Chlorophyll, donor/acceptor properties

Because of its ionic nature magnesium forms very few coordination compounds. Donor-acceptor species in aqueous solutions are short-lived, but magnesium bromide and iodide have sufficient acceptor properties to dissolve in donor solvents such as alcohols and ketones. Magnesium appears to be an essential element for life, occurring in chlorophyll. 

Transient absorption spectroscopy has been used to study isolated Photosystem 2 (PS2) reaction centres stabilised by the use of anaerobic conditions. In the absence of added artificial electron donors and acceptors, the light induced electron transfer properties of the reaction centre are restricted to the formation of the radical pair P680+Pheophytin and charge recombination pathways from this state [1]. This charge recombination has been observed to produce a 23% yield of a chlorophyll triplet state [1]. Attempts to reconstitute these particles with quinone have until now been limited to the observation of a steady state, quinone-mediated photoreduction of the cytochrome b-559 [2]. 

When ferrous iron is inserted into protoporphyrin, the ubiquitous iron porphyrin or heme is formed. Because of the resonating structure, an electron donor or acceptor molecule need not come in contact with the iron atom directly to oxidize or reduce the iron atom it is probably sufficient that contact be made with any portion of the resonating molecule for the iron atom to be oxidized or reduced. The oxidative properties of the iron atom in heme are modified by the iron being held in this ring and are further modified by the heme being attached to specific proteins. In nature the other metal that complexes with protoporphyrin is magnesium Mg protoporphyrin is an intermediate compound in the biosynthetic chain of chlorophyll synthesis. The movements of the ir electrons in the porphyrin are undoubtedly intimately connected with the functioning of the heme and chlorophyll structures, but of this we know very little. 

In order to observe the important structure-dependent anisotropic spin-spin interactions, such as the dipolar interaction, D, within radical pairs and to prevent spin lattice relaxation from destroying the spin polarization, it is necessary to examine the radical pairs in the solid state at low temperatures. Photosynthetic model systems based on chlorophyll or porphyrin electron donors have the interesting, but unfortunate property that the efficiency of their light-initiated, singlet state electron transfer reactions is negligibly low whenever they are dissolved in solid solutions. Stated more precisely, the decay rates of chlorophyll and porphyrin excited singlet states are much faster than the rates of electron transfer from these donors to most electron acceptors in the solid state. 

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