First electron transfer, purple bacteria

The characteristic derivative-shaped feature at g 1.94 first observed in mitochondrial membranes has long been considered as the sole EPR fingerprint of iron-sulfur centers. The EPR spectrum exhibited by [4Fe-4S] centers generally reflects a ground state with S = I and is characterized by g values and a spectral shape similar to those displayed by [2Fe-2S] centers (Fig. 6c). Proteins containing [4Fe-4S] centers, which are sometimes called HIPIP, essentially act as electron carriers in the photoinduced cyclic electron transfer of purple bacteria (106), although they have also been discovered in nonphotosynthetic bacteria (107). Their EPR spectrum exhibits an axial shape that varies little from one protein to another with g// 2.11-2.14 and gi 2.03-2.04 (106-108), plus extra features indicative of some heterogeneous characteristics (Pig. 6d). 

There is some confusion as to which electron acceptor should be called primary. Historically, in purple bacteria the quinone acceptor, Q, was so named. Later it was found that a BPh molecule accepts an electron before Q, and possibly even earlier acceptors, or charge transfer states, exist. Since the latter matter is still under debate (see Chapter 3), one might prudently keep the label primary for the quinone acceptor with the understanding that it is the first stable (on a time scale of ms) acceptor. 

As in the case of the purple photosynthetic bacteria, the more stable electron acceptor of green filamentous bacteria was first detected using instrumentation with millisecond-time resolution and so the rise and decay kinetics of any earlier electron acceptor(s) would be too fast to be detected. The detection of any earlier electron acceptor would require greater time resolution, such as afforded by picosecond spectroscopy. As a framework for further discussion we write the sequence of the primary photochemical and electron-transfer reactions in green filamentous bacteria as follows ... 

Finally, and most importantly, the two chloroplast photosystems differ significantly in their functions only PSII splits water to form oxygen, whereas only PSI transfers electrons to the final electron acceptor, NADP". Photosynthesis in chloroplasts can follow a linear or cyclic pathway, again like green and purple bacteria. The linear pathway, which we discuss first, can support carbon fixation as well as ATP synthesis. In contrast, the cyclic pathway supports only ATP synthesis and generates no reduced NADPH for use in carbon fixation. Photosynthetic algae and cyanobacteria contain two photosystems analogous to those in chloroplasts. 

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