Chemical of the Week 20: Chlorophyll

Anyone who has a drive to work knows the frustration of morning travel and wasted time but recently I’ve discovered a way to avoid the boredom – podcasts ! In particular the Radio 4 In Our Time Science series by Melvyn Bragg and it was the photosynthesis programme that got me thinking about chlorophyll. I’ve never really given the chemical process of photosynthesis much thought as it is firmly rooted in the biology curriculum but all that changed last week.

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The combination of carbon dioxide and water to make glucose and oxygen is no mean feat and the elegant structure of the chlorophyll molecule allows for the absorption of light to drive the reaction. So what is chlorophyll – it has a porphyrin ring structure attached to a protein backbone. The porphyrin is built up of pyrrole molecules – 5 membered aromatic rings which are made of four carbons and one nitrogen atom. This ring system acts as a polydentate ligand and has a magnesium cation at its centre. It is delocalisation of electrons around this aromatic porphyrin system that allows the absorption of light that kick starts the photosynthetic process allowing the production of the energy-storage and transport molecules ATP and NADPH. Off interest is that chlorophyll absorbs light in the blue and red regions of the electromagnetic spectrum reflecting the mid region hence plants are green! You might also notice the similarities of the structure of haemaglobin and chlorophyll, both having porphyrin ring structures. You can not help but be transfixed by these amazing natural macromolecules which are not easy to make synthetically. A few months ago a guest post centred around the great organic chemist R B Woodward and one of the natural products he made was chlorophyll.

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So probably the more important question is how do carbon dioxide and water react to produce an organic molecule? It’s obvious that it’s not a straightforward process and something that we don’t see happening without specific conditions. Water is in a sense acting as a fuel adding electrons to the carbon dioxide to allow it to grow into an organic molecule. The podcast mentioned that it might not have been water originally but iron or hydrogen sulfide acting as the electron donors. During this process water is oxidised forming oxygen, hydrogen ions and electrons 2H2O –> 4H+ + O2 + 4e- (photolysis). The conversion of carbon dioxide to glucose can be followed using the Calvin cycle. Carboxylate/oxygenase enzymes catalyse the ‘carbon dioxide fixing’ allowing it to combine with a five-carbon sugar, ribulose1,5-biphosphate. This intermediate quickly breaks down to give molecules with three carbons ( 3-phosphoglyceric acid ) which go on to build glucose.

Now anyone who knows me knows that this foray into biology is not a comfortable one (that’s my disclaimer!). It’s a pity that chlorophyll doesn’t get a mention on the A2 chemistry spec when we are studying polydentate ligand systems – surely this is the ultimate complex without which the World would be a truly different place! Oh and I loved the fact that when the historical biochemist talked about the evolutionary photosynthetic process, oxygen was originally considered a toxic (reactive) product – how things have changed !

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