Back reactions, electron-transfer approximation

The oxidized P700 is reduced by the copper protein, plastocyanin, present in the lumenal space ofthe thylakoid. Reflecting the overall reaction it supports, the PS-1 reaction center is sometimes called plastocyanin ferredoxin oxidoreductase. Note that in cyanobacteria the corresponding electron donor is cytochrome c552 rather than plastocyanin and some algal species when in a copper-deficient medium synthesize a c-type cytochrome as a replacement for plastocyanin. The complete electron-transport chain ofphotosystem 1 is shown in Fig. 2. The approximate redox potentials and halftimes for forward electron transfers at ambient temperature are also indicated. Perhaps to assure wasteful back reactions are mini-... 

The general kinetic expression for this reaction mechanism is quite involved, but, clearly, the overall velocity will depend on the detailed kinetics of all three steps. If the back electron transfer rate is neglected (i.e., k 3[Ar][Bo] = 0), and the steady-state approximation is applied to the two central complexes, then the reaction velocity, v, may be derived ... 

An approximate value for the rate constant of this process ( 1-2x10 s 1 in methanol) can be measured from the emission decay (multiexponential because of some conformational freedom). Laser flash photolysis at 440 nm, which corresponds to excitation of the Ru(NN)3 chromophore (eq 18), fails to show any transient accumulation of the electron transfer product, simply proving that the back electron transfer process (eq 20), is faster than the forward reaction (eq 19) [83]. 

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