Reaction partitioning model

Mechanisms of Type III Polyketide Synthase Functional Diversity From Steric Modulation to the Reaction Partitioning Model... 

From these specific cases, we can construct a general reaction partitioning model for type III PKS cyclization specificify diat seems likely to ply to all non-reductive polyketide biosynthetic steps. In any iterative type III PKS multi-step reaction sequence, each successive polyketide intermediate has an increasing number of potentially competing reactions available to it, only one of which is on-pathway to any specific final product (See Figure 3). 

Figure 3. Simple schematic diagram illustrating the increasing intrinsic susceptability of each subsequent polyketide intermediate toward alternative competing reactions. Our Reaction partitioning model of diversity states that evolutionary divergence of PKS product specificity results from alternative partitioning of intermediates at key mechanistic branchpoints.

Kamens R, M Jang, K Leach (1999) Aerosol formation from the reaction of a-pinene and ozone using a gas-phase-kinetics-aerosol partitioning model. Environ Sci Technol 33 1430-1438. 

It has already been shown in Chapter 4 (section 4.2.1) that from the thermodynamic point of view the process described by Eq. (16.2) can be modeled by the sum of its partial processes (extraction steps), irrespective of whether they really proceed or not. That is because Gibbs free energy is the function of state and its total change does not depend on the reaction path. According to the complex formation-partition model [76], one can distinguish two main steps in extraction of metal ions ... 

Representative plots of Ma°/Mtot for the dimerization reaction with varying values of the distribution parameters Km and KP are given in Figure 1 as a semilogarithmic plot. Values of the distribution parameters were chosen here in relation to a gel partition model, where the only factor influencing distribution is a molecular size function that is, Kx... 

Numerical values for the structural parameters of QSAR are obtained from an evaluation of the effect of the substituent on the properties (e.g., the rate or equilibrium constants) of a model reaction. The classification of these parameters is therefore model dependent. The model reactions are chosen to represent the most pervasive types of physicochemical phenomena (e.g., dissociation reactions, hydrolysis, substitution reactions, partition between solvents). 

This revised model of PKS mechanistic regulation provides an improved framework for studying the functional divergence of evolutionarily related PKS enzymes. Our model posits that alternative product specificities from a given acyl-CoA starter must initially arise due to the selection of some mutation or set of mutations that specifically alters enzymatic reaction partitioning of key intermediates into other competing reaction pathways. Whether this relative kinetic effect is due to an increase or decrease of enzymatic control over the intrinsic reactivity of key intermediates may not be readily apparent from comparing the overall kinetics of evolutionarily refined CHS-like enzymes, as... 

Several models have been developed to describe reactions between aqueous ions and solid surfaces. These models tend to fall into two categories (1) empirical partitioning models, such as distribution coefficients and isotherms (e.g., Langmuir and Freundlich isotherms), and (2) surface-complexation models (e.g., constant-capacitance, diffuse-layer, or triple-layer model) that are analogous to solution complexation with corrections for the electrostatic effects at the solid-solution interface (Davis and Kent, 1990). These models have been described in numerous articles (Westall and Hohl, 1980 Morel, Yeasted, and Westall, 1981 James and Parks, 1982 Barrow, 1983 Westall, 1986 Davis and Kent, 1990 Dzombak and Morel, 1990). Travis and Etnier (1981) provided a comprehensive review of the partitioning and kinetic models typically used to define sorption of ions by soils. The reader is referred to the cited articles for details of the models. 

The Smith-Ewart nucleation models are based on free-radical partitioning between particles and micelles until all the surfactant is adsorbed on the surface of nucleated particles. The batch reaction nucleation model for this simple system is given by Equation (8.4),... 

Surfactants have also been of interest for their ability to support reactions in normally inhospitable environments. Reactions such as hydrolysis, aminolysis, solvolysis, and, in inorganic chemistry, of aquation of complex ions, may be retarded, accelerated, or differently sensitive to catalysts relative to the behavior in ordinary solutions (see Refs. 205 and 206 for reviews). The acid-base chemistry in micellar solutions has been investigated by Drummond and co-workers [207]. A useful model has been the pseudophase model [206-209] in which reactants are either in solution or solubilized in micelles and partition between the two as though two distinct phases were involved. In inverse micelles in nonpolar media, water is concentrated in the micellar core and reactions in the micelle may be greatly accelerated [206, 210]. The confining environment of a solubilized reactant may lead to stereochemical consequences as in photodimerization reactions in micelles [211] or vesicles [212] or in the generation of radical pairs [213]. 

