Quantum yield total

J. M. Tadic, 1. O. Juranic, and G. K. Moortgat. Photooxidation of n-heptanal in air Norrish type 1 and 11 processes and quantum yield total pressure dependency, J. Chem. Soc. Perkin Trans., 2 135-140 (2002). 

The quantum yield of photosynthesis, the amount of product formed per equivalent of light input, has traditionally been expressed as the ratio of COg fixed or Og evolved per quantum absorbed. At each reaction center, one photon or quantum yields one electron. Interestingly, an overall stoichiometry of one translocated into the thylakoid vesicle for each photon has also been observed. Two photons per center would allow a pair of electrons to flow from HgO to NADP (Figure 22.12), resulting in the formation of 1 NADPH and Og. If one ATP were formed for every 3 H translocated during photosynthetic electron transport, 1 ATP would be synthesized. More appropriately, 4 hv per center (8 quanta total) would drive the evolution of 1 Og, the reduction of 2 NADP, and the phosphorylation of 2 ATP. 

Fig. 3.3.2). The total light elicitable (or the quantum yield of coelenterazine) is nearly constant in a wide range of pH, 6.0 to 10.0, and falls off at both ends. 

To measure the total light emission of a photoprotein sample, it was necessary to add 1-5 pM Fe2+ several times due to the short effective life of Fe2+ under the conditions involved. The total amount of light measured in this manner was always proportional to the weight of photoprotein used, with both CPA and CPC (4.7 x 1013 photons/mg at 25°C Shimomura and Johnson, 1968d). Thus, the quantum yield is estimated at roughly 0.01 for CPA, and 0.015 for CPC. 

To assay the amount of LBP, first an excess amount of luciferin is added to the sample at pH 8 to saturate the binding site of LBP, and then the excess luciferin is removed by gel filtration using a small column of Sephadex G-25 (about 1 ml volume) also at pH 8. The luciferin-bound LBP is eluted at the void volume. To measure the amount of LBP, the following assay buffer is added to a small portion of the elu-ate 0.2 M phosphate, pH 6.3, containing 0.25 mM EDTA, 0.1 mg/ml of BSA, and luciferase (Morse and Mittag, 2000). The total light obtained represents a relative amount of LBP the absolute amount (the weight or the number of molecules) cannot be obtained because the quantum yield of the luminescence reaction is not known. 

There are methods available to quantify the total mass of americium in environmental samples. Mass spectrometric methods provide total mass measurements of americium isotopes (Dacheux and Aupiais 1997, 1998 Halverson 1984 Harvey et al. 1993) however, these detection methods have not gained the same popularity as is found for the radiochemical detection methods. This may relate to the higher purchase price of a MS system, the increased knowledge required to operate the equipment, and the selection by EPA of a-spectrometry for use in its standard analytical methods. Fluorimetric methods, which are commonly used to determine the total mass of uranium and curium in environmental samples, have limited utility to quantify americium, due to the low quantum yield of fluorescence for americium (Thouvenout et al. 1993). 

A series of experiments investigated the effect of laser pulse intensity on the distribution of damage. For each pulse intensity, DNA samples were exposed to three different doses. The quantum yield for the formation of lesions, expressed with respect to total DNA bases, was then calculated by linear regression analyses. At all intensities, the formation of lesions was found to be linear with respect to the applied dose. Oxidized nucleosides, including... 

The overall OD vibrational distribution from the HOD photodissociation resembles that from the D2O photodissociation. Similarly, the OH vibrational distribution from the HOD photodissociation is similar to that from the H2O photodissociation. There are, however, notable differences for the OD products from HOD and D2O, similarly for the OH products from HOD and H2O. It is also clear that rotational temperatures are all quite cold for all OH (OD) products. From the above experimental results, the branching ratio of the H and D product channels from the HOD photodissociation can be estimated, since the mixed sample of H2O and D2O with 1 1 ratio can quickly reach equilibrium with the exact ratios of H2O, HOD and D2O known to be 1 2 1. Because the absorption spectrum of H2O at 157nm is a broadband transition, we can reasonably assume that the absorption cross-sections are the same for the three water isotopomer molecules. It is also quite obvious that the quantum yield of these molecules at 157 nm excitation should be unity since the A1B surface is purely repulsive and is not coupled to any other electronic surfaces. From the above measurement of the H-atom products from the mixed sample, the ratio of the H-atom products from HOD and H2O is determined to be 1.27. If we assume the quantum yield for H2O at 157 is unity, the quantum yield for the H production should be 0.64 (i.e. 1.27 divided by 2) since the HOD concentration is twice that of H2O in the mixed sample. Similarly, from the above measurement of the D-atom product from the mixed sample, we can actually determine the ratio of the D-atom products from HOD and D2O to be 0.52. Using the same assumption that the quantum yield of the D2O photodissociation at 157 nm is unity, the quantum yield of the D-atom production from the HOD photodissociation at 157 nm is determined to be 0.26. Therefore the total quantum yield for the H and D products from HOD is 0.64 + 0.26 = 0.90. This is a little bit smaller ( 10%) than 1 since the total quantum yield of the H and D productions from the HOD photodissociation should be unity because no other dissociation channel is present for the HOD photodissociation other than the H and D atom elimination processes. There are a couple of sources of error, however, in this estimation (a) the assumption that the absorption cross-sections of all three water isotopomers at 157 nm are exactly the same, and (b) the accuracy of the volume mixture in the... 

The photodecarbonylation of a series of dibenzyl ketones was studied by Robbins and Eastman/63 The results of this study are presented in Table 4.5. The data in Table 4.5 indicate that the presence of a p-methyl or a p-methoxy group has little effect on the quantum yield for this reaction. p-Cyano groups, on the other hand, essentially totally eliminated the decarbonylation. Since the reaction could also be quenched (inefficiently) by benzonitrile or biphenyl, it was concluded that the decarbonylation occurs from a short-lived triplet state. The effect of the p-cyano groups then could result from internal triplet quenching. 

Quantum Yields (p) for Product Formation. The quantum yields for formation of the corresponding photoproducts b (arylamine), c (ortho photo-Fries), and d (para photo-Fries) upon photolysis of la-4a are given in Table II. In each case, for photolysis at 254 nm the sum (S>total) of the quantum yield for photoproducts (fy,... 

The spectra are a product of the number of molecules (N), the laser intensity t (at /.(x), the (excitation wavelength dependent) extinction coefficient e , the quantum yield Q, the corrected emission spectra F(A), and the instrument response or gain g(X) that is typically wavelength dependent. If we have Ns molecules that show FRET with an efficiency E, and ND (total) donor molecules and Na (total) acceptor molecules, Eq. (8.A1) can be rewritten ... 

In total, the situation is less favorable in terms of quantum yields than in the preceding case. Indeed, even if the cleavage rate constant is large enough to overcome back electron transfer from the ion pair (kc k act), the maximal value of the quantum yield is 1/(1 + P), which would reach unity only in the unlikely case where the ground-state electron transfer was entirely non-adiabatic (H = 0). 

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