodopsin II activates the G-protein , initiating the visual transduction cascade
- 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 , 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:
- (long-wavelength, red): peak absorption at 564 nm
- (medium-wavelength, green): peak absorption at 533 nm
- (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 (), providing high visual acuity
- Three types of cone cells, each containing a different photopigment (opsin) with distinct peak absorptions:
- 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)
Key Terms to Review (31)
11-cis-retinal: 11-cis-retinal is a light-sensitive molecule that plays a crucial role in the process of vision. It is a specific isomer of the retinal molecule, where the carbon-carbon double bond is in the cis configuration, meaning the two hydrogen atoms are on the same side of the double bond.
All-trans-retinal: all-trans-retinal is the aldehyde form of vitamin A, which plays a crucial role in the process of vision by acting as the light-absorbing chromophore in the retinal photoreceptor cells. This term is closely connected to the topics of conjugation, color, and the chemistry of vision.
Anthocyanins: Anthocyanins are a group of water-soluble pigments found in many plants, particularly fruits and flowers. They are responsible for the vibrant red, purple, and blue colors seen in various plant tissues and play a crucial role in the chemistry of vision and color perception.
Bathochromic Shift: A bathochromic shift, also known as a red shift, is a phenomenon in which the absorption or emission spectrum of a molecule is shifted to longer wavelengths (lower energy) compared to a reference compound. This shift is typically observed in ultraviolet and visible light spectroscopy and is closely related to the concepts of conjugation, aromaticity, and the chemistry of vision.
Bathorhodopsin: Bathorhodopsin is a photochemically-induced intermediate state of the visual pigment rhodopsin, which plays a crucial role in the initial steps of the visual transduction process. It is formed by the absorption of a photon of light by rhodopsin, leading to a series of structural changes that ultimately trigger a neural response in the retina, enabling vision.
Beta-Carotene: Beta-carotene is a type of carotenoid, a naturally occurring pigment found in many fruits and vegetables. It is known for its role in vision, conjugation, and color in organic chemistry.
Chlorophyll: Chlorophyll is the green pigment found in plants that is essential for photosynthesis. It absorbs sunlight and uses its energy to convert carbon dioxide and water into glucose, the primary energy source for plants.
Chromophore: A chromophore is a functional group or conjugated system within a molecule that is responsible for the molecule's color. It is the part of a molecule that absorbs specific wavelengths of light, leading to the observed color of the molecule.
Conjugation: Conjugation refers to the overlap or sharing of atomic orbitals, resulting in the delocalization of electrons across a system of connected atoms. This concept is central to understanding resonance, the stability of certain molecules and ions, and the interpretation of various spectroscopic techniques in organic chemistry.
Electromagnetic spectrum: The electromagnetic spectrum encompasses all types of electromagnetic radiation, ranging from gamma rays with the shortest wavelengths to radio waves with the longest wavelengths. In organic chemistry, it plays a crucial role in structure determination by providing information about molecular vibrations and ion fragmentation patterns.
Electromagnetic Spectrum: The electromagnetic spectrum is the entire range of electromagnetic radiation, which includes various types of energy waves such as radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. This spectrum is organized based on the wavelength and frequency of the different forms of radiation, and it plays a crucial role in various areas of science, including organic chemistry.
Fovea: The fovea is a small, highly specialized area in the center of the retina that is responsible for our sharpest and most detailed vision. It is the region of the eye that provides the clearest, most focused image and is essential for tasks that require high visual acuity, such as reading, driving, and recognizing faces.
HOMO: HOMO, or Highest Occupied Molecular Orbital, is a fundamental concept in molecular orbital theory that describes the highest energy level occupied by electrons in a molecule. This term is crucial in understanding the stability, reactivity, and spectroscopic properties of organic compounds, particularly in the context of conjugated systems, pericyclic reactions, and the chemistry of vision.
Homotopic: In the context of 1H NMR spectroscopy and proton equivalence, homotopic protons are those that can be interchanged by a symmetry operation without changing the molecule's overall spatial arrangement. These protons have identical chemical environments and therefore exhibit identical chemical shifts in NMR spectroscopy.
Indigo Dye: Indigo dye is a natural blue dye derived from the leaves of certain plants, primarily the Indigofera tinctoria plant. It has a long history of use in textile coloration and is known for its distinctive blue hue and its ability to produce deep, rich colors on fabrics through a complex chemical process.
