Chapter 22 – Infographic descriptions

22.0a The Chemistry of Tomatoes

Should tomatoes be stored in the fridge? Chilling damages cell membranes in tomatoes, and inhibits enzyme activity, which can lead to a drastic loss of volatile compounds. Some of these, such as the C6 (six carbon) volatiles, do not contribute significantly to flavour, but others, such as geranial, have a noted impact on factors such as sweetness. Taking tomatoes out of the fridge for 24 hours can lead to some recovery of volatile compounds, however, though only within a week of fridge storage. It’s also worth noting that storing ripe tomatoes in the fridge can obviously be beneficial, to stop them from going off! (Z)-3-Hexenal is also a significant volatile compound in tomatoes.

What causes the colour of tomatoes?

Green tomatoes are also coloured because of the presence of chlorophyll. As they ripen, the pigment lycopene develops; this compound absorbs light across mos tof the visible light spectrum, except the red portion, causing the tomatoes to appear red. It absorbs most visible light as a result of its highly conjugated structure – that is to say, it has lots of alternating double and single bonds. Lycopene absorbs all but the longest wavelengths of visible light.

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22.1a The Chemistry of Spinach

Compared to many other vegetables, spinach does have a higher iron content. However, iron in vegetables tends to have a low bioavailability, meaning it is not easily absorbed in the body.

According to USDA food consumption database; Scrimshaw (1991): Spinach: 2.6mg iron per 100grams, 1.7% absorbed (0.44mg). Sirloin steak: 2.5mg iron per 100 grams, 20% absorbed (0.50mg).

Low absorption of iron is partly due to the polyphenol compounds in spinach binding iron – not due to its oxalic acid content (as previously thought). Though it might not be a great source of iron, it’s a good source of Vitamin A in the form of carotenoids (for example Beta-carotene).

Spinach contains high amounts of oxalic acid which leaves your teeth with a ‘chalky’ feeling. The oxalic acid reacts with the calcium ions in the spinach and your saliva forming poorly soluble calcium oxalate crystals which coat your teeth creating ‘spinach teeth’.

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22.1b The Chemistry of Bell Peppers

Bell peppers go through a spectrum of colours as they ripen.

Chlorophyll, used by plants for photosynthesis, gives bell peppers their initial green colour. As the pepper ripens, chlorophyll decomposes and a range of carotenoid pigments (lutein, violaxathin, and beta-carotene) appear, which give yellow and orange hues. Eventually red carotenoid pigments including capsanthin and capsorubin appear, which are exclusively found in peppers.

Chlorophyll A: [latex]R = -CH_{3}[/latex]
Chlorophyll B: [latex]R = -CHO[/latex]

The aroma of bell peppers also develops as they ripen.

Green peppers: the smell is largely due to 2-methoxy-3-isobutylpyrazine (“bell pepper pyrazine”). Other minor contributors include: (E,Z)-2,6-nonadienal (“cucumber aldehyde”). The concentrations of most volatile compounds drop during ripening, with the exception of (E)-2-hexenal and (E)-2-hexenol, lending a sweeter, fruitier note to the aroma.

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22.3a The 2022 Nobel Prize in Chemistry

The 2022 Nobel Prize in Chemistry, awarded jointly to Carolyn R. Bertozzi, Morten Meldal and K. Barry Sharpless for their development of click chemistry and bioorthogonal chemistry.

Barry Sharpless coined the concept “click” chemistry in 2001. Click chemistry refers to reactions which efficiently snap together small molecules with simple reaction conditions and no unwanted byproducts.

Independently, Barry Sharpless and Morten Mendal developed the first click reaction: a reaction in which an azide is added to an alkyne with a copper catalyst. The two reagents click together to form a single cyclic product, with the copper catalyst making the reaction quick and selective. Chemists could add groups onto the azide and alkyne to change the product formed by the reaction.

Carolyn Bertozzi introduced the concept of bioorthologanal chemistry – chemical reactions that happen in cells without affecting their normal chemistry – in 2003. Copper is toxic to living cells, so she modified the original click reaction to produce a copper-free version. She used this reaction to track molecules called glycans on cell surfaces, which she had been investigating since the early 1990s.

Additional click chemistry reactions have developed, useful in the synthesis of new drugs. Bioorthologonal reactions allow researchers to study biological molecules and help identify targets of new drugs, and are also being trialed to produce ‘clickable’ antibodies to target cancerous tumours.

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22.4a Today in Chemistry History: August Kekulé and the structure of benzene

Today in Chemistry History: August  Kekulé.

Kekulé was born on 7 September 1829.

