Alternative electron donors

When the primary electron donation pathway in photosystem II is inhibited, chlorophyll and p-carotene are alternate electron donors and EPR signals for Chl+ and Car+ radicals are observed.102 At 130 GHz the signals from the two species are sufficiently resolved to permit relaxation time measurements to be performed individually. Samples were Mn-depleted to remove the relaxation effects of the Mn cluster. Echo-detected saturation-recovery experiments were performed with pump pulses up to 10 ms long to suppress contributions from cross relaxation and spin or spectral diffusion. The difference between relaxation curves in the absence of cyanide, where the Fe(II) is S = 0, and in the presence of cyanide, where the Fe(II) is S = 2, demonstrated that the relaxation enhancement is due to the Fe(II). The known distance of 37 A between Fe(ll) and Tyrz and the decrease of the relaxation enhancement in the order Tyrz > Car+ > Chl+ led to the proposal of 38 A and > 40A for the Fe(II)-Car+ and Fe(II)-Chl+ distances, respectively. Based on these distances, locations of the Car+ and Chl+ were proposed. 

In addition to its function in thiol/disulfide exchange, Grx-1 also serves as an alternative electron donor to ribonucleotide reductase (Fernandes and Holmgren 2004), participates in deiodination of thyroxine to triiodothyronine (Takagi et al. 1989), and acts as a dehydroascorbate reductase for regenerating ascorbic acid (Washburn and Wells 1989). However, despite the potential role of Grx in... 

Fig. 7. (A) Photooxidation pathways of the alternate electron donors Cyt b559 and Chiz in PS II. (B) effect of illumination temperature on the photooxidation of competing electron donors in PS II as monitored by EPR signal intensities. See text for discussion. Plot (B) reproduced from Thompson and Brudvig (1988) Cytochrome b559 may function to protect photosystem II from photoinhibition. Biochemistry 27 6656.

Nomoto T, Fukumori Y, Yamanaka T (1993) Membrane-bound cytochrome c is an alternative electron donor for cytochrome aa3 in Nitrobacter winogradskyi. J Bacteriol 175 4400-4404... 

Under particular conditions, P-carotene (1) serves as an alternative electron donor in the PS2 reaction centre, coping with highly oxidised intermediates formed during water oxidation. This redox role of P-carotene (1) in PS2 is unique amongst photosynthetic reaction centres. An overview on the electron transfer processes in PS2, emphasising those involving carotenoids, has recently been presented [97]. Methods employed for studies of the P-carotene cation radical (12) in PS2 include Resonance Raman [36,98,99], FT-IR [100], ENDOR [38] and EPR [37] spectroscopy, see under Methods below. 

It reduces the intermolecular CTC between alternating electron donor (diamine) and electron acceptor (dianhydride) moieties. In general, fluorinated poly-imides exhibit lighter color and greater transparency than non-fluorinated analogues [42]. 

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