Introduction
β-Carotene and lycopene are two strong pigments mostly found in vegetables and fruits. β-Carotene is most commonly known for giving carrots their distinct orange color, while lycopene is responsible for the red coloring of tomatoes. Both of these molecules fall under the carotene group, which are classified as unsaturated, fat-soluble hydrocarbon substances that can be converted into Vitamin A in the body1.
As we can see from their structures, both β-Carotene and lycopene contain long chains of alternating single and double bonds. This bond-pattern is known as conjugation, and the conjugated chain in each structure plays a very important role in terms of the molecules’ colorful properties.
Coloring
These pigments absorb frequencies of electromagnetic energy in the visible region of the electromagnetic spectrum and in turn reflect color, the color that your eyes see. In order to understand why you see the colors you see in the world, it is important to understand the color wheel, a simple concept that is shown below.
As shown by the color wheel, a molecule that absorbs light of wavelength 620 nm to 800 nm is said to absorb red light. However, the light that a compound absorbs is not the color that is reflected back to your eyes. Rather, the light reflected off the compound and thus, the color you see, is a mixture of all of the wavelengths of light that are not absorbed by the compound. For a compound that absorbs light of 620-800 nm, the color that is reflected back to your eyes is a mixture of colors that range from wavelengths of 400-620 nm. The electromagnetic spectrum shown below shows the range of wavelengths that correspond to each color of visible light.
Experimental spectroscopy for β-Carotene shows the absorption for this compound to be strongest from 400-500 nm, which lies in the green/blue part of the electromagnetic spectrum2. Hence, precisely due to the absorbance at the green/blue range, the red/yellow colors of visible light are reflected back and carrots appear orange. As seen on the color wheel, the position of blue-green, the color absorbed by these pigments, is exactly complementary to red-orange, the color which is reflected.
The absorption spectra of β-carotene (left) and lycopene (right).
Likewise, lycopene’s experimental spectroscopy shows absorption from 400-650 nm, which indicates that it absorbs light of all wavelengths except those corresponding to the color red, which ranges around 700 nm3. Because the red is not absorbed by lycopene, it is reflected back, and this is the color that you see.
When an electron in a system absorbs a photon of light of the right wavelength, it can be promoted to a higher energy level. With every double bond added to the conjugated portion of the molecule, the molecule absorbs photons of longer wavelength (and lower energy), and the compound ranges from yellow to red in color4. Compounds that are blue or green are typically not composed of many conjugated double bonds because they absorb photons of higher energy than highly conjugated compounds.
Conjugation
In molecules, conjugation refers to alternating single (sigma) and double (pi) bonds. These bonds form a system of connected p-orbitals that help to delocalize electrons across the entire conjugated portion of the molecule. The delocalization of charge in such molecules causes molecules with extensive conjugation to be more stable than molecules with, say, only single bonds5.
The highlighted parts of the molecules above show the conjugated portions. These long chains of alternating single and double bonds add to the delocalization of charge amongst the p-orbitals in each molecule. With every double bond that is added to a conjugated system, the molecule will absorb light of longer wavelengths and lower energy. This trend is due to the stabilizing effect caused by the increased delocalization of charge that each additional conjugated p-orbital adds to the system5. For this reason and due to the electronic transitions described in the next section, molecules with more conjugation absorb light of longer wavelengths than molecules with less conjugation.
Counting the number of alternating single and double bonds in β-carotene and lycopene shows that both molecules have eleven areas of alternating double and single bonds. Thus, both of these molecules have extensive conjugation, which causes these molecules absorb electromagnetic radiation in the visible region of the electromagnetic spectrum. This is why tomatoes and carrots appear colored!
Modeling the Molecules
Because these models are highly conjugated about one long chain of carbon-carbon bonds, it is easiest to use the particle-in-a-box (PIB) model to explain their pigmentation. The particle in a box model is appropriate for these two structures because the path of electrons is limited to the length of the conjugation, thereby establishing energy barriers. These energy barriers are known the PIB model as the potential energy walls, where V=∞; and thus the electron is “trapped” in this path6. The energy barriers are included in the model to describe an ideal confined area in which the electron is unable to go. In reality, the potential energy barrier is finite, and some electrons do sometimes acquire enough energy to overcome the energy barrier.
The particle-in-a-box model uses a box equal to the length of the conjugated chain of bonds in lycopene and β-carotene. The potential at each boundary is infinite and the particle can only be found within the box.
As exhibited by the image, the infinite potential energy is represented by an insurmountable wall, indicating that the particle can only be found within these walls. Another important note regarding the image is that there is no potential energy from 0 to L on the x-axis, meaning that there is equal probability to find the particle anywhere in the box6. This conjugated chain that acts has insurmountable boundaries on the end of the conjugation for the electron is why we can use the PIB model and ultimately treat it as a particle in a one-dimensional path.
Electronic Transitions
Electronic transitions and how they relate to the strong pigments exhibited by lycopene and β-carotene rely strongly on the conjugation of a system. When an electron in the system absorbs a photon of light that has the correct wavelength for excitation, the electron can be promoted to a higher energy level as described by the particle-in-a-box model (see above).
