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



Chlorophyll in plants has two main functions. First, it facilitates energy transfer of an absorbed photon"s energy via the many other chlorophylls that constitute the antenna pigment-protein complex to the reaction centre. This means that with $> 95~\%$ efficiency the energy of a single absorbed photon reaches the reaction centre which consists of a dimer of chlorophyll molecules (the special pair), two pheophytins ($ce2H$ replace $ceMg$) and two other chlorophylls. The second role is in electron transfer. Electron transfer begins from the excited special pair and the electron is passed to other nearby chlorophyll and pheophytin molecules and then to quinones/iron sulfur complexes depending on the particular type of reaction centre. The exciton interaction between the two chlorophyll in the special pair means that it can be treated as a single entity. This interaction lowers its excited state energy and so traps the energy arriving from the antenna.

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It is possible to make chlorophylls with 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 reform the ground state. Consequently very little if any energy transfer will occur. This means that insufficient energy will reach the reaction centre and so the efficiency of photosynthesis will be dramatically reduced.

Magnesium chlorophyll has a unique property which is that the absorption and emission spectra overlap exceptionally well meaning that energy transfer to nearby molecules by the Forster (resonance) mechanism is very favourable. As shown in the figure below the energy may move among $approx 100 ceChl$ molecules and reaches the reaction centre in a few ($ Chl -> Phe -> Quinine }$ with as high an efficiency as possible (the whole process takes only $approx pu250ps$ in bacteria, and ten times faster in plants), but the rate of each back reaction on each step has to be as low as possible. Other metals in Chl will change the redox and even a change of $0.1~mathrmV$ will be important, and so make this process less efficient. The reason for this is that in normal photosynthesis electron transfer is almost at the peak of the Marcus rate vs. free energy curve, thus increasing or decreasing the exothermicity will make the electron transfer less efficient.

All of this illustrates that photosynthesis is, via natural selection, highly adapted, and so small changes can have dramatic effects. Under different conditions, it may be possible for alternative schemes to do the same job.

The figure shows the antenna (light harvesting) chlorophylls in the protein complex of plant photosystem 1 and the reaction centre. (The protein residues have been removed for clarity). The reaction centre is at the centre, shown within the dotted line with the special pair shown edge on. The final electron acceptors are $ceFeS$ complexes but they are not shown and would lie directly below the special pair.

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The molecules to either side of the special pair are the accessory chlorophylls through which the electron from the special pair travels. The structure is modified from PDB entry 1JBO.