Electron artificial donors

Fig. 3. Schematic illustration of two different modes of photoinhibition. (A) structural model for the isolated PS-II reaction-center (B) secondary electron transport in the presence of an artificial donor and acceptor (C) acceptor-side induced photoinihibition and (D) donor-side induced photoinhibition. See text for discussion.

The effects of cadmium and lead (Pb) has been assessed in chloroplasts isolated from lucerne (Medicago sativa). The presence of Cd and Pb led to the inhibition of the PSII activity. The inhibition appeared to be located on the oxidizing side of PSII because the addition of artificial donors restored the electron flow. 

In the absence of the or Q quinone molecules in the complex the detection of net electron transfer activity required addition of a compound which could act as an electron acceptor from reduced pheophytin. Silicomolybdate was found to act in this way (22) and with DPC, Mn or other artificial donors present, a light dependent rate of silicomolybdate reduction was measureable (10). 

SiMo is an active electron acceptor which allows not only the photoaccumulation of P680 and of oxidised secondary donors but also electron transport from artificial donors such as MnCL, hydroxylamine, etc. (3). Although various quinones can be reduced photocnemically by PSII reaction... 

Scenedesmus obliquus wild type (WT) and a number of low-fluorescent (LF), non-02 evolving mutants were examined by thermoluminescence (TL) ana EPR spectroscopy. Three of the mutants (LF-1, LF-8, and LF-9) exhibit electron transport across the reaction center (RC) if artificial donor and acceptor molecules are supplied [1]. Three other mutants (LF-18, LF-22, and LF-23) do not exhibit such activity in the presence of artificial donors and acceptors indicating that the RC itself is seriously affected [1], The purpose of this work is to gain more information about what point on the donor side of PSII that the mutants are affected and to examine sources of charge recombination in the mutants. 

Structural and functional control by spectroscopy. In order to study interactions and dynamics in the native and modified reaction centers the entire arsenal of spectroscopy has to be employed. The interpretation of dynamic data profits from more general developments in electron transfer theory [13] and new experimental results in the field of artificial donor/acceptor systems where specific features can be influenced in a parametric way. (Scherer et al., Wasielewski et al.). 

Some aspects of the Lowe-Thomeley mechanism for nitrogenase action, which has served us well over the past 15 years, are being called into question. In particular, the necessity for protein-protein dissociation after each electron transfer, the rate-determining step with dithionite as reductant, is being questioned when the natural electron donor flavodoxin or other artificial systems are used. Some aspects of the mechanism should be reinvestigated. 

Incorporation of an artificial flavin nucleobase and of a cyclobutane pyrimidine dimer building block into DNA DNA double strands, DNArPNA hybrid duplexes, and DNA-hairpins, provided compelling evidence that an excess electron can hop through DNA to initiate dimer repair even at a remote site. The maximum excess electron transfer distance realised so far in these defined Donor-DNA-Acceptor systems is 24 A. New experiments are now in progress to clarify whether even larger transfer distances can be achieved. 

In artificial photosynthetic models, porphyrin building blocks are used as sensitisers and as electron donors while fullerenes are used as electron acceptors. Triads, tetrads, pentads and hexads containing porphyrins and Qo have been reported in the literature (see the Further Reading section). 

Figure 12.12 Building blocks of an artificial photosynthesis system for hydrogen production using a chromophore (C), an electron acceptor (A) and a sacrificial donor(D)...

The activity of active enzyme is usually assayed with artificial electron acceptors or donors (usually dyes). It has been shown that when the A. vinosum enzyme is directly attached to an electrode, its hydrogen-oxidizing activity is much higher than that obtained with dyes (Pershad et al. 1999). Even under 10% hydrogen, the diffusion of hydrogen to the active site was shown to be the rate-limiting step. This means that in normal assays, the reaction with dyes is probably rate limiting. It also indicates that electron transfer and the ejection of H+ by the enzyme are fast processes. 

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