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A general kinetic model for catalysis by interfacial enzymes is developed. It couples the Michaelis-Menten catalytic turnover cycle at the interface with that in the aqueous phase through the distribution equilibria between the interface and the surrounding aqueous phase. Analysis under two limiting conditions fully describes the steady-state kinetics of hydrolysis and resolves the allosteric effects from apparent modes of interfacial activation in terms of the primary rate and equilibrium parameters for pig pancreatic phospholipase A2 (PLA2). One limit is observed in dispersions of anionic phospholipid vesicles, in which intervesicle exchange of enzyme, substrate, and hydrolysis products is absent and reaction occurs only on vesicles containing enzyme. A complete analysis at this highly processive limit, called kinetics in the scooting mode, has been published [Berg et al. (1991) Biochemistry 30, 7283]. Here is reported the analysis in the other limit, PLA2-catalyzed hydrolysis of zwitterionic micelles of short-chain phosphatidylcholines, at which substrate and products are in rapid exchange. Hydrolysis occurs either in bulk aqueous solution with phospholipid monomers or at the micellar interface. Above the critical micelle concentration (cmc), the hydrolysis rate shows a hyperbolic dependence on the bulk substrate concentration present as micelles. This dependence, characterized by the fitting parameters KMapp and VMapp, is analyzed in terms of the primary rate and equilibrium constants. The kinetic analysis is based on the assumption that the microscopic steady-state condition is satisfied because substrate replenishment in the micro-environment of the enzyme is fast relative to the catalytic turnover time. Added NaCl and anionic interface increase the hydrolysis rate in zwitterionic micelles dramatically. The overall interfacial rate enhancement is attributed to three factors: (a) promotion of PLA2 binding by net anionic charge of the interface, (b) enhancement of substrate affinity of PLA2 at the interface (Ks* allostery), and (c) stimulation of the rate-limiting chemical step (kcat* allostery).

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Product yields measured by high-performance liquid chromatography from chemical trapping of Cl -, Br -, and H2O by an aggregate-bound arenediazonium ion in cetyltrialkylammonium halide {(CTRA)X, R ) Me, Et, n-Pr, and n-Bu; X ) Cl, Br}, micelles are used to estimate, simultaneously, interfacial counterion, Xm, and water, H2Om, concentrations as a function of [(CTRA)X] and tetramethylammonium halide concentrations, [(TMA)X]. The results are interpreted by using a two-site pseudophase model. Values of Xm and H2Om are estimated by assuming that when the product yields from reaction of a long-tail aggregatebound arenediazonium ion in micelles are the same as the product yields from reaction of its short chain analogue in an aqueous quaternary ammonium ion salt solution, then Xm ) [Xw] and H2Om ) [H2Ow] in those solutions. The results show that Xm and H2Om are functions of headgroup size, surfactant concentration, and aqueous counterion concentration and type. Plots of Xm against [(CTRA)X] at a series of salt concentrations fall on separate curves. Xm increases gradually with added (CTRA)X and almost incrementally with added (TMA)X. However, plots of Xm and H2Om are essentially continuous functions of the aqueous counterion concentration, [Xw], at constant degree of micelle ionization, R. Three factors affect the shapes of these profiles. (a) An initial rapid increase in Xm is attributed to a salt-induced contraction of the micellar interfacial volume. (b) Above ca. 0.1 M [Xw], an incremental (slope of 1) increase in Xm with added counterion for (CTEA)Br, (CTPA)Br, and (CTBA)Br micelles is attributed to free movement counterions and co-ions between the interfacial region of the micelles (up to the micellar core) and the aqueous pseudophase. (c) Xm increases markedly for (CTMA)Br and (CTMA)Cl at their respective sphere-to-rod transitions ca. 0.1 M [Brw] and ca. 1.2 M [Clw]. The increases in Xm are accompanied by concomitant decreases in H2Om. Specific salt-induced rod formation is attributed to dehydration (partial) and tight ion pair formation between surfactant headgroups and counterions. Phase separation of (CTBA)Br micelles in 0.5 M (TMA)Br occurs when the interfacial water concentration is too low to maintain micelle stability. The dependence of Xm on [Xw] contradicts assumptions in the original pseudophase ion exchange model for aggregate effects on chemical reactivity that can be corrected, in part, by setting Xm equal to the sum the aqueous and interfacial counterion concentrations.

Two important and unsolved problems in the food industry and also fundamental questions in colloid chemistry are how to measure molecular distributions, especially antioxidants (AOs), and how to model chemical reactivity, including AO efficiency in opaque emulsions. The key to understanding reactivity in organized surfactant media is that reaction mechanisms are consistent with a discrete structures-separate continuous regions duality. Aggregate structures in emulsions are determined by highly cooperative but weak organizing forces that allow reactants to diffuse at rates approaching their diffusion-controlled limit. Reactant distributions for slow thermal bimolecular reactions are in dynamic equilibrium, and their distributions are proportional to their relative solubilities in the oil, interfacial, and aqueous regions. Our chemical kinetic method is grounded in thermodynamics and combines a pseudophase model with methods for monitoring the reactions of AOs with a hydrophobic arenediazonium ion probe in opaque emulsions. We introduce (a) the logic and basic assumptions of the pseudophase model used to define the distributions of AOs among the oil, interfacial, and aqueous regions in microemulsions and emulsions and (b) the dye derivatization and linear sweep voltammetry methods for monitoring the rates of reaction in opaque emulsions. Our results show that this approach provides a unique, versatile, and robust method for obtaining quantitative estimates of AO partition coefficients or partition constants and distributions and interfacial rate constants in emulsions. The examples provided illustrate the effects of various emulsion properties on AO distributions such as oil hydrophobicity, emulsifier structure and HLB, temperature, droplet size, surfactant charge, and acidity on reactant distributions. Finally, we show that the chemical kinetic method provides a natural explanation for the cut-off effect, a maximum followed by a sharp reduction in AO efficiency with increasing alkyl chain length of a particular AO. We conclude with perspectives and prospects.

Antioxidant (AO) efficiencies are reported to go through maxima with increasing chain length (hydrophobicity) in emulsions. The so-called "cutoff" after the maxima, indicating a decrease in efficiency, remains unexplained. This paper shows, for gallic acid (GA) and propyl, octyl, and lauryl gallates (PG, OG, and LG, respectively), that at any given volume fraction of emulsifier, the concentrations of antioxidants in the interfacial region of stripped corn oil emulsions and their efficiency order follow PG > GA > OG > LG. These results provide clear evidence that an AO's efficiency correlates with its fraction in the interfacial region. AO distributions were obtained in intact emulsions by using the pseudophase kinetic model to interpret changes in observed rate constants of the AOs with a chemical probe, and their efficiencies were measured by employing the Schaal oven test. The model provides a natural explanation for the maxima with increasing AO hydrophobicity.