Molecule of the weekLycopene

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Introduction Lycopene is a red carotenoid pigment that colours tomatoes (Figure 1) as well as many other red, orange and yellow fruits and vegetables. The name lycopene comes from that of the genus Lycopersicum which includes tomatoes. Lycopene has recently been the focus of extensive research reflecting growing awareness of its various roles and particularly its potential health benefits.

Chemical properties Because the molecular structure of lycopene consists entirely of carbon and hydrogen (Table 1) it is a non-polar hydrocarbon [1] with a high melting point. Not only is it completely insoluble in water, it has relatively low solubility even in non-polar solvents. These characteristics can be understood if we consider the arrangement of the atoms and the bonding within a lycopene molecule (Figure 2).

Table 1 The molecular properties of lycopene [1,2]

Molecular formula C40H56

Carbon content by weight 89.49%

Hydrogen content by weight 10.51%

Polarity Non-polar

Molecular weight (MW) 536.87

Water solubility insoluble

Melting point (mp) 172-173C

Molecular structure Lycopene is polyunsaturated and in its natural form, all of the double bonds are in the

Figure 1 Ripe tomatoes are rich in lycopene

The significance of conjugation The extensive conjugation and extended molecular orbital help us to understand the characteristics, as well as the special and varied roles, of lycopene in our foods.

The elongated linear shape of the molecule is quite inflexible compared to that of many other structures because there is no free rotation about the double bonds in the conjugated system. The intermolecular forces between lycopene molecules are essentially only van der Waal’s interactions, which are very weak bonds. In addition its relatively high molecular weight limits its solubility even in non-polar solvents [1,2].

The ability of the molecule to absorb particular wavelengths of visible light so that it appears a bright colour. The eleven conjugated double bonds, covering much of the overall structure of the molecule allows the

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trans- rather than the cis- form. As a result, the shape of the molecule is extended and straight, constrained by its system of eleven conjugated double bonds.

In this context the term conjugation refers to the pattern of alternating double and single bonds that can be seen in the structure. In lycopene there are 22 carbon atoms in the conjugated system. This means that the bonding electrons for each of these carbons become “delocalised”. They are shared between all 22 of the carbon atoms which participate in this system and become a part of a molecular orbital.

There are two ways in which this is quite unlike the arrangement seen in polyunsaturated fatty acids (including those that are - 3 and -6). Firstly, in the latter double bonds occur naturally in the cis- rather than trans- configuration. Secondly in fatty acids there is no pattern of conjugation and the double bonds are described as isolated.

Figure 2 Molecular structure of lycopene (with C atoms shown in grey and H in blue) [1]

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Food molecule: Lycopene page 2 of 2

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absorption of all but the longest wavelengths of light which is why it appears red [1,3,4].

Generally double bonds are quite reactive so that they contribute to the instability of many molecules including unsaturated fats. However in lycopene, the formation of the extended molecular orbital bestows a much higher degree of stability on the molecule than we might have predicted [3,4].

Sterically there are as many as 72 possible geometric isomers of the molecule. When it is exposed to heat or light, lycopene can undergo isomerisation to any number of these cis-isomers, which have a bent rather than linear shape [4].

Significance of lycopene Within plants, the molecule is an important intermediate in the biosynthesis of many other carotenoids including -carotene. It is often referred to as a phytochemical (simply meaning plant molecule) involved both in photosynthesis as well as photoprotection through its ability to minimise oxidative damage to the structure of the chlorophyll molecule [4,5].

Although not an essential nutrient for humans, lycopene is a powerful antioxidant and consumption is associated with reduced cancer risks and enhanced well-being. It is also amongst the phytochemicals thought to reduce the risks of cancers of the oesophagus, lungs, prostate and stomach [3-7]. To obtain the benefits of antioxidants in plants, we should eat plenty of fruit and vegetables each day [3,5].

Lycopene in food A variety of fruits and vegetables have high lycopene contents (Table 2) [8,9]. More data are available in the US food composition database [8] particularly for processed foods for which tomatoes are an ingredient. Low levels are found in animal foods including chicken liver (Table 2). And even green vegetables can be significant sources. Asparagus, parsley and basil are examples where the red colour of lycopene is masked by the chlorophyll present. Other fruits that appear red, including strawberries and cherries contain anthocyanins rather than lycopene.

While gac has the highest concentration of lycopene [9], it is rarely found outside of Southeast Asia, and therefore, tomatoes account for about 85% of the dietary intake for most people. Lycopene is found tightly bound to vegetable fibre. It is soluble in oil but not in water, which is why processing tomatoes and serving them in oil-rich dishes increases the bioavailability of lycopene [4].

Lycopene is sometimes used to colour food and it is an approved food additive in some but not all countries [2,10]. In Australia and Europe it has the designation of 160d (E160d).

Health benefits of antioxidants Antioxidants are responsible for keeping high-energy reactions from damaging essential DNA and proteins [3,5]. Lycopene and other carotenoids operate by a singlet quenching

Table 2 Dietary sources of lycopene [8,9]

Source Content (per 100g)

Gac (baby jackfruit) 200-300 mg

Sundried tomato 45.9 mg

Tomato sauce 16.7 mg

Watermelon 4.5 mg

Fresh tomato 2.6 mg

Papaya 1.8 mg

Pink grapefruit 1.4 mg

Baked beans 51 g

Asparagus (cooked) 30 g

Chicken liver 20 g

Carrots (raw) 1 g

mechanism, which is a well-recognised mechanism of antioxidant action. They react with singlet oxygen, 1O2, returning it to the relatively unreactive triplet state [3]:1O2 + carotenoid 3O2 + carotenoid*(excited)

carotenoid* carotenoid + heat

Lycopene may be the most powerful carotenoid quencher of singlet oxygen [6]. Test tube studies indicate that it is 100 times more efficient at quenching singlet oxygen than vitamin E. Singlet oxygen produced during exposure to ultraviolet light is a primary cause of skin aging [7].References and further reading[1] Budavari S. 2006. The Merck index. 14 th ed.

Whitehouse Station, NJ: Merck.

[2] Smith J, Hong-Shum L. 2011. Food additives data book. 2nd ed. Oxford, UK: Wiley-Blackwell.

[3] Coultate TP. 2009. Food: the chemistry of its components. 5th ed. Cambridge: Royal Society for Chemistry.

[4] Damodaran S, Parkin KL, Fennema OR, (Eds.). 2008. Fennema’s food chemistry, 4th ed. CRC Press/Taylor & Francis: Boca Raton.

[5] McGee H. 2004. On food and cooking. 1st Scribner revised ed. New York, US: Simon & Schuster.

[6] Di Mascio P, Kaiser S, Sies H. 1989. Archives of Biochemistry and Biophysics 274(2): 532–8.

[7] Berneburg M, et al. 1999. Journal of Biological Chemistry 274(22):15345–9.

[8] USDA. 2012. USDA Nutrient Database for Standard Reference. [Online. Internet.] Available from: http://ndb.nal.usda.gov/.

[9] Ishida BK et al. 2004. Journal of Agricultural and Food Chemistry 52:274-9.

[10] ANZFSC standards 1.2.4 and 1.3.1.

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