Quenching of excited chlorophyll A in vivo by nitrobenzene

Effect of Solvent Quality on Laminar Slip Flow Penetration of Poly(N-isopropylacrylamide) Films with an Exploration of the Mass Transport Mechanism

The effect of solvent quality on the slip flow penetration of polymer films was evaluated by monitoring small-molecule mass transport under varying laminar flow rates using Förster resonance energy transfer in combination with total internal reflectance fluorescence microscopy (FRET-TIRFM). For thin films of poly(N-isopropylacrylamide) (pNIPAM), solvents with solvent quality ranging from good to poor were studied. The solvents used were composed of varying mole ratios of methanol and water in order to take advantage of the unique cononsolvency phenomenon of pNIPAM such that differences in the physicochemical properties of these solvents were insignificant for fluorescence detection. FRET quenching of a donor fluorophore covalently tethered on the substrate surface at the bottom of the pNIPAM film by a solution-confined acceptor was monitored as a function of time. Quenching curves were fit to a combined Taylor-Aris-Fickian mass transport model for the acceptor, rhodamine B (RhB) or 2-nitrobenzylaclohol (2-NBA), allowing apparent diffusion coefficients to be determined and used to assess slip flow penetration into the polymer film. An increase in the apparent diffusion coefficient of tracer molecules was observed with increasing laminar flow rate for all solvents, indicating that mass transport processes in the pNIPAM film are significantly perturbed by laminar slip flow penetration. In going from poor solvents, 31 mol % MeOH/H₂O and 20 mol % MeOH/H₂O, to the theta solvent, 13 mol % MeOH/H₂O, and finally to a good solvent, 100% methanol, the slip length increases from 25 to 37 to 70 to 128 nm, with the corresponding percentage of the film penetrated by slip flow increasing from 19 to 27 to 42 to 57%, respectively. The apparent diffusion coefficients of the two acceptors, RhB and 2-NBA, which differ substantially in size and charge, in pNIPAM films under identical conditions were found to be of the same order of magnitude, albeit with a small difference (∼10%) due to inherently different diffusive properties. Therefore, the dominant mechanism for the mass transport of small molecules in densely grafted thin pNIPAM brush films is suggested to be linear Fickian diffusion under the chosen laminar flow conditions with linear flow velocities ranging from 192 to 2952 μm/s. High-quality fits to a Taylor-Aris-Fickian diffusion model of the experimental breakthrough curves obtained with both acceptor molecules further substantiate the proper use of this model and the validity of the FRET-TIRFM method.

A critical review of the literature on nitrobenzene toxicity

This literature review encompassing information available through 1980 and limited coverage in 1981, emphasizes results useful in assessing the potential toxic effects of nitrobenzene to man. Nitrobenzene exposure in man or experimental animals is most often associated with methemoglobinemia. Histopathologic changes also are observed in the hemato-lymphoreticular system, central nervous system, and liver. In addition, lesions have been reported in adrenals and testes. No information was found on carcinogenic or teratogenic potential, fertility, or reproductive effects of nitrobenzene. Results from Ames Salmonella assay are negative but test procedures are questionable; metabolites give positive results. Metabolism of nitrobenzene involves either oxidation or reduction yielding p-aminophenol and p-nitrophenol, and other reduced intermediates. From the foregoing, several aspects of nitrobenzene toxicity have been identified which warrant further study. Recommendations are made for chronic and subchronic exposure of test animals via inhalation to assess various toxicological endpoints. In addition, the relationship of nitrobenzene metabolism to its toxicity needs to be established. Genotoxic effects of nitrobenzene also need study.

Transfer and trapping of excitation energy in photosystem II as studied by chlorophyll alpha 2 fluorescence quenching by dinitrobenzene and carotenoid triplet. The matrix model

1. The curves representing the reciprocal fluorescence yield of chlorophyll alpha of Photosystem II (PS II) in Chlorella vulgaris as a function of the concentration of m-dinitrobenzene in the states P Q and P Q-, are found to be straight parallel lines; P is the primary donor and Q the primary acceptor of PS II. In the weakly trapping state P Q- the half-quenching of dinitrobenzene is about 0.2 mM, in vitro it is of the order of 10 mM. The fluorescence yield as a function of the concentration of a quencher is described for three models for the energy transfer between the units, and the matrix model. If it is assumed that the rate constant of quenching by dinitrobenzene is high and thus the number of dinitrobenzene molecules per reaction center low, it can be concluded that the pigment system of PS II in C. vulgaris is a matrix of chlorophyll molecules in which the reaction centers are embedded. Theoretical and experimental evidence is consistent with such an assumption. For Cyanidium caldarium the zero fluorescence yield phi 0 and its quenching by dinitrobenzene were found to be much smaller than the corresponding quantities for C. vulgaris. Nevertheless, our measurements on C. caldarium could be interpreted by the assumption that the essential properties (rate constants, dinitrobenzene quenching) of PS II are the same for these two species belonging to such widely different groups. 2. The measured dinitrobenzene concentrations required for half-quenching in vivo and other observations are explained by (non-rate-limiting) energy transfer between the chlorophyll alpha molecules of PS II and by the assumptions that dinitrobenzene is approximately distributed at random in the membrane and does not diffuse during excitation. 3. The fluorescence kinetics of C. vulgaris during a 350 ns laser flash of variable intensity could be simulated on a computer using the matrix model. From the observed fluorescence quenching by the carotenoid triplet (CT) and the measurement of the the number of CT per reaction center via difference absorption spectroscopy, the rate constant for quenching of CT is calculated to be kT = 3.3 . 10(11)s-1 which is almost equal to the rate constant of trapping by an open reaction center (Duysens, L.N.M. (1979) CIBA Foundation Symposium 61 (New Series), pp. 323--340). 4. The fluorescence quenching by CT in non-treated spinach chloroplasts after a 500 ns laser flash (Breton, J., Geacintov, N.E. and Swenberg, C.E. (1979) Biochim, Biophys. Acta 548, 616--635) could be explained within the framework of the matrix model when the value for kT is used as given in point 3. 5. The observations mentioned under point 1 indicate that the fluorescence yield phi 0 for centers in trapping state P Q is probably for a fraction exceeding 0.8 emitted by PS II.