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JCE Featured Molecules
William F. Coleman Wellesley College Wellesley, MA 02481
Molecular Models of Lycopene and Other Carotenoids February Featured Molecules Over the past decade or so the phrase “emerging research suggests” has entered the argot of advertising, and that phrase has been applied to this month’s Featured Molecule, lycopene, particularly with regard to potential health benefits of tomatoes. The paper by Jie Zhu, Mingjie Zhang, and Qingwei Liu (1) describes an extraction and purification of lycopene from tomato paste using an emulsion rather than the traditional solvent-based extraction. Lycopene is a member of the family of molecules called carotenoids, the most familiar of which is beta-carotene. This family of natural products includes more than 500 members that have been isolated and whose structures have been determined. Professor Hanspeter Pfander’s research group at the University of Bern maintains a Web site with a significant amount of information on carotenoid structure, synthesis, and activity (2).
Figure 1. The dihedral angle of beta-carotene.
Dihedral Angle / deg 0
100
200
300
9398 9398 9399
Energy
/ (kcal/mol)
9400 9401 9402 9403 9404 9405 9406
Structurally one can think of carotenoids as consisting of three segments, a relatively rigid conjugated central portion with end groups. The end groups are, in general, flexible with respect to rotation about the bond connecting them to the central portion. For example, in beta-carotene, the dependence of total energy on the dihedral angle shown in Figure 1, displays a very broad range of essentially isoenergetic conformations (Figure 2). The energies shown in Figure 2 were calculated at the PM3 level using Hyperchem 7.5 (3). Calculations at the HF/631-G(d,p) level, with many fewer data points, show a similar trend. Many of the health benefits derived from various carotenoids are attributed to their antioxidant activities. Carotenoids react with singlet-oxygen in a physical, diffusion-controlled, quenching process that results in ground state triplet-oxygen and, following a non-radiative relaxation, ground state carotenoid. Of the various carotenoids that have been studied, lycopene and beta-carotene show the greatest quenching rate constants (4). The carotenoids provide us with countless explorations by students and teachers looking for connections between fundamental chemical concepts and real-world applications. Structure, reactivity, chemical synthesis, biosynthesis, and stereochemistry are just a few of the concepts involved in understanding the manifest important roles that these molecules play. Literature Cited 1. Zhu, Jie; Zhang, Mingjie; Liu; Qingwei. J. Chem. Educ. 2008, 85, 256–257. 2. Pfander Group; http://dcb-carot.unibe.ch/default.htm (accessed Dec 2007). 3. HyperChem; http://www.hyper.com/ (accessed Dec 2007; HyperChem is a registered product of Hypercube, Inc.). 4. Cantrell A.; McGarvey, D. J.; Truscott, T. G.; Rancan, F.; Böhm, F. Arch Biochem Biophys. 2003, Apr 1; 412 (1), 47–54.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Feb/abs320.html
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Full text (HTML and PDF) with links to cited URLs and JCE articles
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Supplement Find Molecular Models of Lycopene and Other Carotenoids in the JCE Digital Library at http://www.JCE.DivCHED.org/ JCEWWW/Features/MonthlyMolecules/2008/Feb/
Figure 2. Dependence of total energy on the dihedral angle of betacarotene (shown in Figure 1).
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lycopene: a bright red carotenoid pigment found in tomatoes and other red fruits
Journal of Chemical Education • Vol. 85 No. 2 February 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
