Rise kinetics

Figure 5.2. Grabowski s model of TICT formation in DMABN the locally excited (LE) state with near-planar conformation is a precursor for the TICT state with near perpendicular geometry. The reaction coordinate involves charge transfer from donor D to acceptor A. intramolecular twisting between these subunits, and solvent relaxation around the newly created strong dipole. Decay kinetics of LE and rise kinetics of the TICT state can be followed separately by observing the two bands of the dual fluorescence. For medium polar solvents, well-behaved first-order kinetics are observed, with the rise-time of the product equal to the decay time of the precursor, but for the more complex alcohol solvents, kinetics can strongly deviate from exponentiality, interpretable by time-dependent rate constants. 52 ...

Figure 6 (a) Transient absorption spectra for NOSI5 irradiated with 388-nm 180 fsec light in 1-butanol, (b) Transient rise kinetics for NOSI5 in 1-butanol. 

Time profiles of the formation of fullerene radical anions in polar solvents as well as the decay of 3C o obey pseudo first-order kinetics due to high concentrations of the donor molecule [120,125,127,146,159], By changing to nonpolar solvents the rise kinetics of Go changes to second-order as well as the decay kinetics for 3C o [120,125,133,148], The analysis of the decay kinetics of the fullerene radical anions confirm this suggestion as well. In the case of polar solvents, the decay of the radical ion absorptions obey second-order kinetics, while changing to nonpolar solvents the decay obey first-order kinetics [120,125,127,133,147]. This can be explained by radical ion pairs of the C o and the donor radical cation in less polar and nonpolar solvents, which do not dissociate. The back-electron transfer takes place within the ion pair. This is also the reason for the fast back-electron transfer in comparison to the slower back-electron transfer in polar solvents, where the radical ions are solvated as free ions or solvent-separated ion pairs [120,125,147]. However, back-electron transfer is suppressed when using mixtures of fullerene and borates as donors in o-dichlorobenzene (less polar solvent), since the borate radicals immediately dissociate into Ph3B and Bu /Ph" [Eq. (2)][156],... 

Fig. 5 shows a kinetic model of the bacteriorhodopsin photocycle. It accounts quantitatively for time-resolved difference spectra measured between 100 ns and 100 ms after flash excitation at temperatures between 5 and 30°C. The kinetic coimection between M, L, and K through reversible reactions introduces two additional time constants into the rise kinetics of M, and the reverse reactions which connect M, N, and O likewise affect the decay of M. An important part of this scheme is the existence of two consecutive M substates, Mi and M2, connected by an irreversible reaction [114]. 

The mutant, B6, was obtained following FdUrd treatment of wild type cells (6). Genetic analysis indicated that B6 is a chloroplast mutant (Roitgrund and Mets, in preparation). Measurements of fluorescence induction showed typical fluorescence rise kinetics as expected for cells possesing an active photosystem II, but impaired in the oxidation of reduced plastoquinol (Fig. i). To test the possibility that the cytochrome subunits are present but inactive, cyt. bjf components were assayed by TMBZ heme staining and by immunoblotting with specific antibodies. 

Schapendonk and Vredenberg (A.H.C.M. Schapendonk et al. 1979), this P515 bandshift has components contributed by two different reactions called reaction 1 and reaction 2 (insert Fig. 1). Reaction 1 is characterised by a fast (ps) rise and a subsequent single exponential dark decay with a half life of about 75 ms (under these conditions). The response obtained by subtracting reaction 1 from the overall P515 response has been attributed to that of reaction 2. The rise kinetics of reaction 2 show a slow absorbance increase within 150 ms after the flash and a subsequent dark decay with a first order rate constant of the order of 1 s K... 

The bimodal profile observed at low catalyst concentration has been explained by a combination of two light generating reactive intermediates in equihbrium with a third dark reaction intermediate which serves as a way station or delay in the chemiexcitation processes. Possible candidates for the three intermediates include those shown as "pooled intermediates". At high catalyst concentration or in imidazole-buffered aqueous-based solvent, the series of intermediates rapidly attain equihbrium and behave kineticaHy as a single kinetic entity, ie, as pooled intermediates (71). Under these latter conditions, the time—intensity profile (Fig. 2) displays the single maximum as a biexponential rise and fall of the intensity which is readily modeled as a typical irreversible, consecutive, unimolecular process ... 

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