Eosin rates

However, some data have been more difficult to incorporate into the mechanism shown in Figs. 8 and 9. As reported 21) in Section II,B the Fe protein can be reduced by two electrons to the [Fe4S4]° redox state. In this state the protein is apparently capable of passing two electrons to the MoFe protein during turnover, although it is not clear whether dissociation was required between electron transfers. More critically, it has been shown that the natural reductant flavodoxin hydroquinone 107) and the artificial reductant photoexcited eosin with NADH 108) are both capable of passing electrons to the complex between the oxidized Fe protein and the reduced MoFe protein, that is, with these reductants there appears to be no necessity for the complex to dissociate. Since complex dissociation is the rate-limiting step in the Lowe-Thorneley scheme, these observations could indicate a major flaw in the scheme. 

Approximate Rates of Intersystem Crossing (sec.-1) for Eosin Di-Anion and Proflavine Hydrochloride... 

It will have been obvious from the previous discussion that the data available for eosin and proflavine hydrochloride are far less complete than could be desired. More extensive data in various solvents as a function of temperature (including the region below 77 °K.) would allow more reliable values of intersystem crossing rates to be calculated. It would be interesting to compare these rates for a variety of dyestuffs, or... 

The delayed fluorescence produced by triplet-triplet quenching is to be sharply differentiated from that observed with eosin or proflavine hydrochloride. The latter type has the same lifetime as the triplet and its intensity is proportional to the first power of the rate of light absorption. It is produced by thermal activation of molecules from the triplet level to the excited singlet level and can occur with any substance for which... 

Fluorescein is an energy acceptor for chromophores such as naphthalene and anthracene and acts as energy donor toward Eosin and Rhodamine, so derivatives have been used for singlet-singlet energy transfer studies. According to Forster s theory [68] the rate constant for energy transfer increases... 

Determination of translational diffusion rates of proteins requires measurements at longer timescales, one-tenth of a second to several minutes. Eosin derivatives are also commonly used to measure translational diffusion coefficients using the Fluorescence Recovery After Photobleaching technique [138-141],... 

Photooxidation of Eosin with periodate ion has been used to initiate the polymerization of acrylonitrile in aqueous solution [187]. Addition of acrylonitrile to a periodate solution shifts the absorption maximum from 220 to 280 nm. This spectral change is interpreted as being due to complex formation between the monomer and oxidizing agent. The rate of photopolymerization increases linearly with the absorbed light intensity and monomer concentration. The observed intensity dependence indicates the main chain terminator is not produced photochemically. Polymer is not formed when the concentration of periodate ion is lower than 0.5 mM and the rate of polymerization is independent of its concentration for higher values. 

Table 3 summarizes our results obtained with Eosin lactone using different exposures and incident laser intensities. These results indicate that the rate of growth of the spike increases linearly with the incident intensity. The data show that the polymerization depth depends on the total incident energy and not on the rate at which the energy is delivered. The system is said to obey reciprocity since irradiation with a high intensity for a short period of time produces the same effect as irradiation with a low intensity for a long time, provided the total energy delivered is the same in both cases. 

We, as well as Chesneau and Fouassier, find that the photospeed increases linearly with light intensity. From this observation one can conclude that chain termination reaction is not the usual interaction between two macroradicals. We have measured the initial rate of photopolymerization using thin foil calorimetry and find a linear relationship between the rate of photopolymerization at low conversions (less than 15%) and the absorbed light intensity. Using the same monomer but with a different photoinitiator (to be discussed in detail later) we observe an order of one half with respect to light intensity both by thin foil calorimetry and by measuring the polymer spike. Therefore we conclude that the linear dependence observed for the Eosin-triethanolamine system is real and not an artifact of the technique employed to determine the photospeed. 

We have performed laser flash photolysis experiments in ethyl acetate/20% methanol under conditions in which triplet-triplet annihilation is not important. We achieve these conditions by using Eosin concentrations higher than 1 x 10 5 M and low pulse energies in order to obtain clean exponential decays for the triplet. Determination of the triplet lifetime at different Eosin and amine concentrations allows us to obtain the rate constants collected in Table 5. A summary of our observations is as follows ... 