Consider the energetically most favorable electronic transition: π π*. This transition promotes an electron in the highest energy occupied bonding pi-orbital to the lowest energy occupied anti-bonding pi-orbital7. Larger differences in energies between these orbitals require photons of higher energy and shorter wavelengths to excite electrons, and thus result in shorter wavelengths of light absorbed. As demonstrated in the particle-in-a-box model, conjugation brings the energy differences between these two orbitals closer together5. That is, more delocalization of charge in a molecule results in more closely spaced energy levels. These closely spaced energy levels make absorption of light of lower energy and longer wavelengths more probable and result in colors that our eyes are able to detect!
This diagram shows the effect of conjugation on the spacing of orbital energy levels. Molecules with conjugation have more closely spaced energy levels than molecules without conjugation and thus require less energy to excited electrons to higher energy levels. Such molecules absorb electromagnetic radiation in the visible region of the electromagnetic spectrum and appear colored.
As the diagram above shows, extensive conjugation in a molecule brings the energy levels of electronic transition closer together. The closer spacing of these energy levels makes it easier to excited electrons to the higher energy levels and thus, it requires electromagnetic radiation of lower energy and longer wavelengths to promote electrons in conjugated systems8. For this reason, systems with extensive conjugation absorb electromagnetic radiation of energies that correspond to the visible region of the electromagnetic spectrum and compounds like lycopene and β-Carotene appear colored as described in the “Coloring” section above.
So What Does This Mean To Me?
You like colors, right? Think of this: very few compounds held together by single bonds (sigma bonds) alone are colored. This is because the electrons in such molecules are localized and are held so tightly by the molecule that it takes a photon of extremely high energy to excite them7. Due to this, these molecules absorb electromagnetic radiation from the portion of the electromagnetic spectrum that is higher in energy than human eyes can detect.
Using the particle-in-a-box model, we are able to explain how the conjugation creates colors by facilitating electronic transitions that require lower amounts of energy. Organic molecules of eight or more conjugated bonds in length generally appear colored because to excite electrons they require electromagnetic radiation of an energy that is detectable to the human eye5. On the color wheel, the wavelengths of light that pigments absorb is the complement on the color wheel to the colors that you see. Molecules made up of only single bonds absorb electromagnetic radiation of a frequency much higher than conjugated systems3. These molecules absorb electromagnetic radiation that is higher in energy than human eyes can detect and generally do not appear vibrantly colored. Without conjugation, your dinner plate would not be as colorful and lively!
References
1) Evens, Martha. "β-carotene Home." School of Chemistry - Bristol University - UK. 28 Oct. 2011. <http://www.chm.bris.ac.uk/motm/carotene/β-carotene_home.html>.
2) H., Keiler M. "A UV-Vis Spectroscopic Study." University of Wisconsin-La Crosse, 2006. 24 Oct. 2011. <http://www.uwlax.edu/faculty/loh/pdf_files/chm313_pdf/JPChemLab/JPCL_F06_pdf_files/JPChemLab_F06_1_KS.pdf>.
3) Clark, Jim. "UV-visible Absorption Spectra." Chemguide: Helping You To Understand Chemistry. 2007. 30 Oct. 2011. <http://www.chemguide.co.uk/analysis/uvvisible/theory.html>.
4) "Visible Spectra of Conjugated Dyes." 9 Aug. 2007. 26 Oct. 2011. <http://www.tau.ac.il/~phchlab/experiments/Conjugated/theory.pdf>.
5) "Conjugated Systems." Chemical Education at University of Wisconsin. University of Wisconsin, Apr. 2010. 29 Oct. 2011. <http://chemed.chem.wisc.edu/chempaths/GenChem-Textbook/Conjugated-Systems-1043.html>.
6) Dios, Angel C. "Particle in a Box." Angel C. De Dios, Department of Chemistry, Georgetown University. Georgetown University. 24 Oct. 2011. <http://bouman.chem.georgetown.edu/S02/lect13/lect13.htm> .
7) "Electronic Spectroscopy and Electronic Structure Models." 2010. 24 Oct. 2011. <http://www.elmhurst.edu/~ksagarin/pchem/spectralab.pdf>.
8) Volland, Walt. "Electronic Energy Level Changes and Color." Bellevue Community College, 1999. 27 Oct. 2011. <http://www.800mainstreet.com/elsp/Elsp.html>.
Pictures:
Beta-Carotene Molecular Structure: http://www.chm.bris.ac.uk/motm/carotene/beta-carotene_structure.html
Carrot: http://fellowshipofminds.wordpress.com/2010/08/08/carrot-egg-or-coffee/
Color Wheel: http://www.wou.edu/las/physci/ch462/tmcolors.htm
Dinner Plate: http://www.sparkpeople.com/mypage_public_journal_individual.asp?blog_id=3504501
Electromagnetic Spectrum: http://www.antonine-education.co.uk/physics_gcse/Unit_1/Topic_5/topic_5_what_are_the_uses_and_ha.htm
Lycopene Molecular Structure: http://www.chemicalbook.com/ChemicalProductProperty_EN_CB5213951.htm
Particle-In-A-Box Model: http://www.everyscience.com/Chemistry/Physical/Quantum_Mechanics/.images/box.gif
Tomato: http://www.benglued.com/tomato-festival-in-bangalore/
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