Reaction-transport coupling

D. The Role of Metabolism and Intestinal Secretion Reaction-Transport Coupling... 

Meyerhoff and Rechnitz (1976) developed a potentiometric creatinine sensor by inclusion of creatinine iminohydrolase between the gas-permeable membrane of an ammonia electrode and a dialysis membrane. Since the specific activity of the enzyme used was very low, 0.1 U/mg, only 43 mU could be entrapped at the electrode. Therefore the sensor was kinetically controlled and reacted to addition of the enzyme activator tripolyphosphate by an increase in sensitivity from 44 mV to 49 mV per concentration decade and a corresponding decrease of the detection limit. These effects agree with theoretical considerations of reaction-transport coupling. The samples were treated with a cation exchanger to remove endogenous serum ammonia. 

In the second approach, the spatially homogeneous chemical slow manifold is used, and the method must somehow accoxmt for reaction-transport coupling. For a chemical timescale to be defined as fast in a reactive flow system, the Damkohler number, which is defined as the ratio of the flow timescale tf and the chemical timescale Tc, must be large ... 

The phosphorylation of cytoplasmic sugar and the facilitated diffusion from the cytoplasm to the periplasm are catalyzed by the E-IIs under conditions where they are also active in the vectorial phosphorylation reaction. Therefore, the former two activities should be integral parts of any kinetic scheme representing the mechanism of E-IIs. Such a scheme should explain how vectorial phosphorylation, transport coupled to phosphorylation, is still achieved while the uncoupled pathways are integral parts of the scheme. 

Shaw, J., Turner, C., Miller, B., Harper, M., Reaction and transport coupling for liquid and liquid/gas microreactor systems, in Ehreeld, W., Rinaed,... 

The reduction of O2 in W by hydroquinone derivatives (QH2) in O is a subject of interest, since the reaction might offer the fundamental information on the electron transport coupled with the proton transport at a biomembrane realized by the respiration [2,3,56]. 

Morrison et al. (1995) investigated the adsorption of on ferrihydrite and coupled the data with a reaction/transport model for contaminated groundwater which included economic factors. 

Morrison, S. J., Tripathi, V. S. Spangler, R. R. 1995. Coupled reaction/transport modeling of a chemical barrier for controlling uranium (VI) contamination in groundwater. Journal of Contaminant Hydrology, 17, 347-363. 

Detailed overviews of all aspects of multiphase mass transport coupled with reactions are given elsewhere [6],... 

Recovery of metals such as copper, the operation of batteries (cells) in portable electronic equipment, the reprocessing of fission products in the nuclear power industry and a very wide range of gas-phase processes catalysed by condensed phase materials are applied chemical processes, other than PTC, in which chemical reactions are coupled to mass transport within phases, or across phase boundaries. Their mechanistic investigation requires special techniques, instrumentation and skills covered here in Chapter 5, but not usually encountered in undergraduate chemistry degrees. Electrochemistry generally involves reactions at phase boundaries, so there are connections here between Chapter 5 (Reaction kinetics in multiphase systems) and Chapter 6 (Electrochemical methods of investigating reaction mechanisms). 

When there is no net electron flow — that is, when no water is consumed and no oxygen is produced — the electrons from reduced ferredoxin are transported back to chlorophyll (or an associated unidentified compound) through a series of reactions which are not yet known. Electron transport in these reactions is coupled to the synthesis of ATP from ADP + Pi by cyclic photophosphorylation (Eq. 9). 

This final reaction accounts for most of the known oxygen consumption by aerobic organisms. The cascade of redox reactions that couples the oxidation of organic substrates to reduction of molecular oxygen in biological systems is called electron transport, and is often presented schematically as shown in Figure 14-1. When substrates are oxidized by such a system, the rate and extent of substrate oxidation is directly dependent on, and can be measured by, the decrease in concentration of molecular oxygen, as will be done in this experiment. 

