Electron transport rates

A compound which is a good choice for an artificial electron relay is one which can reach the reduced FADH2 active site, undergo fast electron transfer, and then transport the electrons to the electrodes as rapidly as possible. Electron-transport rate studies have been done for an enzyme electrode for glucose (G) using interdigitated array electrodes (41). The following mechanism for redox reactions in osmium polymer—GOD biosensor films has... 

It was reabzed early on that because of their high electron transport rates, the charging rates of conducting polymer films would be controlled predominantly by the rate at which charge-compensating ions [Eq.(l)] could be extracted from, or ejected into, the bathing electrolyte solution.160,161 However, these and some other studies employing chronoam-... 

III (column 3 versus column 5), inhibition of the uncoupled electron transport rate was only partially relieved. Thus, the compounds appear to have two effects (a) the more sensitive is an effect on the ATP-generatlng pathway and (b) a second, but weaker, effect involved the electron-transport pathway. 

In summary, the distance dependence of electron transport dynamics varies as r p, where r is the bridge length (or the number of bridge units in the case of a charge hopping process) and p is less than 2. Consequently, electron transport rates display a very weak dependence on bridge length. 

The existence of an undissipated proton gradient in cyanobacteria, and its possible effect on photosynthetic electron transport rates, is less clear and, to our knowledge, is not currently being addressed by any research group. 

If one assumes that the ratio H /e is 2, then an ATP/2 e of 1.66 can be calculated. Izawa and Good [91] (see also Ref. 92) have established that a ratio ATP/2 e of 2 can be calculated for phosphorylating electron transport, if one subtracts the independently occuring electron transport rate observed in the absence of ADP-Pji obviously, such a correction cannot be made if the purpose is to evaluate the quantum requirement of photosynthesis, but it is important in the investigation of the efficiency of energy coupling. 

If the model proposed by Andersson and Anderson [109] of total separation of PS I and PS II in the granal chloroplasts were to be accepted, electron transport from the PS II acceptors to P-700 would require a mobile electron carrier(s) which should diffuse laterally in the membrane fast enough to account for the observed electron transport rate. Plastoquinone [112] and plastocyanin are the candidates of choice for this role. The former has been shown to be present at approximately the same activity in the partitions and in the stroma-exposed membranes [43], while PC is known to be located in the intrathylakoid space [113],... 

Other binding groups have been used, such as dipyridophenazine ligands in polymer 34, which were found to increase the electron transport rate. LEDs prepared from this material containing Ru at various levels indicated once again energy transfer to the metal complex at higher Ru content [71]. 

FIGURE 7.4. Three possible modes of electrochemical film conversion that can be differentiated by ellipsometric measurements. Model A corresponds to a process of charge injection limited by electron transport Model B to a rate limited by insertion of ions from solution, and Model C to a case of high levels of both ionic and electronic transport rates within the film. 

Ground state absorption spectra and electron transport rates indicate that the PSII RC material was sufficiently active to attempt fast kinetic studies [1,3,6,7]. Figure 1 shows transient absorption difference spectra of isolated PSII RC complex at both 277 and 15K. 

CX>2 fixation, 20 M DEMIB (8) was used. In the presence of both DBMIB and OyiQ, addition of 10 itiM H003 to the depleted sairple also stimulated the electron transport rate by a factor of 4. A maximum HOO3" effect was found at approximately pH 6.7 (Fig. 5). This result is consistent with the conclusion that both CO2 and HOO3", not OO2 or CX)3"" alone, may be the active species in the stimulation of Hill reaction. Blubaugh and Govindjee (9) have shown that HC3O3 is the active species involved in spinach thylakoids. 

Thus, the fast component of dark relaxation of variable fluorescence is associated with electron transport from Q to Photosystem 1, whereas the slow one reflects the electron efflux from Q to the donor side of Photosystem 2, The contribution of these components in the dark relaxation and their rates depend on the electron transport rates on the acceptor side of Photosystem 1 and the donor side of Photosystem 2. 

The wild diploid and tetraploid species of wheat possess much higher rates of flag leaf photosynthesis than the cultivated hexaploids (1) The high rates of photosynthesis per unit leaf area have been correlated with several morphological and anatomical traits (2>3). Attempts have also been made to correlate the in vitro electron-transport rates with leaf photosynthesis. Zelenskii et al (4) found that the chloroplasts of wild diploid species had 42 per cent higher rates of uncoupled Hill reaction as compared with cultivated hexaploid. This was later confirmed by Miginiac-Maslow et al (5). However,... 

Prolonged illumination of higher plants with intense visible light leads to photoinhibition as observed by diminution of O2 evolution and electron transport rate through photosystem II (PS II) and by a decrease of variable fluorescence yield (for review see Ref. 1 and 2). The molecular mechanism of photoinhibition is not yet clear and there is still some discussion about the exact site(s) of the primary events. Most authors localized inhibition within the PS II reaction center or at the Qg-binding site. In addition for chloroplats with already inhibited oxygen evolution capacity, the involvement of PS II donor side components has been reported. 

Table 2. Light saturated electron transport rates of spheroplasts isolated from LLG and HLG S. platensis before and after photoinhibitory treatment...

Fig. 1(a) Electron transport rates of PS2 enriched particles from control pea plants and those which have been frozen at -18°C overnight, (b) Photolnhlbltlon (2000 //E/m /s, 30 fig, Chl/ml, 10 min) of electron transfer In PS2 particles from control and frozen pea plants. CaClj (10 mM), NaCl (20 mM) or DPC (0.5 mM) were added before light treatment to some samples as Indicated. Electron transport to the artificial electron acceptor,... 

FIGURE 1 Effect of high concentrations of quinacrine (left) and 9-amine acridine (right) on the electron transport rate at different pH values. 

It has been noticed that stimulation of the electron transport rate by quinacrine and 9-amine acridine had an inverse relationship to the pH values studied, as seen at the Fig. 1. Taking in account the pK of dissociation of the utilized acridines (9-AA 10.0 QA 7.9 and 10.4), with both compounds the increase of the electron transport velocity presented a saturation behavior which reached higher values when the more protonated forms were present. Thus, the reported effect could not be explained by permeation of the unprotonated form to the vesicle lumen in a similar mechanism concerned to the ammonium uncoupling. 

Fig. 1, below, left - Schematic time course of absorption changes at 433 nm induced by red actinic light in the presence of far-red background light. The actinic light intensity is 500 Wm-2, producing approx. 90 % of the maximal electron transport rate. 

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