The electron donor side of PS II

The primary electron donor of PS II was detected as a flash-induced absorption change attributable to chlorophyll a oxidation [160]. Its bleaching maximum is close to 680 nm and it is thus designated P-680 (it is also called Chi an). The fluorescence of the light-harvesting chlorophylls interferes with measurements at this wavelength, thus many kinetic studies of P-680 have been done by measuring the smaller broad absorption increase at around 820 nm [161,162]. This broad absorption in the near infrared is probably responsible for the fact that P-680 is a quencher of chlorophyll fluorescence. 

Since P-680 is rapidly reduced by the native electron donor system on a time scale which ranges from 50 to 250 ns (see the next section), most studies of P-680 have been done under conditions in which the secondary donors are inhibited (e.g. Tris-washing, NH2OH treatment, detergent treatment, extremes of pH). Even under these conditions P-680 is short-lived, being reduced either more slowly by the 

The very high oxidizing power of P-680 leads to it oxidizing a number of different donors. Some of the components oxidized by P-bSO are not functional as electron donors under physiological conditions but are oxidized when the native rapid donor system is not operational. Such donors include Cyt 6-559, Chi and carotenoid and their properties are dealt with in a subsequent section. 

The non-physiological oxidation of close-by components by P-680 has led to some confusion. In particular photoinduced free radical EPR signals were at first misassigned to P-680. However, time-resolved EPR spectra of P-bSO have been reported and the g value of g = 2.0027 and linewidth of 8 G [164,165] are compatible with oxidation of a single Chi a molecule. 

An EPR signal attributable to the spin-polarized triplet state of P-680 ( P-680) formed by recombination of P-680 Pheo was detected by illuminating PS II preparations at liquid helium temperature [141], The zero-field splitting parameters are identical to those of triplets of monomeric Chi in vitro. The triplet minus singlet spectrum of P-680 measured by absorption-detected magnetic resonance indicated a monomeric triplet but a dimeric ground state [166]. 

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