Flash spectroscopy, donor-acceptor

Binding of the components in a neutral aqueous solution was confirmed by potentiometric titrations. The feasibility of electron transfer between the components was predicted by cyclic voltammetry and an efficient outer-sphere fast electron transfer was foreseen. Fluorescence spectroscopy measurements showed that the formation of a defined donor-acceptor complex worked even in water at neutral pH. Electron transfer as the quenching mechanism was proved by laser flash photolysis. 

All the above EPR spectroscopy was carried out in the steady state. With the use of fast-response spectrometers, however, it was discovered a decade ago that when measured early (at room temperature a few /u,s) after a light flash, the EPR spectra of primary reactants in PS I [105,106] and in bacterial RC [107] showed EPR lines characteristic of systems out of Boltzmann equilibrium (Table 2). Part or all of these so-called spin-polarized lines may then either be in emission or show absorption that is enhanced or decreased compared to the equilibrium absorption (Fig. 7). Electron spin polarization occurs through magnetic interactions between two simultaneously induced donor-acceptor radicals (reviewed in Ref. R14). Thus, a study of spin-polarized EPR lines yields information on these magnetic interactions and therefore on the configuration (distance, relative orientation, etc.) of the radicals (see e.g. Ref. R14). 

Flash spectroscopy has turned up the existence of donor-acceptor complexes of iodine atoms (produced by the flash) and benzenoid hydrocarbons (XIII) The existence of some sigma complex in these systems can probably... 

In polar solvents, such as acetonitrile, organic donor-acceptor systems such as those listed in Table 6.2 show only the fluorescence due to A no new fluorescence appears as in exciplex formation. Flash spectroscopy shows absorption spectra characteristic of the hydrocarbon radical anion and the amine radical cation. The product in these solvents is either an ion-pair or two free ions, stabilised no doubt by solvation, and the reaction is a complete transfer of an electron from one molecule to another, rather than exciplex formation. The reaction goes effectively to completion, and so (with only one fluorescence lifetime to be considered) the kinetic equations for the intensity and lifetime reduce to the simple Stem-Volmer forms (Equations (6.16) and (6.19)). The rate constants for the reactions of aromatic hydrocarbons with various amines in acetonitrile are found to be correlated with the free-... 

Comparison of the flash-induced T-S spectrum [9] with the ADMR-detected T-S spectrum at 509 or 982 MHz, shows that both spectra have an identical shape around the Qy maximum, including the shoulder at 785 nm. The positive contribution between 805 and 815 nm in the ADMR-detected T-S spectrum is not seen in the flash-induced T-S spectrum. The ADMR-detected T-S spectrum shows more structure than the flash-induced T-S spectrum, which can easily be explained by the fact that the selectivity with resonant microwaves used by ADMR allows better discrimination than flash spectroscopy when several triplets are present with triplet lifetimes in the same order of magnitude. Smit et al. [5] and Kleinherenbrink et al. [9] attributed the flash-induced T-S spectrum to the triplet state of the primary donor, formed by radical recombination. As the ADMR signal increases when the sample is frozen under illumination our measurements confirm this conclusion. The appearance of the acceptor signal at 670 nm, which is only seen in the spectra recorded at 473, 509 and 982 MHz, is further evidence that these transitions belong to the reaction center triplet. 

In purple bacteria a number of different lines of evidence led to the conclusion that bacteriopheophytin (BPh) acts as an electron carrier between the primary donor and Qa (Chapter 3). When is reduced illumination results in the photoaccumulation of reduced bacteriopheophytin, detected by its characteristic absorption changes and by an EPR signal split due to its interaction with Q Fe. At temperatures too low for rapid photoaccumulation of BPh to take place, illumination results in formation of a triplet state of the primary donor P-870 which has a polarization pattern characteristic of its formation by recombination of a radical pair. When BPh is reduced this triplet state cannot be formed. The most direct proof that BPh acts as a primary acceptor comes from the direct observation by absorption spectroscopy of BPh reduction within a few picoseconds after the flash. The BPh is reoxidized in 200 ps by electron transfer to or, if is already reduced, by recombination in 14 ns (see Chapter 3). 

Fig. 8. Kinetics of optical changes induced by 150-fs, 850-nm laser flashes (XoxJ in Rb. sphaeroides reaction centers. Changes are monitored at several wavelengths (top row) specifically absorbed by various pigment molecules ( tmon)- Solid traces are for the measured absorbance changes and the dotted lines represent the best fit corresponding to a relaxation time of 2.8-ps. Figure source Martin, Breton, Hoff, Migus and Antonetti (1986) Femtosecond spectroscopy of electron transform the reaction center of the photosynthetic bacterium Rhodopseudomonas sphaeroides R-26 Direct electron transfer from the dimeric bacteriochloro-phyllprimary donor to the bacteriopheophytin acceptor with a time constant of 2.8 0.2 ps. Proc Nat Acad Sci, USA. 83 958-960.

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