Vesicular rate constant

This limited amount of kinetic evidence suggests that the kinetic models developed for reactivity in aqueous micelles are directly applicable to reactions in vesicles, and that the rate enchancements have similar origins. There is uncertainty as to the appropriate volume element of reaction, especially if the vesicular wall is sufficiently permeable for reaction to occur on both the inner and outer surfaces, because these surfaces will have different radii of curvature and one will be concave and the other convex. Thus binding, exchange and rate constants may be different at the two surfaces. 

Most of the characteristics invoked to explain rate accelerations and rate retardations by micelles are valid for vesicles as well. For example, the alkaline hydrolysis of A-methyl-A-nitroso-p-toluenesulfonamide is accelerated by cationic vesicles (dioctade-cyldimethylammonium chloride). This rate acceleration is the result of a higher local OH concentration which more than compensates for the decreased polarity of the vesicular pseudophase (compared to both water and micelles) resulting in a lower local second-order rate constant. Similar to effects found for micelles, the partial dehydration of OH and the lower local polarity are considered to contribute significantly to the catalysis of the Kemp elimination " by DODAB vesicles. Even the different... 

Chaimovich and coworkers have prepared large unilamellar vesicles of DODACl by a vaporization technique which gives vesicles of ca 0.5 pm diameter. These vesicles are much larger than those prepared by sonication, where the mean diameter is 30 nm, and their effects on chemical reactivity are very interesting. The reaction of p-nitrophenyl octanoate by thiolate ions is accelerated by a factor of almost 10 by DODACl vesicles (Table 2), but this unusually large effect is due almost completely to increased concentration of the very hydrophobic reactants in the small region of the vesicular surface and an increased extent of deprotonation of the thiol. There is uncertainty as to the volume element of reaction in these vesicles, but it seems that second-order rate constants at the vesicular surface are similar to those in cationic micelles or in water (Cuccovia et al., 1982b Chaimovich et al., 1984). 

Figure 19 Dependence of the observed rate constant (ktp) for the cleavage of PNPP on the ligand concentration in different vesicular blends (25 °C, [copper(II)] = 1.8xlO- M), (20) and 2Ci6Br, A CTABr, O 2C,6GlyBr, A (21) and 2C (,Br, 2C]5GlyBr, . The inset shows the time course of the absorbance increase observed upon addition of Cu(NO3)2 to the two different vesicular blends (20), (21), O- Only vesicles made of (20) and 2CigBr show clearly biphasic behavior. The fast uptake of copper(II) by the ligand on the outer layer of the bilayer is followed by a much slower process, probably copper(II) permeation...

It will be clear that encapsulation of the reactant(s) is a prerequisite for vesicle-induced rate effects. In case of unimolecular organic reactions, the change in rate constant relative to the rate constant in bulk aqueous solution, is determined by the change in reaction environment going from water to the reactant binding sites in the vesicle. Several studies have suggested that the reaction environment in the vesicular phase is often less polar than that in micelles. Of course, the kinetic effect is a function of the distribution of the reactant over the aqueous and vesicular pseudophases. If the medium effects on the reaction are understood in some detail, the vesicle-induced rate effects provide information about the nature of the reactant encapsulation process. 

Akinetic study was also performed in a variety of vesicular solutions (DDAB, DODAB, DODAC [NaOH] = 2.25mM, 25 °C). Interestingly, the vesicles possess stronger catalytic reaction environments than the micelles. The rate-determining proton transfer from carbon to the hydroxide ion was accelerated up to 850 fold in di- -dodecyldimethylam-monium bromide (DDAB) vesicles. This is evidence that the reaction sides are less aqueous than those in micelles, as anticipated. Application of the pseudophase model afforded the bimolecular rate constants in the vesicles (kves). For the different vesicles, ves is significantly higher (ca. 12 times for DODAB) than the second-order rate constant in water. This shows that the catalysis is due to both a medium effect and a concentration effect. It was assumed that there was a fast equilibrium for substrate binding to the inner and outer leaflets of the bilayer, in accord with the fact that no two-phase kinetics were found. 

Addition of cholesterol leads to two counteracting effects on the rate constants. The first is a smaller counterion binding, reducing the rate constants. The other is a rate enhancing effect resulting from the less polar vesicular binding sides. The overall effect depends on the exact reaction conditions. ... 

Rate constants for the reactions in the presence of overall positively charged vesicles are about ten times larger than those in the absence of vesicles. The effect was ascribed to an increase in the reactivity of water. In case the water molecules at the vesicular interphase are in part replaced by the glucose groups of CiaGlu, the catalytic efficiency of the vesicles decreases significantly. 

The bimolecular rate constants for the reaction with bronude ions are smaller at the vesicular interphase, and independent of the presence of CiaGlu. These results indicate that this reaction, not involving water as a reactant, is not sensitive towards partial dehydration of the binding sites of the organic substrate. 

If initial solute uptake rate is determined from intestinal tissue incubated in drug solution, uptake must be normalized for intestinal tissue weight. Alternative capacity normalizations are required for vesicular or cellular uptake of solute (see Section VII). Cellular transport parameters can be defined either in terms of kinetic rate-time constants or in terms of concentration normalized flux [Eq. (5)]. Relationships between kinetic and transport descriptions can be made on the basis of information on solute transport distances. Note that division of Eq. (11) or (12) by transport distance defines a transport resistance of reciprocal permeability (conductance). 

As the vesicular concentration is equivalent to tnoles/volume and V = 4/3nr, so r is proportional to the cubed root of the vesicle content [8,20], Thus, the data for each release event can be replotted as the frequency of release events versus the cubed root of vesicular amount (in moles). A second explanation suggests that a lognormal distribution of vesicular amount will result if multiplicative deviations from the mean occur such as if integral differences in the number of uptake transporters per vesicle result in multiplicative transmitter accumulation rates [21], Thus, release event data can also be plotted as the frequency of release events vs. the log transform of quantal size. When the amperometricaUy recorded data are plotted after either of the aforementioned transformations, a dramatically different histogram with a nearly gaussian distribution results that is said to reflect the vesicular size distribution. Small deviations in the gaussian shape are possibly due to variations in the vesicular concentration, which is assumed to be constant. 

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