TABLE 5 Triplet Decay Rate Constants for Eosin in Ethyl Acetate/20% Methanol"... 

It has been suggested in the literature that the a-amino radical is the species that initiates polymerization [210], This view is supported by our observation that, in spite of the relatively high quenching rate constant of Eosin triplet by triphenylamine (Table 5), the system Eosin-triphenylamine does not sensitize the photopolymerization of multifunctional acrylates. Thus, it is necessary that the amine contains a hydrogen at the a-carbon to be released as a proton after oxidation of the amine by the dye triplet. This deprotonation prevents the back electron transfer and forms a carbon radical that is sufficiently long-lived to be captured by the monomer. 

Recently Fouassier and Chesneau [219] studied the photochemistry of the system Eosin-PDO-MDEA in aqueous acetonitrile using steady-state irradiation and laser flash photolysis. The photopolymerization of methyl methacrylate (MMA) sensitized by the photoreduction of Eosin is investigated in acetronitrile to understand the mechanism of initiation and the enhancement in the rate of polymerization caused by the presence of PDO, 3. Rates, quantum yields of photopolymerization, and number average molecular weights of the polymer are determined with MMA (7 M), Eosin (3 x 10 5 M), and MDEA (0.1 M) in the presence and in the absence of 2 x 10-3 M PDO. 

Irradiation in the presence of MDEA completely inhibits the formation of products. The amine quenches the fluorescence of Eosin with a rate constant of 8 x 108 M-1s-1 and quenches the Eosin triplet with a rate two orders of magnitude lower. A summary of rate constants for the decay of the triplet is presented in Table 8. In addition to the reactions shown in Scheme 3, with Am = (V-methyl diethanolamine, the rate constants for reaction of PDO with Eosin triplet and semioxidized Eosin radical in aqueous solution (Eqs. 19 and 20) are included in the table. 

To explain the reduced rate of termination observed in the presence of PDO, the authors propose that one of the termination reactions involves the protonated form of the semireduced Eosin radical which, by reaction with PDO, is reduced to the leuco dye (Eqs. 21 and 22). 

Fouassier and Chesneau [219] is not consistent with the experimental observations. From the values of the rate constants of triplet decay presented in Table 8, and taking into account that k3/k2 = 0.23 (as determined by Kasche and Lindqvist), we calculate the quantum yield of D + under the polymerization conditions. For Eosin (3 x 10 5 M) and MDEA (0.1 M) the yield of semioxidized Eosin radical is 4 x 10 3 M in the presence or in the absence of 2 x 10 3 M PDO. From the values for the quantum yield of photopolymerization and the molecular weight in the absence of PDO we calculate a quantum yield of initiation between 0.086 and 0.17, the actual value depending on the mode of termination. Therefore, we conclude that formation of a-amino radicals according to Scheme 10 represents only a minor contribution to the quantum yield of initiation observed in the presence of PDO. 

The decrease in the termination rate is attributed to the efficient capture of semireduced Eosin radical by diphenyliodonium. The rate constant of the reaction shown in Eq. (30) was determined by laser flash photolysis as 1 x 1010 M-1s-1. 

Diphenyliodonium, at a concentration of 0.05 M, does not quench the Eosin triplet. It decreases the fluorescence intensity by a factor of 20 and shifts the fluorescence wavelength 10 nm toward the red. The calculated rate constant for singlet quenching is 5 times larger than for a diffusion controlled... 

Fig. 12.3. Epidermal measurements, mitotic figures, and apoptotic keratinocytes in a chronic proliferative dermatitis mutant (Sharplncpdm/Sharplncpdm) mouse. Routine hematoxylin- and eosin-stained paraffin histologic sections can be used to determine proliferation rates based on mitotic index (number of mitotic figures, circled in the figure, in the stratum basale per 1000 cells) or the presence and numbers of apoptotic epidermal keratinocytes (dotted arrows) when present. Epidermal thickness can be measured at high dry magnification (40x) to include the malpigian, living cell, layer (M), the stratum corneum thickness (SC), or the full thickness of the epidermis (M+SC).

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