Rate Of Back Reaction Research Articles

Pathway of proton transfer in bacterial reaction centers: replacement of glutamic acid 212 in the L subunit by glutamine inhibits quinone (secondary acceptor) turnover.

The mechanism of proton transfer in the reaction centers (RCs) from Rhodobacter sphaeroides was investigated by site-directed mutagenesis. Replacement of Glu-212 of the L subunit, a protonatable residue located near the secondary acceptor (QB) binding site, by glutamine reduced the in vitro electron turnover from cytochrome c to 2,3-dimethoxy-5-methylbenzoquinone (UQ0) by a factor of 25. The electron transfer rate to QB remained essentially unimpaired. Consequently, it is postulated that the reduced turnover in the mutant is due to a reduced rate of proton transfer to QB2-. The lack of pH dependence of the forward electron transfer rate DQA-QB----DQAQB- and the back reaction rate D+QAQB- ----DQAQB (where D = primary donor and QA = primary acceptor) in the mutant RC indicate that the observed pH dependence in the native RC is due to Glu-212, which has an anomalously high pKa value of 9.5 +/- 0.3. These results support the involvement of Glu-212 as a proton donor to reduced QB.

Equilibrium states of diffusion-limited reactions.

Diffusion-limited reactions A+B\ensuremath{\rightarrow}inert, with input A,B, or the black reaction (inert\ensuremath{\rightarrow}A+B), are studied. I have found that in equilibrium, segregation phenomena can occur below or in two dimensions, with typical size of A-rich and B-rich clusters \ensuremath{\xi}\ensuremath{\sim}1/${R}^{2/2\mathrm{\ensuremath{-}}d}$ for d2 and \ensuremath{\xi}\ensuremath{\sim}exp(1/${R}^{2}$) for d=2, respectively, where R is the A and B input rate or back-reaction rate.

Photoinduced hydrogen evolution in micellar system

Photoreduction of viologens by the irradiation of the system containing NADPH, zinc mesotetraphenylporphyrintrisulfonate (Zn-TPPS 3 3−), viologen and colloidal platinum has been investigated in the presence of surfactant micelles. In the presence of either cationic micelles or anionic micelles, a remarkable increase in the accumulation of the reduced form of viologen was observed. The existence of the micelles depressed both the quenching rate of the photoexcited Zn-TPPS 3 3− by viologen and the back reaction rate, recombination rate of the oxidized Zn-TPPS 3 3− and reduced viologen. Compared with both reactions, it was clarified that the recombination rate strongly influenced the viologen reduction rate. The effect of the micelles was explained by the electrostatic effect among the charges of the micellar surface, Zn-TPPS 3 3− and viologen. By the addition of colloidal platinum to the system, photoinduced hydrogen evolution was also studied.

Concentration and solvent effects on proline— p-benzoquinone complexes

Spectrophotometric studies have been used to show that the concentration of proline has a marked effect on its interaction with p-benzoquinone in pH 7 buffer. As the concentration is decreased, both the back reaction rate constant and the equilibrium constant increase whereas the entropy change decreases very rapidly. Similar effects are observed in a constant proline concentration but adding an increasing amount of dimethyl sulphoxide (DMSO). The extinction coefficient for the proline- p-benzoquinone complex is lower in the presence of DMSO. Extinction coefficients and equilibrium constants obtained by both Benesi—Hildebrand and Rose—Drago methods are identical. The interaction is shown to be first order with respect to each component and second order overall. The reaction is both temperature and dilution reversible. Rate constants and thermodynamic parameters have been calculated over a wide range of conditions.

Effects of non-electrolyte polymers on photoinduced electron transfer reactions. Influence of aqueous poly(vinylalcohol) on quenching of tetraanionic porphyrins by methyl viologen

The association of water soluble tetraanionic porphyrins with poly(vinylalcohol) (PVA) occurs in dilute solutions, as evidenced by absorption and emission spectral changes. Quenching of the emission of the free porphyrin fluorescence and the palladium(II) complex phosphorescence by methyl viologen is suppressed upon addition of PVA ( M = 78000). Flash excitation of the Pd(II) complex in solutions containing viologen (1 mM) and PVA ( > 0.05%) results in charge separation of photooxidized porphyrin and reduced viologen. The charge separation yield increases to a maximum of 15% and the back reaction rate decreases with the addition of polymer until a plateau is reached at approximately 0.3% PVA.