Enzyme-catalyzed reactions, including the electron transport chain and proton translocation, are composed of series of elementary reactions that proceed forward and backward. One of the methods in describing this thermodynamically and mathematically coupled complex chemical reaction-transport system is the nonequilibrium thermodynamic model, which does not require the detailed knowledge of the system. 

SQR (respiratory complex II) is involved in aerobic metabolism as part of the citric acid cycle and of the aerobic respiratory chain (Saraste, 1999). QFR participates in anaerobic respiration with fumarate as the terminal electron acceptor (Kroger, 1978 Kroger etal., 2002) and is part of the electron transport chain catalyzing the oxidation ofvarious donor substrates (e.g., H2 or formate) by fumarate. These reactions are coupled via an electrochemical proton potential (Ap) to ADP phosphorylation with inorganic phosphate by ATP synthase (Mitchell, 1979). 

In many cases the electron-exchange reaction and coupled reactions are slow at the potential at which the transport of electrons to and from the electrode is equal, i.e., the reversible potential, and it is then necessary to apply an extra potential, an overvoltage, to obtain a reasonable rate of reaction. The overvoltage is dependent on many parameters, and is has not been possible to predict it on theoretical grounds. The potentials to be used in electrolysis may thus be found empirically, e.g., from current-voltage curves of micro-electrodes. 

Equations (19)-(24) completely define the CO/H2 reaction-transport model by describing reactant concentrations within catalyst pellets and along the reactor position. They also form a system of coupled ordinary equations and boundary value problems in the unknowns x and y the equivalent dimensional form and details about its numerical solution can also be found elsewhere (4,5). 

Coenzyme Q is a quinone derivative with a long isoprenoid tail. The number of five-carbon isoprene units in coenzyme Q depends on the species. The most common form in mammals contains 10 isoprene units (coenzyme Qio) For simplicity, the subscript will be omitted from this abbreviation because all varieties function in an identical manner. Quinones can exist in three oxidation states (Figure 18.10). In the fully oxidized state (Q), coenzyme Q has two keto groups. The addition of one electron and one proton results in the semiquinone form (QH ). The semiquinone form is relatively easily deprotonated to form a semiquinone radical anion (Q ). The addition of a second electron and proton generates ubiquinol (QH2), the fiilly reduced form of coenzyme Q, which holds its protons more tightly. Thus, ybr quinones, electron-transfer reactions are coupled to proton binding and release, a property that is key to transmembrane proton transport. 

After it became clear that the reduction of CH3-S-C0M to CH4 consists of two reactions, one of these, the reduction of the heterodisulfide of CoM-S-H and H-S-HTP with H2 (Reaction 7 of Table 2), was considered to be the coupling site for ATP synthesis [14,71], Indeed, it was shown that everted vesicles of G61 also catalyzed the reduction of the heterodisulfide with H2 or with chemically reduced factor F420, F420H2, to H-S-CoM and H-S-HTP and that this reaction was coupled with the synthesis of ATP via the mechanism of electron transport phosphorylation [114-117] (Fig. 5) (i) the reduction of the heterodisulfide was associated with primary proton translocation at a ratio of up to 2H /CoM-S-S-HTP proton translocation was inhibited by protonophores rather than by DCCD (ii) reduction of the heterodisulfide was stimulated by protonophores and inhibited... 

The mechanism by which the exergonic methyl-transfer reaction is coupled with vectorial electrogenic Na translocation across the membrane is not known. An electron transport chain appears not to be involved in Na transport. Sodium ion transport... 

An updated and expanded version of PHREEQC (version II) was published by Parkhurst and Appelo (1999). Version II has all of the capabilities of version I, and includes new routines for kinetically controlled reactions, solid-solution equilibria, fixed-volume gas-phase equilibria, variation of the number of exchange or surface sites in proportion to a mineral or kinetic reactant, diffusion or dispersion in one-dimensional (ID) transport, ID transport coupled with diffusion into stagnant zones, and isotope mole balance in inverse modeling. 

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