The kinetic data are essentially always treated using the pseudophase model, regarding the micellar solution as consisting of two separate phases. The simplest case of micellar catalysis applies to unimolecTilar reactions where the catalytic effect depends on the efficiency of bindirg of the reactant to the micelle (quantified by the partition coefficient, P) and the rate constant of the reaction in the micellar pseudophase (k ) and in the aqueous phase (k ). Menger and Portnoy have developed a model, treating micelles as enzyme-like particles, that allows the evaluation of all three parameters from the dependence of the observed rate constant on the concentration of surfactant". ... 

The catalytic effect on unimolecular reactions can be attributed exclusively to the local medium effect. For more complicated bimolecular or higher-order reactions, the rate of the reaction is affected by an additional parameter the local concentration of the reacting species in or at the micelle. Also for higher-order reactions the pseudophase model is usually adopted (Figure 5.2). However, in these systems the dependence of the rate on the concentration of surfactant does not allow direct estimation of all of the rate constants and partition coefficients involved. Generally independent assessment of at least one of the partition coefficients is required before the other relevant parameters can be accessed. 

Table 5.2. Analysis using the pseudophase model partition coefficients for 5.2 over CTAB or SDS micelles and water and second-order rate constants for the Diels-Alder reaction of 5.If and 5.1g with 5.2 in CTAB and SDS micelles at 25 C.

In contrast to SDS, CTAB and C12E7, CufDSjz micelles catalyse the Diels-Alder reaction between 1 and 2 with enzyme-like efficiency, leading to rate enhancements up to 1.8-10 compared to the reaction in acetonitrile. This results primarily from the essentially complete complexation off to the copper ions at the micellar surface. Comparison of the partition coefficients of 2 over the water phase and the micellar pseudophase, as derived from kinetic analysis using the pseudophase model, reveals a higher affinity of 2 for Cu(DS)2 than for SDS and CTAB. The inhibitory effect resulting from spatial separation of la-g and 2 is likely to be at least less pronoimced for Cu(DS)2 than for the other surfactants. 

An emulsion model that assumes the locus of reaction to be inside the particles and considers the partition of AN between the aqueous and oil phases has been developed (50). The model predicts copolymerization results very well when bulk reactivity ratios of 0.32 and 0.12 for styrene and acrylonitrile, respectively, ate used. 

Table 10.4 lists the rate parameters for the elementary steps of the CO + NO reaction in the limit of zero coverage. Parameters such as those listed in Tab. 10.4 form the highly desirable input for modeling overall reaction mechanisms. In addition, elementary rate parameters can be compared to calculations on the basis of the theories outlined in Chapters 3 and 6. In this way the kinetic parameters of elementary reaction steps provide, through spectroscopy and computational chemistry, a link between the intramolecular properties of adsorbed reactants and their reactivity Statistical thermodynamics furnishes the theoretical framework to describe how equilibrium constants and reaction rate constants depend on the partition functions of vibration and rotation. Thus, spectroscopy studies of adsorbed reactants and intermediates provide the input for computing equilibrium constants, while calculations on the transition states of reaction pathways, starting from structurally, electronically and vibrationally well-characterized ground states, enable the prediction of kinetic parameters. 

The MOR is a surface-sensitive reaction [Clavilier et al., 1981a]. The influence of surface crystallography on the partitioning between the direct and the indirect pathways was investigated by Housmans and co-workers using electrochemistry, mass spectrometry, and modeling [Housmans et al., 2006 Housmans and Koper, 2003]. 

CO2 increased from 50% to 80%. It would be highly desirable to investigate MOR on model nonporous substrates with DBMS in order to determine the influence of particle size on various reaction steps and the partitioning between the direct and indirect pathways. 

Although the Lewis cell was introduced over 50 years ago, and has several drawbacks, it is still used widely to study liquid-liquid interfacial kinetics, due to its simplicity and the adaptable nature of the experimental setup. For example, it was used recently to study the hydrolysis kinetics of -butyl acetate in the presence of a phase transfer catalyst [21]. Modeling of the system involved solving mass balance equations for coupled mass transfer and reactions for all of the species involved. Further recent applications of modified Lewis cells have focused on stripping-extraction kinetics [22-24], uncatalyzed hydrolysis [25,26], and partitioning kinetics [27]. 

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