The quinone-iron complex

The normal one-electron reduction of occurs with a midpoint potential lower than 0 mV but the actual value is still a subject of some controversy (see Section 3.3.3 below). The value often cited by those not wishing to get bogged down in that controversy is that obtained by measuring the redox potential dependence of Cyt b-559 photooxidation at 77 K in chloroplasts [106]. A value was obtained which was pH-dependent at pH values below pH 8.6. The value at and above pH 8.6 (the p of Qa ) was -130 mV. This value is usually considered the operative E, since Qa is not protonated on a functional time scale. This assumption was also made earlier for the E of Qa/Qa in purple bacteria [107]. Arguments for and against the use of the E -pK are discussed in detail in a recent review [108]. 

In forward electron transfer, is reduced very rapidly after a flash. Direct measurements of the kinetics of formation have not been reported, although estimations of hundreds of picoseconds have come from indirect measurements [112]. 

The redox properties of Qg are also unlike those of plastoquinone in the pool. The semiquinone form, Qb . is tightly bound to a protein of the reaction centre and is thus stabilized. Qb is much more stable than Qa . since forward electron transfer does not take place from Qb - The lifetime of Qb, like that of Qa in the presence of DCMU, is determined by the stability and availability of positive charges on the donor side. For example, Qb recombination occurs with S2 or S3 (Ref. 115, and see section 3.5) with a ty2 of approximately 30 s [116] but when Qb is present in centres where the stable S states, S,) and Sj, are present, Qb is stable for hours. This probably explains why a certain proportion of Qg is present even in PS II which has been dark-adapted for long periods. A number of measurements have indicated the involvement of proton uptake when Qb is reduced to semiquinone form [117], although the optical spectrum is more compatible with Qb being the unprotonated anion. This can be explained by the protonation of a group on the protein close to Qg, as first proposed in purple bacterial reaction centres to explain similar phenomena [118]. 

Unlike Qa , Qb can accept a second electron in a physiological reaction. The kinetics of electron transfer from Qa to Qg are faster than those of Qa to Qg [116,119]. Half-times of = 100 and — 200 /xs have recently been reported for Qa to Qb and Qa to Qb respectively [116] however, values significantly different from these have also been reported [119]. 

The second reduction of Qg is accompanied by a true protonation forming the 

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