Silviamar: Vegetables, chemistry and colour    
 Vegetables, chemistry and colour9 comments
picture 16 Oct 2005 @ 17:02, by Silvia Martínez

An object is coloured because of the light that it reflects. White light from the sun contains all the wavelengths, but when it impacts on an object some of its wavelenghts are absorbed and some reflected. When we see an object of a certain colour, it's because it's reflecting more of that particular wavelength. For example red objects reflect 'red' light, which is light with a long wavelength.

Many vegetables and fruits are strongly coloured because they contain an especial kind of chemical compounds named carotenoids. These compounds have an area called choromophore, which absorbs and gives off particular wavelengths of light, generating the colour that we then perceive.

The chromophore is formed by a sequence of linear carbon-carbon double bonds (represented as C=C), much stronger than simple bonds (represented as C-C), so the atoms remain closer to each other. In general, it's necessary at least seven linear conjugated double bonds for a carotenoid to produce a colour. Besides, the bigger the number of bonds conjugated, the bigger the wavelength of the light absorbed and also the more red the vegetable, as you can see in this picture of the light spectrum:



The tomato is red because of the carotenoid lycopene, which contains 11 conjugated carbon-carbon double bonds. You can count these bonds in the picture below, they are selected in red (the atom carbons are omitted, only the bonds are shown). This compound is generated by the plant to protect itself from the air oxidation. So it's a good antioxidant useful for us too, protecting our cells against the action of free radicals (potent oxidants), which are one of the main responsibles of cardiovascular diseases, cancer and aging.



The pigment present in carrots is the betacarotene, with 9 linear conjugated double bonds, less than in lycopene so they are no red but orange (smaller wavelength than red, check it in the spectrum picture). This compound is also a potent antioxidant and besides it's transformed in our body into vitamin A, very important for the maintenance of healthy skin, good vision and a robust inmune system.



Spinachs, parsley and plants in general are green because they contain chlorophyll, a pigment which enables the plant to carry on photosynthesis, transforming solar energy and carbon dioxide into chemical energy in the form of carbohydrates and oxygen. This is a process essential for life.
As you can see in the pic below, the structure of chlorophyll is very complicated, so let's simpy say that it contains a big ring with a magnesium atom in the center. Curiously, the structure of hemoglobine (the carrier of oxygen in our blood) is pretty similar to chlorophyll, though it has an atom of iron instead of magnesium in its center.
The chlorophyll masks the other colours in vegetables and as its amount decreases the rest of colours become evident. This explains for example why tomatoes are initially green and then become red when they ripen.



This is an example of how Chemistry is everywhere, sometimes more evident, and sometimes much less :-).

Versión en español aquí.


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9 comments

16 Oct 2005 @ 18:00 by melztripp : i've just added a brain fold!!!
hey sylvia, i absolutely love to learn things like this...which is why i should have been a chemist instead of a nurse. i think we spoke about this before. unfortunately, there is only 1 or 2 people that i can explain this to, who will actually care!!! and the ones i tell, will probably already know this anyway!! oh well, thanks for the chemistry lesson. hope all is well with you, melanie!

*************
Hi Melanie! Yes, we've talked about it, and I'm pretty sure that you're learning many things on your own in this field. I always wanted to know about this things, I still remember getting shocked one day when I was 8 because I read the ingredients of a coconut cookies and realized that they didn't have any coconut but they tasted to it! :-)  

16 Oct 2005 @ 18:33 by jstarrs : I just wish I'd had Silvia
as a chemistry teacher when I was younger.
The only kicks I got back then in chemistry were making stink bombs...
I don't know the color of stink bombs.

**********
Thank you Jeff. Well, you were more fortunate than me, I didn't learn to make stink bombs at school. Probably the teacher knew that if we learnt to make them, we would have used them in the class all the time :-D  

29 Nov 2005 @ 15:05 by JMG @156.35.192.3 : teaching
Silvia, are you a teacher? I wish all teachers explained as clearly as you, you could do a living teaching chemistry...

*********
Thank you, you're very kind :-). Actually, I teach Chemistry at university, though I mainly do research. Teaching is what motivates me more.  

20 Dec 2005 @ 15:41 by Ruthie @80.230.216.7 : lycopene uv- visible spectrum
Hello
I'm trying, in vain, to find the uv-visible absorption spectrum of lycopene.
I would appreciate receiving a relevant link or the spectrom itself if you have it
thanks
Ruthie  

21 Dec 2005 @ 00:38 by silviamar : Ruthie, this is the light absorption
spectrum of lycopene at room temperature: Link
Hope it helps! :-)  

23 Apr 2006 @ 08:12 by bachhue @125.234.65.217 : Lycopene
Would you give me a UV vis spectrum of Lycopene
Thank you very much

********
Hello Bachue, in the message just above yours I gave the link to the spectrum of Lycopene between 340- 560nm (Link), with the main absorbance peaks at 446, 476 (the max absorbance) and 504 nm. I don't have it below that range.
By the way, there is a great free NMR, IR and UV spectra database of many compounds here: Link, lycopene is not there, but it can be useful for you for other compounds.  

18 Jun 2006 @ 12:11 by Joost @145.53.55.157 : lycopene NMR spectrum
I'm looking for a nmr spectrum of lycopene
Do you know where to find it?
thank you

******
You can find the 1H-NMR here: [link]
It's in table 2, first column. And one of the references included there (Helv. Chim. Acta. 75, 1848–1865) gives you the characterization of all the isomers of lycopene.  

1 Nov 2007 @ 09:57 by adinarayana mundra @202.63.102.42 : chemistry
I do make an effort to understand lycopene antioxidant activity  

9 Jan 2008 @ 01:15 by Sydney @12.216.169.160 : Thanks!!
this really came in handy with one of my chemistry project, you are officially my hero  

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