First electron transfer absorbance change

The rate of Sc -promoted photoinduced electron transfer from Ceo to CI4Q determined from the decay rate of the absorbance due to Ceo at 740 nm (inset of Fig. 11) obeys pseudo-first-order kinetics and the pseudo-first-order rate constant increases linearly with increasing the p-chloranil concentration [CI4Q] [135]. From the slope of the linear correlation, the second-order rate constant of electron transfer ( et) in Scheme 15 was obtained. The A et value increases linearly with increasing the Sc + concentration. This indicates that CUQ produced in the photoinduced electron transfer forms a 1 1 complex with Sc + (Scheme 15) [78]. When CI4Q is replaced by p-benzoquinone (Q), the value for electron transfer from Ceo to Q increases with an increase in [Sc " ] to exhibit a first-order dependence on [Sc ] at low concentrations, changing to a second-order dependence at high concentrations, as shown in Fig. 13 (open circles) [135]. Such a mixture of first-order and second-order dependence on [Sc ] was also observed in electron transfer from CoTPP (TPP = tetraphenylporphyrin dianion) to Q... 

Optical spectra of transferrin C-lobe docked with the transferrin receptor showed a characteristic broad absorption band centred at 465 nm, just as in the receptor-free /zo/o-protein (Figure 2.1 inset). The intensity of this absorbance band declined as more negative potentials were applied in a spectroelectrochemistry experiment, but did not qualitatively change in its overall features. An EPR spectrum of the Fec/TfR complex at pH 5.8, recovered from the OTTLE cell after completion of spectroelectrochemical studies allowed us to conclude that the first coordination shell of Fe " in transferrin is intact and unperturbed when C-lobe is complexed with TfR. Consequently, we assume that C-lobe and Fec/TfR complex have similar if not identical Fe " and Fe binding constants, and so we take for binding of Fe " in the protein-receptor complex to be 10 M as calculated for free Tf. This value was used to correct the observed Nernst plot data by accounting for the dissociation of Fe that occurs upon reduction. Nernst plots for the observed spectroelectrochemical data for FccTf/TfR, and data corrected for Fe dissociation, are presented in Figure 2.7. The corrected plot exhibits typical Nernstian behaviour for a one-electron transfer and a E1/2 value of —285 mV. 

It was found that when the second flash in a pair of flashes is applied 500 ps after the first, the full measure of the absorbance change due to Cyt c553 oxidation is readily seen, but as the interval between the two flashes is decreased, the effect of the second flash eventually becomes smaller. When the time interval is reduced to 60 ps, the effect of the second flash is only half that of the first flash. A simple interpretation of these observations is that when the reduced acceptor produced by the first flash has not yet transferred its electron to Qb, the radical pair [P T ] produced by the second flash simply recombines to reform [P I]. Therefore, cytochrome oxidation can take place by the second flash only if has transferred its electron to Qb and is thus ready to accept an electron from E formed in the second flash so... 

Fig. 17 (B) shows flash-induced absorbance changes measured at 480 as well as 580 nm. The anteima transients described above were again subtracted to yield the net signals. These new results show more clearly complete electron transfer from photosystem I to ferredoxin. Furthermore, some new features emerge in the transients at 580 nm. At higher time resolution, Setif and Bottin found three fast, first-order components in the absorbance changes with ty, of 500 ns, 20 jus and 100 jus, respectively. Separately measured spectra in the 460-600 nm region show that the three phases are all attributable to electron transfer from [FeS-A/B] to Fd. The different phases have been accounted for on the basis of structurally different, Fd-binding sites in the PS-I reaction center Possible sites may be seen in the newly determined, three-dimensional structure of the PS-I reaction center . ...

Figure 1. PQ - PQ difference spectra measured in chromatophores of Rb. capsulatus. The filled squares mark the absorbance changes associated with the formation of PQa > which was generated following a laser flash using 200 /iM ferrocene as an electron donor to P", and with 3 mM orthophenanthroline to block electron transfer to Qg. The plot shows the absorbance changes measured 60 ms following excitation. The empty squares mark the absorbance changes associated with the formation of PQg , which was measured with the addition of 200 uM ferrocene, and as the absorbance difference measured 400 ms following the first laser flash and that measured 400 ms following a second flash spaced. The two flashes were spaced 400 ms apart. The first flash generates PQg and the second flash removes this by formation of the double reduced quinone.

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