Measurement of electron transfer

Picosecond Measurements of Electron Transfer in Bacterial Photosynthetic Reaction Centers... 

ESTABLISHING A QUANTITATIVE MEASURE OF ELECTRON TRANSFER RATE ATTENUATION... 

It seems likely that the magnitude of p represents the "extent of electron demand at phosphorus , in the T. S. or in other words is a measure of "electron transfer" in the T.S., a term (=z) which appears in the semi-empirical equations describing the nucleophilic reactivity of tricoordinated phosphorus. This concept is reinforced by (i) the observation (from the data of Bokonov. and Goetz ) that p-values based on the pKg values of protonated phosphines increase with increasing pKa, i.e. increase with a shift of the equilibrium (eqn. 4) to the left and... 

New anionic photoreductions initiated by excited / -naphtholate anions have been presented recently. According to measurements of electron transfer quenchings of the jS-naphtholate excited singlet state [96], it was shown that this powerful reducing species is able to reduce all the benzenic compounds bearing two or more chlorine substituents and all naphthalenes and biphenyls bearing at least monochlorinated. This means of course that this process is applicable to the polychlorobiphenyls, well known for their undesired persistence in the environment. When applied to chloronaphthalene or monochlorobiphenyl, this anionic photodechlorination presents characteristics comparing favorably with other sensitized processes [175]. 

The need to combine conventional electrochemical measurements of electron transfer (ET) with spectroscopic probes was recognized over 40 years ago [1, 2]. It was quickly realized that optical spectroscopies, first applied to the visible... 

Heroic techniques are not required [40] for the measurement of electron-transfer rates, even when they are close to the diffusion limit. The principles involved in ESR and NMR line broadening are very similar, but different frequencies are involved and the timescales differ. The ESR method allows the measurement of very short lifetimes (10 " to 10 s). 

Measurement of Electron Transfer Rate Constants Involving Surface-Confined Redox Reagents... 

Gaigalas, A.K. and Niaura, G. (1997) Measurement of electron transfer rates between adsorbed azurin and a gold electrode modified with a hexanethiol layer. Journal of Colloid and Interface Science, 193, 60-70. 

Time-resolved measurements of electron transfer times for quantum well photoelectrodes which can be compared with hot electron relaxation times, have not yet been reported. Only some excitation spectra, i.e. photocurrent vs. photon energy for MQWs and single quantum wells (SQWs), have been published so far [2]. In both cases, the photocurrent spectra show distinct structures corresponding to transitions between the hole and electron wells as shown for SQW electrodes in Fig. 9.32. The... 

Measurements of electron transfer rates by pulse radiolysis studies of Co -P -V show a diffusion-controlled reduction of the viologen chromo-phore, followed by exergonic (AG = -0.80 eV) electron transfer to the cobalt, with rate constants of 630 and 360 s for the 16-mer and 20-mer complexes, respectively (Figure 5). The reaction proceeds according to Scheme II. The... 

Electron affinities of molecules are of interest not only in gas-phase reactions, e.g. in negative chemical ionization mass spectrometry, but also in the field of condensed-phase chemistry. It is characteristic that negative ions are by far not studied to the same level of detail as the corresponding positive ions. However, during the last decade a large number of EA determinations based on measurements of electron transfer equilibria utilizing pulsed high-pressure mass spectrometry have been reported . ... 

Measurement of Electron Transfer Rate using ER Measurement I 87... 

Figure 7.14. Schematic diagram of SECM measurement of electron transfer through DNA dupiexes that aiiowed to provided a possible mechanism for the differences in charger transfer resistances before and after Zn + addition to dsDNA films. B. Liu, A. J. Bard, C.-Z. Li and H.-B. Kraatz ]. Phys. Chem. B 2005,109, 5193-5198. Cop50 ight 2005 American Chemical Society.

The photochemical activity of PS II reaction centers associated with primary charge separation has been documented by direct measurements of electron transfer by substrate donors (including water) or acceptors as well as by spectroscopic methods involving optical absorption, fluorescence and EPR. On the picosecond time scale it appears that the sequence of events and even the kinetics associated with the earliest steps are very similar between PS II and the purple bacterial reaction centers (15). Nevertheless, other aspects of this similarity remain to be demonstrated whether the primary electron donor of PS II consists of a special pair of chlorophylls, whether the PS II reaction center possesses a structural two-fold symmetry together with a functional asymmetry and whether there is a portion of the PS II complex that corresponds to the H polypeptide. 

A theoretical analysis of charge distribution within supercomplexes (or clusters in which the movement of diffusible carriers is restricted) has been developed by Lavergne et al [4]. This theory predicts the evolution of the redox state of the carriers under continuous illumination or flash excitation for any cluster stoichiometry. The predictive power of this treatment is illustrated by the analysis of the light-induced oxidation of primary and secondary donors in isolated centers of Rhodopseudomonas viridis (Fig. 3). In this case, it is definitely established that the secondary donors (cytochromes) are irreversibly bound to the reaction center. In the absence of mediators, no electron exchange is expected to occur between photocenters. In the presence of 200yM ascorbate, only two of the four cytochromes (cyt 556 and cyt 559) are in their reduced state prior to the illumination. As expected, the apparent equilibrium constant between P and the cytochromes measured during the course of illumination is much lower than that computed from the value of the redox potentials (K = 50 for cyt 559 and K 1500 for cyt 556). The fit between the experimental data and the theoretical simulation (dashed lines) is excellent and clearly demonstrates that the measurement of electron transfer reactions under weak illumination is a powerful tool to characterize the degree of structuration of a photosynthetic electron transfer chain. 

Artes JM, Diez-Perez I, Sanz F, Gorostiza P (2011) Direct measurement of electron transfer distance decay constants of single redox proteins by electrochemical tunneling spectroscopy. ACS Nano 5 2060-2066... 

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