Chlorophyll has a $\ceMg^2+$ ion. Why is it preferred over various other ions? for example, what happens if over there is $\ceZn^2+$ or $\ceCa^2+$ or any kind of other (divalent) cation instead of $\ceMg^2+$?


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Chlorophyll in plants has two key functions. First, that facilitates energy transfer that an soaked up photon"s power via the many other chlorophylls that constitute the antenna pigment-protein complex to the reaction centre. This way that v $> 95~\%$ effectiveness the power of a single absorbed photon get the reaction center which is composed of a dimer of chlorophyll molecules (the unique pair), 2 pheophytins ($\ce2H$ replace $\ceMg$) and also two other chlorophylls. The second function is in electron transfer. Electron transfer begins from the excited distinct pair and the electron is passed to other adjacent chlorophyll and also pheophytin molecules and then to quinones/iron sulfur complexes relying on the particular kind of reaction centre. The exciton interaction in between the 2 chlorophyll in the special pair means that it deserve to be treated together a single entity. This interaction lowers its excited state energy and so catch the energy getting here from the antenna.

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It is possible to make chlorophylls with various other metals instead of magnesium, but if these contain heavy or paramagnetic ones they will competitively quench the chlorophyll"s excited state (via a spin-orbit coupling or "heavy atom effect") and also reform the floor state. Subsequently very small if any kind of energy transfer will occur. This means that insufficient power will reach the reaction centre and also so the performance of photosynthesis will be dramatically reduced.

Magnesium chlorophyll has a distinctive property i m sorry is the the absorption and also emission spectra overlap particularly well definition that power transfer to surrounding molecules by the Forster (resonance) mechanism is an extremely favourable. As shown in the figure below the energy may move among $\approx 100\ \ceChl$ molecules and also reaches the reaction centre in a couple of ($ Chl -> Phe -> Quinine }$ with as high an performance as feasible (the whole procedure takes only $\approx \pu250ps$ in bacteria, and ten times quicker in plants), however the price of each back reaction on each step has to be as low together possible. Other steels in Chl will adjust the redox and also even a readjust of $0.1~\mathrmV$ will certainly be important, and also so do this procedure less efficient. The reason for this is the in common photosynthesis electron carry is nearly at the height of the Marcus rate vs. Totally free energy curve, therefore increasing or decreasing the exothermicity will certainly make the electron transfer less efficient.

All that this illustrates that photosynthesis is, via natural selection, very adapted, and so small changes have the right to have dramatic effects. Under various conditions, it may be feasible for alternative schemes to do the very same job.

The figure shows the antenna (light harvesting) chlorophylls in the protein complicated of tree photosystem 1 and also the reaction centre. (The protein residues have been removed for clarity). The reaction center is in ~ the centre, presented within the dotted line through the special pair presented edge on. The final electron acceptors are $\ceFeS$ complexes but they space not shown and also would lie directly listed below the special pair.

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The molecules to either side of the distinct pair are the accessory chlorophylls v which the electron from the special pair travels. The framework is modified indigenous PDB entry 1JBO.