Pheophytin- the intermediate electron acceptor

In purple bacteria a number of different lines of evidence led to the conclusion that bacteriopheophytin (BPh) acts as an electron carrier between the primary donor and Qa (Chapter 3). When is reduced illumination results in the photoaccumulation of reduced bacteriopheophytin, detected by its characteristic absorption changes and by an EPR signal split due to its interaction with Q Fe. At temperatures too low for rapid photoaccumulation of BPh to take place, illumination results in formation of a triplet state of the primary donor P-870 which has a polarization pattern characteristic of its formation by recombination of a radical pair. When BPh is reduced this triplet state cannot be formed. The most direct proof that BPh acts as a primary acceptor comes from the direct observation by absorption spectroscopy of BPh reduction within a few picoseconds after the flash. The BPh is reoxidized in 200 ps by electron transfer to or, if is already reduced, by recombination in 14 ns (see Chapter 3). 

Rash kinetic absorption measurements have provided further support for its role as an intermediate acceptor. In PS II in which was reduced, the formation of some Pheo which decayed in only a few nanoseconds was observed [142] and recently a change attributed to Pheo decaying in 200 ps was reported in particles in which was oxidized before flash illumination [112]. 

The reduced minus oxidized difference spectrum of Pheo photoaccumulated in PS II, showing large changes in the blue and red and in particular a small bleaching at 545 nm, is typical of Pheo reduction compared to spectra obtained in vitro [109,140]. The EPR spectrum from Pheo is split by when it is present 

The photoaccumulation of Pheo in PS II is accompanied by a large decrease in the level of fluorescence [140,109]. This observation led to the hypothesis that var- 

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