Photolysis of water over metallized powdered titanium dioxide

Photolysis of gas-phase water has been carried out on NaOH-coated Rh/TiO2 and Pd/TiO2 catalysts. Hydrogen and oxygen were simultaneously produced on both catalysts and the stoichiometric composition (H2/O2= 2) was obtained for most of the catalysts. The rate of water photolysis strongly depended upon the amount of NaOH loaded and the pressure of water vapour. The photolysis rate on the Rh/TiO2 catalyst was maximized at 15 wt % of NaOH coating and at ca. 2 Torr of water-vapour pressure. The photocatalytic activity depended upon the kind of metal deposited as well as the source of TiO2. Rh was more active but Pd was less active than Pt, which was tested previously. The highest formal quantum yield was ca. 29% when the Rh/TiO2 catalyst was used. The dependences of the photolysis rate on light intensity, wavelength and thermal back reaction rate have also been studied.

The distance dependence of electron transfer reactions: rate maxima and rapid rates at large reactant separations

It has been found that the dependences of the electronic coupling element and of the solvent reorganization energy on distance have important implications for intramolecular electron transfers and for the forward and back reaction rates and cage-escape yields in light-induced electron-transfer processes. An equation is presented for the net second order rate constant for bimolecular transfer. A derived difference in the optimum distance for the forward and back reactions is pointed out to have important implications for the formulation of steady-state quenching schemes and for the interpretation of cage-escape yields.

AMPHIPHILIC COPOLYMERS AS MEDIA FOR LIGHT‐INDUCED ELECTRON TRANSFER‐II. ELECTROSTATIC EFFECT ON THE BACK ELECTRON TRANSFER

Abstract— Photosensitized reduction of zwitterionic viologen (SPV) and methyl viologen (MV2+) was investigated using an amphiliphilic copolymer having phenanthryl and sulfonate groups (APh) as photosensitizer in aqueous solutions. In the presence of triethanolamine the accumulation of SPV * (photoproduct) was found to be faster than that of MV+. This attributed to the electrostatic repulsion between SPV. and anionic segments of APh. Such difference between SPV and MV2+ was minimized in the case of the related monomer model. Retardation of the back reaction for the APh‐SPV system was also demonstrated by laser photolysis, kb= 8.7 × 107M‐1 s‐1 for the polymer system as compared to kb= 2.8 × 109M‐1 s‐1 for the monomer model system. Strong salt‐effects on the yield of the photoreduction and the rate of back reaction confirm the strong electrostatic interaction between the photoproducts and polyanions. This remarkable electrostatic effect of the polyanions was simulated by electrochemical redox reactions by using a graphite electrode coated with APh.

Electron-transfer quenching of ruthenium(II) photosensitizers by mercury(II) in aqueous nitrate media

Excited-state interactions of tris(..cap alpha..-diimine)ruthenium(II) photosensitizers with Hg/sup 2 +/ were studied in aqueous nitrate media by using luminescence quenching and flash photolysis methods. Quenching proceeds via oxidative electron transfer to yield Ru(III) and a Hg(I) free radical with high efficiency. Regardless of the excited-state reducing power of the photosensitizer, quenching was near but below the Marcus diffusion-controlled limit. Dimerization of the Hg(I) free radical to give Hg/sub 2//sup 2 +/ competes effectively with the diffusion-limited back-electron-transfer reaction of the free radical with the Ru(III) species. The back-reaction rate of Hg/sub 2//sup 2 +/ and Ru(III) is much slower and depends on E/sup 0/ (Ru(III/II)). The efficiency of electron-transferred-product separation is sensitive to E/sup 0/ (Ru(III/II)). The mechanism of the oxidation of Hg/sub 2//sup 2 +/ by Ru(III) is discussed.

Diffusion and binding constants for acetylcholine derived from the falling phase of miniature endplate currents.

In previous papers we studied the rising phase of a miniature endplate current (MEPC) to derive diffusion and forward rate constants controlling acetylcholine (AcCho) in the intact neuromuscular junction. The present study derives similar values (but with smaller error ranges) for these constants by including experimental results from the falling phase of the MEPC. We find diffusion to be 4 X 10(-6) cm2 s-1, slightly slower than free diffusion, forward binding to be 3.3 X 10(7) M-1 s-1, and the distance from an average release site to the nearest exit from the cleft to be 1.6 micron. We also estimate the back reaction rates. From our values we can accurately describe the shape of MEPCs under different conditions of receptor and esterase concentration. Since we suggest that unbinding is slower than isomerization, we further predict that there should be several short "closing flickers" during the total open time for an AcCho-ligated receptor channel.