L-cones: L-cones are one of the three types of color-sensitive photoreceptor cells in the human retina, responsible for detecting and processing long-wavelength (red) light. They play a crucial role in our ability to perceive color and are closely linked to the topics of conjugation, color, and the chemistry of vision.
Lowest unoccupied molecular orbital (LUMO): The LUMO is the lowest energy molecular orbital that does not contain electrons but can accept them during chemical reactions or excitations. It plays a crucial role in determining the reactivity and properties of molecules, especially in conjugated systems analyzed by ultraviolet spectroscopy.
Lumirhodopsin: Lumirhodopsin is a key intermediate in the visual transduction process, which is the series of events that occur when light enters the eye and is converted into a neural signal that the brain can interpret. It is a critical step in the chemistry of vision and is closely related to the concept of conjugation and color in organic chemistry.
LUMO: LUMO, or Lowest Unoccupied Molecular Orbital, is a fundamental concept in molecular orbital theory that describes the energy level of the highest-energy orbital that is not occupied by electrons in the ground state of a molecule. The LUMO is crucial in understanding the stability and reactivity of conjugated systems, as well as the behavior of molecules in various photochemical and pericyclic reactions.
Lycopene: Lycopene is a naturally occurring red pigment found in certain fruits and vegetables, particularly tomatoes. It is a carotenoid, a class of plant-based compounds known for their antioxidant properties and potential health benefits related to vision, skin, and cancer prevention.
M-cones: M-cones, also known as medium-wavelength sensitive cones, are one of the three types of color-detecting photoreceptor cells found in the human retina. These cones are responsible for perceiving medium-wavelength light, which corresponds to the green portion of the visible light spectrum.
Metarhodopsin: Metarhodopsin is a photochemical intermediate formed during the visual cycle, the series of biochemical reactions that occur in the retina in response to light exposure. It plays a crucial role in the chemistry of vision, color perception, and the phenomenon of conjugation in organic molecules.
Opsin: Opsin is a light-sensitive protein found in the retina of the eye that plays a crucial role in the process of vision. Opsins are the key components of the visual pigments that allow us to perceive color and light.
Photoisomerization: Photoisomerization is a photochemical process in which a molecule undergoes a structural change, typically involving the rotation around a carbon-carbon double bond, in response to the absorption of light energy. This phenomenon is central to understanding the chemistry of vision and various photochemical reactions.
Photopic Vision: Photopic vision is the visual perception that occurs in the presence of bright light, which is mediated by the cone photoreceptors in the retina. This type of vision allows for the perception of color and fine detail, and is essential for day-time activities and tasks that require high visual acuity.
Rhodopsin: Rhodopsin is a light-sensitive pigment found in the retina of the eye that plays a crucial role in the process of vision. It is a member of the G-protein coupled receptor family and is responsible for initiating the visual transduction cascade, which converts light energy into electrical signals that the brain can interpret as vision.
S-cones: S-cones, also known as short-wavelength cones or blue cones, are a type of photoreceptor cells found in the retina of the human eye. These specialized cells are responsible for perceiving short-wavelength light, primarily in the blue region of the visible light spectrum, and play a crucial role in color vision and the chemistry of vision.
Scotopic Vision: Scotopic vision refers to the visual perception that occurs in low-light conditions, primarily mediated by the rod photoreceptors in the retina. This type of vision is essential for our ability to see in dim environments, such as at night or in dark rooms, and is closely related to the concepts of conjugation, color, and the chemistry of vision.
Scotopic vision is distinct from photopic vision, which is our daylight vision mediated by the cone photoreceptors and responsible for color perception and high-acuity sight.
Spectral Sensitivity: Spectral sensitivity refers to the ability of a photoreceptor, such as the rods and cones in the human eye, to respond to different wavelengths of light. It is a crucial concept in understanding the chemistry of vision and how the visual system perceives color.
Transducin: Transducin is a G-protein that plays a crucial role in the process of vision by converting light signals into electrical signals in the retina. It is a key component in the visual transduction pathway, which is the process by which light is detected and converted into a neural response that can be interpreted by the brain.
Visual Phototransduction: Visual phototransduction is the process by which light energy is converted into electrical signals in the retina, enabling the visual system to detect and respond to light. This process is a crucial component of the chemistry of vision and is closely linked to the concepts of conjugation and color.