Photochemical electron transfer in PS II - an overview

Upon light excitation of dark-adapted PS II, the primary charge separation takes place, forming P-680 and Pheo. This probably happens in a small number of picoseconds. Electron transfer from Pheo to occurs in a few hundred picoseconds, stabilizing the separated charges [112]. If is already reduced the (P-680 Pheo ) radical pair can still be formed, although perhaps with a low quantum yield (see Ref. 145), but now it lasts for a few nanoseconds [142] and gives rise to some recombination luminescence or, at low temperature, populates the triplet state of P-680 [141], which itself decays with a of around 1 ms [166]. 

When (Z P-680 Pheo Qa ) is formed the forward electron transfer from Qa to Qb takes place with kinetics of = 100 /as at room temperature [116]. When Qb is present in the dark (in 30% of the centres in dark-adapted chloroplasts) the Q to Qb transfer kinetics have a ti/2 200 /as [116]. 

At temperatures below approximately -30°C (240 K) electron transfer from Qa to Qb becomes gradually blocked [198]. The transitions from Si to S2, S2 to S3 and S3 to So are blocked by low temperature at 140 K, 240 K and 250 K respectively [199]. Donation from Z to P-680 is reported to be blocked at 240 K [200] but since this measurement was done with repetitive flashes this value could reflect the electron transfer block on the acceptor side between Qa and Qb- It is more reasonable to suppose that donation from Z can occur at much lower temperature, since Si can donate (presumably via Z) down to 140 K. Below 140 K, if Cyt b-559 is oxidized prior to illumination, chlorophyll is oxidized [195,196]. Under some conditions Signal II can be stably photoinduced at low temperature this has been attributed to Z [201]. Most of these observations can, however, probably be attributed to D.  

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