Use of 8-analino-1-naphthalene sulfonate as a monitor for possible phase transition involving water at low temperatures in photoreaction center from Rhodospirillum rubrum

The increase in the rate of the primary back reaction on cooling the photoreaction center from Rhodospirillum rubrum was interpreted in terms of a model in which the peculiar temperature dependence of the rate results from a phase transition involving water. The primary back reaction is defined as the return of the electron from the reduced primary ubiquinone to the oxidized bacteriochlorophyll molecules following illumination. The dye 8-anilino-1-naphthalene sulfonate was used to detect the state of the water solvent as it transforms on cooling from a liquid to a solid glass. We inferred from studies with air-dried films of photoreaction center that the water which may be responsible for the unusual temperature dependence of the rate of the primary back reaction is not on the surface but is bound within the photoreaction center protein.

A Tris-induced change in the midpoint potential of Z, the donor to photosystem II, as determined by the kinetics of the back reaction

A new method of measuring the rate of the back reaction from the state Z + P680 Q A − in Tris-washed chloroplasts is described. By using ratios of back reaction rates we demonstrate a Tris-induced change in the equilibrium between Z and P680 and attribute this change to an alteration of the midpoint potential of Z by Tris treatment. We also demonstrate that the previously observed inhibition of the back reaction by ADRY reagents can be localized at Z and understood in terms of electron donation to Z + by ADRY reagents.

Structure of Rhodopseudomonas viridis reaction centers. Absorption and photoselection at low temperature

The polarization excitation spectrum of Rhodopseudomonas viridis reaction centers is similar to those obtained with Rps. sphaeroides R-26 and 2-4-1 reaction centers. However, the absorption spectrum and photoinduced absorption changes are much better resolved, especially at low temperature, than in bacteriochlorphyll (BChl) a-containing reaction centers. This allows the separation of band shifts from bleaching in the light-induced difference spectra. In particular, the absorption band bleaching observed at 850 nm is shown to occur perpendicular to the 1000 nm long-wavelength band. This lends strong support to the assignment of the 850 nm band to the second excitonic component of the special pair. ‘Voyeur’ BChl absorption changes (presumably band shifts) upon photoformation of state P + Q − 1 are difficult to analyze quantitatively. The polarization values for these absorption changes are not easily reconcilable simply with absorption band shifts. We suggest that the relative orientation between the voyeur BChl transition moments and the long-wavelength transitions changes after charge separation. This reorientation may be responsible for the slow rate of back reaction between P + and Q − 1. At 4.2 K two bands are apparent in the long-wavelength absorption band. We propose that this structure is due to a charge-transfer state between the special pair and B, the primary BChl acceptor. Finally, we show that the low extinction coefficient of the absorption band at 808 nm, and the appearance of the long-wavelength band (1300 nm) for the neutral BChl molecule left after photooxidation of one molecule of the special pair, can be explained in terms of quantum mechanics.

Poly(pyridine)ruthenium(II)-photoinduced redox reactions of bipyridinium cations, poly(pyridine)rhodium complexes, and osmium ammines