Kekulé is best known for work on the structure of benzene. He claimed he deciphered its ring-shaped structure after a daydream about a snake eating its own tail. He depicted the benzene ring as a flat molecule with alternating single and double bonds between carbon atoms. Original structure was an irregular hexagon with different bond lengths between the carbon atoms, which we now know isn’t the case, and it did not fully explain benzene’s reactivity and stability.

In 1929 Kathleen Lonsdale used X-ray crystallography to show all carbon-carbon bonds in a benzene ring are the same length, meaning they are delocalised. This explains benzene’s stability. Lonsdale proved it was flat molecule.

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22.4b Benzene Derivatives in Organic Chemistry

A wide variety of important organic compounds are derived from benzene, by replacing one of the hydrogens with a different functional group. They can have both common and systematic names, which can be confusing.

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22.4c The Science of Sunscreen & How it Protects Your Skin

Types of UV radiation:

• UVA: (wavelength 320-400nm) accounts for 95%  of solar UV radiation reaching Earth’s surface. Penetrates deepest into skin, contributes to skin cancer via indirect DNA damage. UVA blockers: Avobenzone, Menthyl Anthranilate, Ecamsule.
• UVB: (wavelength 290-320nm) accounts for 5% of solar UV radiation reaching Earth’s surface. Causes direct DNA damage, one of main contributors to skin cancer. UVB blockers: Octyl Methoxycinnamate, Homosalate, PABA, Padimate O, Cinoxate, Octyl Salicylate, Trolamine Salicylate, Octylocrylene, Ensulizole. UVB blockers not approved in the USA: Octly Triazone, Enzacamene, Amiloxate.
• UVC: (wavelength 290-100nm) filtered by ozone in Earth’s atmosphere and does not reach the surface, as a result does not cause skin damage.

Inorganic chemicals in sunscreen (ie. Zinc oxide, titanium oxide) both absorb and scatter UV light.

Organic chemical also used – the chemical bonds absorb UV radiation, with the chemical structure affecting whether they absorb UVA, UVB or both. Several different chemicals are used in sunscreen to ensure full protection.

UVA and UVB blockers, all approved in EU, Canada, Australia: oxybenzone, Sulisobenzone,  Dioxybenzone. UVB blockers not approved in the USA, all approved in EU, Canada, Australia: Mexoryl XL, Tinososorb S, Tinosorb M, Neo Heliopan AP, Uvinual A Plus, UVAsorb HEB.

17 sunscreen active ingredients approved in the USA.

28 sunscreen ingredients approved in the EU.

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22.4d The Chemicals Behind the ‘New Car Smell

60+ is the approximate number of volatile organic compounds (VOCs) detected in the interior of new cars. Not a;; of these ill have odours, but many may be contributors to the characteristics of ‘new car smell’, no it’s not the result of one specific compound.

20% is the approximate percentage decay of total VOC levels per week in new car interiors. They can also vary depending on conditions – concentrations will be decreased by ventilation of the car, but can be increased by increased temperatures within the car.

The most common found compounds: Toluene, Ethylbenzene, Styrene, Xylenes (p-Xylene, m-Xylene, and o-Xylene), Trimethylbenzenes (1,3,5-, and 1,2,4-Trimethylbenzene), various alkanes.

They come from: plastic, moldings, carpets, upholstery, adhesives, lubricants, gasoline, leather and vinyl treatments.

A 2007 study found little toxicity in the new car odours under lab conditions, However, although concentrations of these compounds are still very low, they can be above recommended indoor guidelines for VOCs for the first 6 months after a car’s manufacture. This could lead to headaches, sensory irritations and minor allergic responses. Manufacturers are now taking measure to reduce the levels of these compounds in new cars.

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22.4e The Chemistry of Glow Sticks

When glow sticks are bent the inner glass tube is broken, releasing hydrogen peroxide solution. This then reacts with diphenyl oxalate, producing 1,2-dioxtanedione; this product is unstable, and decomposes to carbon dioxide, releasing energy. The energy is absorbed by electrons in dye molecules, which subsequently fall back to their ground state, losing excess energy in the form of light.

• Rhodamine B produces red colour.
• 5,12-BIS(Phenylethynyl)Naphthacene produces orange colour.
• Rubrene produces yellow colour.
• 9,10-BIS(Phenylethynyl)Anthracene produce green colour.
• 9,10-Diphenylanthracene produce blue colour.

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22.5a Aromatic Chemistry Reactions Map

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Attribution & References

Compound Interest infographics are created by Andy Brunning and licensed under CC BY-NC-ND

Except where otherwise noted, content on this page has been created as a textual summary of the infographics used within our OER. Please refer to the original website (noted below each description) for further details about the image.