Quantum yield kinetic factors

In photosynthetic systems, some electron transfer processes exhibit nonexponential kinetics at low temperature, which are generally attributed to the existence of different conformations of the system. While the differences between the reaction rates corresponding to these conformations do not exceed a factor of four in some cases [157,158,159], they are sufficient to lead to different quantum yields in others [160, 161]. Sometimes, the heterogeneous character of the kinetics disappears at room temperature, which probably reflects a fast exchange between the conformations that are frozen at low temperature [157, 158]. A systematic study of all these effects, similar to that performed in Ref [159], could give useful information about the nature of the conformational differences. 

The Norrish Type II reaction of aliphatic and aromatic ketones in isotropic solvents has been studied in considerable detail (26,43), and several aspects of the reaction depend on the conformational mobility of the excited ketone or the 1,4-biradical intermediates formed by y-hydrogen abstraction. In the case of aromatic ketones for example, the triplet lifetime can provide an indication of the facility with which the proper geometry for hydrogen abstraction can be obtained (29,43), the distribution of fragmentation O-cleavage) and cyclization products obtained depends on the conformations available to the triplet 1,4-biradical intermediate and their relative kinetic behavior prior to intersystem crossing (27-30,43-47), and the total quantum yield for the reaction is a function of both of the above factors. For practical reasons, product ratios are usually the easiest aspect of the reaction to monitor, and this is the approach that has been used most commonly in studies of Norrish II reactivity in ordered media (27-30,45). The pertinent features of the triplet state reaction arc illustrated in Scheme 1 (30). 

If constant intensity of irradiation with time is assumed, only the photo-kinetic factor F has to be included in the time variable. It depends on the progress of the reaction normally, as stated above depending on the absorption of the sample. Then the product of irradiation intensity and photochemical quantum yield forms a constant equivalent to the rate constant of thermal reactions. The dependent variable is a product of the factor F and the irradiation time t combined as a variable 0. This introduction of a transformation in the time axis allows formal kinetics to be applied to thermal and photochemical reactions as well. It even allows the handling of solutions which cannot be homogenised, or solid samples in which the concentration varies locally because of decreasing irradiation intensity in the direction of irradiation by the turnover of the reactants. 

In this work, the solutions of human serum albumin (HSA) (>96%, Sigma) and of bovine serum albumin (BSA) (>98%, MP Biomedicals) in a phosphate buffer (0.01 M, pH 7.4) have been used. The proteins concentrations were lO- (absorption spectra measurement) and 10- M (fluorescence measurement at the nanosecond laser fluorimeter). All of the experiments were performed at a temperature of 25 1 °C. The structure and biological functions of HSA and BSA can be found in (Peters, 1996). Tryptophan, tyrosine, and phenylalanine (with relative contents of 1 18 31 in HSA and 2 20 27 in BSA) are the absorption groups in these proteins (as in many other natural proteins). The tyrosine fluorescence in HSA and BSA (as in many other natural proteins) is quenched due to the effect of adjacent peptide bonds, polar groups (such as CO, NH2), and other factors, and phenylalanine has a low fluorescence quantum yield (0.03) (Permyakov, 1992). Therefore, the fluorescence signal in these proteins is determined mainly by tryptophan groups. In that case the fluorescence, registered in nonlinear and kinetic laser fluorimetry measurements, correspond to tryptophan residues (this fact will be used in Section 6.1). 

We have performed detailed quantitative numerical simulations based on the above qualitative ideas. These simulations which closely follow those published by us for the tetrazine/argon system,2b generate quantitative predictions of the spectral quantum yields and the kinetic behavior of all observed features. The calculated and observed intensities agree quite well -within a factor of two. The calculated and observed kinetics are also in good agreement - again within a factor of two in nearly all instances. 

Quantitative predictions of the model can be made from the simulation procedure described above. The results of these simulations, in terms of kinetics and spectral quantum yields, are compared with the experimental results in Tables I and II. In all cases the agreement is quite good within a factor of about two. 

Apart from this kinetic factor, a thermodynamic factor can also be important. If the catalytic reaction in question is thermodynamically favorable, many catalytic cycles can occur for every active site generated and therefore for every photon absorbed by the complex. In this way the quantum yield (C>) for the reaction can be very large, the light merely initiating the reaction. If the reaction is thermodynamically unfavorable, then the role of the light may be to supply the energy deficit that would otherwise prevent the reaction from taking place. In this case, O will probably be less than unity because each cycle will require a photon. 

If we assume a kinetic chain length of 100, which is a reasonable value, we get a quantum yield for chain initiation of 7 x 10 ". P increases with decreasing temperature from 300 K to 180 K by nearly a factor of five. If TS is perdeuterated P also increases by a factor of 2.5 [44, 54]. 

---