The quenching of *RuL/sub 3//sup 2 +/ (L is a 2,2'-bipyridine or 1,10-phenanthroline derivative) emissions by three classes of oxidants Q has been examined. The results are discussed in terms of the Marcus electron-transfer model recast in a preequilibrium formalism. Substituted bipyridinium cations (methyl viologen and related compounds) undergo thermodynamically favorable reduction with a driving force ranging from 0.1 to 0.7 eV and rate constants in the range (0.4-2.0) x 10/sup 9/ M/sup -1/s/sup -1/, consistent with a diffusion rate constant of 2.0 x 10/sup 9/ M/sup -1/s/sup -1/ and an exchange rate constant of approx.10/sup 6/ M/sup -1/s/sup -1/ for the Q-Q couples. The yields of the separated redox products RuL/sub 3//sup 3 +/ and Q/sup -/ (typically 0.1 mol einstein/sup -1/ per quenching act) require K/sub 30/, the ''intramolecular'' back-reaction rate constant (to re-form ground-state RuL/sub 3//sup 2 +/ and Q), to be (2-4) x 10/sup 10/ s/sup -1/. Since K/sub 30/ increases weakly with driving force in this series, there is no evidence for inverted behavior despite the fact the ..delta..G/sup 0//sub 30/ is approx.-2 eV. With Q = Rh(4,4'-(CH/sub 3/)/sub 2/bpy)/sub 3//sup 3 +/, the quenching rate constants, k/sub q/ = (0.001-1.0) x 10/sup 9/ M/supmore » -1/s/sup -1/, exhibit a great sensitivity to the reducing power of *RuL/sub 3//sup 2 +/ and have been fitted to k/sub 11/ approx. = 2 x 10/sup 9/ M/sup -1/s/sup -1/ and E/sup 0//sub Q,Q/sup -// = -0.97 V for the RhL/sub 3//sup 3 +/-RhL/sub 3//sup 2 +/ couple. This E/sup 0/ value is strikingly similar to that (-0.9 V vs. aqueous SCE) obtained via cyclic voltammetry in acetonitrile. The cyclic voltammetry of the RhL/sub 3//sup 3 +/ complexes in water has been reexamined, and it is concluded that the irreversibility observed is due to ligand loss from rhodium(I).« less

The estimation of a factors for atom fissions in polyatomics

AbstractIt is shown that A factors for atom fissions in polyatomics can be reliably estimated by an improved version of the restricted free rotor model (RFRM). No adjustable parameters are used in the evaluation of A factors for the dissociation of the ROF bond (R: NO2, SF5, CF3, FSO2), and the authors' predictions are compared with high‐pressure kinetic data for these reactions. In the case of O2NOF, for which full spectroscopic information is available, RFRM reproduces the observed value Am well within a factor of 2. Similar agreement is reached for F5SOF and FO2SOF, assigning reasonable limits to their OF torsional frequencies. For F3COF, Am and the calculated back reaction rate constant, which is independent of the model, are both much too small to be plausible, suggesting falloff effects rather than a failure of RFRM.

Current efficiency and back reaction in aluminium electrolysis

The rate of back reaction in aluminium electrolysis is discussed in terms of mass transfer theory. The rate of dissolution of aluminium seems to be the most important step in this reaction. By using the physical data which are available in the literature, current efficiency variations have been calculated as a function of the most important parameters in the electrolytic cells. These calculated variations agree reasonably well with experimentally determined variations.

A SCIENTIFIC BASIS FOR REGULATION OF INDIRECT FOOD ADDITIVES FROM PACKAGING MATERIALS

ABSTRACTThe existence of nonlinear desorption isotherms of the Freundlich type have been demonstrated for very low concentrations of the monomers acrylonitrile and vinylchloride in their polymers. Furthermore, it is postulated and experimentally shown that for a finite time period, desorption ceases to be measurable since the free energy input requirement becomes too high for formation of a freely diffusable species. Thus, a thermo‐dynamically and kinetically “effective zero” migration state can be reached at finite but very low concentrations. This state results when the enthalpy for desorption is so high that the back reaction rate becomes too slow for measurement in real time.Thus, a definition of essentially zero migration can be made on a theoretical model supported by experimental evidence obtained at very low solute concentrations (below 1 ppm in polymer).

Kinetic behavior of antihapten antibody of restricted heterogeneity by stopped flow fluorescence polarization kinetics

Fluorescein, antifluorescein reactions have been studied by initial fluorescence polarization and intensity rate methods. Novel rapid stopped flow instrumentation (msec time range) as well as slower but more sensitive instrumentation has yielded second order rate constants over wide ranges in concentration and time. A bimolecular mechanism for the fluorescein, univalent antifluorescein system has been proposed in which two types of site interactions occur. This mechanism accounts for the fact that fluorescence polarization data consistently yield second order rate constants which are a factor of 10 higher than those obtained from fluorescence quenching data. Comparisons of kinetic behavior of free fluorescein and of fluorescein covalently attached to a normal IgG carrier reacting with Fab antifluorescein have suggested a “carrier effect” of a steric nature. The large equilibrium association constant for fluorescein, antifluorescein systems has been found to be due to extremely small values of the back reaction rate constants; i. e. 10 −3 to 10 −4 as great as those reported for DNP, anti DNP systems.