Reaction rates, vesicles

The permeability coefficient of 2.6x 10 locm/s at 296 K measured by Deamer is sufficient to supply the enzyme in the liposomes with ADP. How could it be shown that RNA formation actually does take place in the vesicles The increase in the RNA synthesis was detected by observing the fluorescence inside the vesicles. In the interior of the liposomes, the reaction rate is only about 20% of that found for the free enzyme, which shows that the liposome envelope does limit the efficiency of the process. The fluorescence measurements were carried out with the help of ethidium bromide, a fluorescence dye often used in nucleic acid chemistry. 

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. 

These microdroplets can act as a reaction medium, as do micelles or vesicles. They affect indicator equilibria and can change overall rates of chemical reactions, and the cosurfactant may react nucleophilically with substrate in a microemulsion droplet. Mixtures of surfactants and cosurfactants, e.g. medium chain length alcohols or amines, are similar to o/w microemulsions in that they have ionic head groups and cosurfactant at their surface in contact with water. They are probably best described as swollen micelles, but it is convenient to consider their effects upon reaction rates as being similar to those of microemulsions (Athanassakis et al., 1982). 

Phospholipid vesicles (and bilayers) composed of phospholipids with well-defined fatty acid side chains undergo a sharp transition from a crystallinelike state to an amorphous state as the temperature is raised.107 The transition temperature depends on the nature of the fatty acid side chains. For example, for C12 saturated fatty acid chains on lecithin the transition temperature is 0° and for C18 saturated fatty acid chains it is 58°C for unsaturated lecithins the transition temperature is below zero.107 For real membranes sharp phase transitions are not observed, because of the heterogeneous composition of the membrane. In the case of /3 hydroxybutyrate dehydrogenase, the enzymic activity apparently is not influenced by this phase transition as judged by the temperature dependence of the reaction rate. However, for some membrane-bound proteins, a plot of the reaction rate versus the reciprocal temperature... 

On the anti-HSA-IgG-spacer-AL-2 latex, 16,000 antibodies were attached. The forward rate constant reduced for one antibody molecule (k =k /number of antibodies on the latex surface) was estimated to be 810 M s, which is much smaller than the observed reaction rate constant for the antigen-antibody reaction in the homogeneous system. Wolff et al. reported that 1.3 molecules of IgG per vesicle is enough for interaction of the vesicle with antigen-carrying cells (2i) ... 

The rate of a chemical reaction taking place inside a solitary giant liposome (or any other reactor) can be controlled by decreasing and increasing the vesicle volume, where a larger (smaller) volume leads to a lower (higher) reactant concentration (given a constant number of molecules) and hence, reduced (increased) reaction rates. As an example, consider the second-order equilibrium reaction... 

A simple method to measure the membrane permeability to specific molecules has been presented by G. Battaglia and coworkers [141], The authors encapsulated highly hydrophilic 3,3, 3//-phosphinidynetris-benzenesulfonic acid (PH) into polyethylene oxidc)-co-poly(butylene oxide) (EB) vesicles and monitored its reaction with 5,5/-dithiobis-2-nitrobenzoic acid (DTNB) penetrating the membrane from the exterior. The reaction rate (amount of the formed product as a function of time after DTNB addition) measured with IJV/Vis was directly correlated to the permeability of the permeating molecule. A comparison of these results with the permeability of egg yolk phosphatidylcholine (PC) vesicles showed that EB membranes have a more selective permeability toward polar molecules than the phospholipids membranes. Also in this case the permeability appeared to depend on the membrane thickness as predicted by Fick s first law. 

In Chapter 29, Bunton presents a brief review of micellar effects on nucleophilicity, and he describes recent work of his own in this area. A major contribution of Bunton s has been his development of a quantitative model for calculating nucleophile concentration in the pseudophase of the micelle thus, calculation of rate constants in the micelle is possible. Using this model, Bunton finds that the reaction rates in micelles are very similar to those in water. Thus, micellar accelerations result from reactant concentration. Bunton notes that this conclusion also applies to microemulsions, vesicles, and inverse micelles. A second important contribution of this chapter is a summary of the large amount of experimental work on the contrasting effects of cationic and anionic micelles on reactions of anionic and neutral nucleophiles and on hydrolyses. 

We recently showed that low-frequency alternating currents (ac) through Na, K-ATPase vesicle suspensions change enzyme activity (21). The ac signals decrease adenosine 5 -triphosphate (ATP) splitting by the normal enzyme, with the maximum effect at about 100 Hz, and increase the enzyme activity when the activity is lowered in different ways, including the introduction of ouabain. Both inhibition and activation by ac signals can be explained by variations in ion activation (22, 23). The frequency dependence is related to the ion mobilities and reaction rate constants in the electrical double layers (20). 

M. K. Kawamuro, H. Chaimovich, E. B. Abuin, E. A. Lissi, 1. M. Cuccovia, Evidence that the effects of synthetic amphiphile vesicles on reaction-rates depend on vesicle size, J. Phys. Chem., 1991, 95, 1458-1463. 

Now, in order to make the reaction possible, the second reactant Y must be added. In Figure 17.4A it is shown the addition of Y in the external phase, followed by the diffusion of Y inside the vesicles. The final state is the product Z, synthesized inside the vesicle. This way is the simplest one, but requires that the Y permeability coefficient differs from zero. Moreover, if the entrance of Y in the vesicles becomes rate-limiting, the observed reaction rate will be limited by this slow step. 

Abstract Vesicles prepared with synthetic amphiphiles constitute useful microreactors, where reaction rates can be delicately controlled. Here we review our work on quantitative analysis of reaction rates in vesicles and show that reaction at several vesicular sites can be probed and controlled. Vesicles prepared with dialkyldimethylammonium halides, (DODA)X, can accelerate bimolecular reactions by more than a million fold. Quantitative analysis of the vesicular effect on ester thiolysis, using a pseudophase ion exchange formaUsm, suggests that the rate increase is primarily due to reagent concentration in the bilayer and interfacial effects on ion distribution, as well as contributions from enhanced nucleophile reactivity. Vesicle-containing solutions exhibit a variety of potential reaction sites the inner and outer surfaces, bilayer and internal aqueous compartment. 

Site dissection and reagent distribution has been accomplished, in several cases. For this purpose we have prepared vesicles of various sizes, determined some of their physical properties and developed theoretical and experimental tools for probing vesicular sites. The reactivity of OH in the internal compartment is identical to that in bulk solution. Moreover, the reaction rates of OH at the inner and outer vesicular interface are also comparable. However, since OH" permeation through the bilayer can limit reaction rate, the relative inner/outer rate ratios can be controlled by changing the composition of the external and/or internal medium. 

Key words Vesicles - reaction sites in vesicles - kinetic analysis in vesicles - vesicular catalysis - reaction rate -control with vesicles... 

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