Active patches

One test that was carried out when trying to compare levamisole receptor properties in SENS and LEVR isolates was to count the numbers of patches of recorded membranes that contained active channels. Figure 21.7 shows the proportion of patches from SENS and LEVR isolates that contained active patches at different concentrations of levamisole. It was... 

Fig. 21.7. Percentage of active patches at each levamisole concentration for SENS ( ) and LEVR ( ) parasites. The difference between SENS and LEVR is significant (chi-squared P < 0.001).

Plasma nicotine samples showed no difference dne to body weight in blood nicotine concentration following active patch pretreatment, as expected, confirming eqnal dosing across snbjects. 

FROM "ACTIVE PATCHES" TO MEMBRANE TRANSPORT PROTEINS... 

As a general rule, the presence of active patches does not alter the X-ray diffraction pattern of a metal. This may be only because the patches on the surface, are of insufficient thickness to give an X-ray pattern of their own—it does not prove that their structure is identical with the normal space-lattice of a metal. Electron diffraction, which reveals the structure of much smaller thicknesses of solids than X-rays, indicates that possibly there may be a difference in structure between catalytically active and inactive platinum.2 It also shows that the structure of metals on supports of other metals, or of other materials, may not be the same as the normal structure of these metals alone8 and thus promoters and supports for metals may influence their catalytic power by altering the spacing of the atoms in the metal on the surface. 

Linear, one-dimensional interfaces. Perhaps the most interesting theory of promoter action is that the linear boundary on the surface between two surfaces of different composition is the seat of the catalysis, i.e. the active patch. There is abundant evidence that such linear interfaces often possess unusual reactive powers. They are undoubtedly... 

Schwab and Ketsch4 have developed a theory of the kinetics of contact catalysis based on the assumption that the active patches are located at such interfaces, using the term adlineation for adsorption there but though their assumption seems a very probable one, at any rate for many cases of promoter action, it is scarcely possible to test it by comparison of rates of reaction with a theoretically derived equation there are too many adjustable constants, and other factors. One of these is the rate of surface diffusion to the active linear interfaces, or other active patches, which as many writers have pointed out, may be very considerable.5... 

Such surface diffusion provides a mechanism whereby the active patched, covering but a small fraction of the surface, may be supplied with molecules of a reacting gas very much more rapidly than they could be supplied by direct hits from the vapour phase surface diffusion (see Chap. VI, 4) may bring molecules up to any kind of active patch. 

Catalysis by solid surfaces is often very specific, however. The molecular architecture of the active patches is certainly a very important factor in catalysis. This may be accounted for by deformation of the molecules caused by the adsorption.3 Suppose, for instance, that the... 

The inevitable conclusion from these last two observations seems to be that the peculiar structure of the active patches of copper atoms is built up after, not before, the decomposition has occurred, and that it is the result of sudden freezing of the liberated copper atoms, within a time which is probably extremely short, following their liberation as the compound decomposes. The temperature of decomposition controls the amount of mobility possessed by these atoms after liberation, and the structures formed seem to be controlled by the cohesive forces between the copper atoms, binding them into different formations which are determined by a delicately adjusted balance between the motions of the copper atoms, the time during which they remain mobile, and the possible space-lattice structures and orientations which the atoms can assume. 

The surface will certainly contain crystal fragments of copper, and perhaps also some amorphous material,1 in all possible orientations. The activating power of the various parts of the surface may well depend on the spacing of the surface atoms of copper, on which the alcohol molecules are adsorbed, and on the amount of distortion (electrical or mechanical) imposed on the adsorbed molecules as they are made to fit the surface of the crystalline fragments in their various orientations. This distortion probably lowers the additional amount of energy, or the heat of activation, required to activate the adsorbed molecules. Though the heat of activation on any particular active patch could not be determined, the... 

By varying the method of preparation of the catalyst, the relative rates of the two reactions could be much altered partial sintering of the copper seems to aid the isomeric change. Clearly, then, different active patches are involved in the two reactions. 

Blood charcoal was found to have about 1 per cent, of the carbon-iron surface of fiftyfold activity, but apparently none of the still more active iron-carbon-nitrogen surface, which is somewhat surprising, as there is plenty of nitrogen in this charcoal. It was estimated, from the percentage of iron in this charcoal, that there was about one iron atom to every six atoms of carbon in the most active patches of the blood charcoal. This ratio of six to one suggested that the iron atom may promote the catalytic activity by reason of a combination of the co-ordinated kind but this is scarcely proved as yet. 

It was shown, by a process of selective poisoning identical in principle with that used on solid catalysts, that the activations occur at specially active patches, and that these patches have specificity, or the power of discriminating between different substrates, through two separate mechanisms the power of a given patch to adsorb different substrates depends on their chemical constitution, so that only a few substances are adsorbed and of those substances which are adsorbed, only a small proportion are activated. 

This class of poisons appears to act simply by competing with the substrate for the space available for adsorption on the active centres. If the concentration of the poison is diminished and that of the substrate increased, the activation is restored. Indeed the poison and the substrate probably compete with one another on approximately equal terms for adsorption. The difference lies in the fact that the active patches on a toluene-treated organism can only activate one (or a few) out of the class of substances which are adsorbed. 

This points to two separate factors in the specificity of the activating mechanism. The structure of the active patch must first be one which will adsorb the particular substance to be activated, and second, it must activate the substance, once adsorbed. Emil Fischer s comparison of the action of an enzyme on its substrate, to the highly specific relation between a lock and key, now seems capable of being analysed further the key must not only be capable of entering the keyhole (adsorption), but it must be capable, once inserted, of operating the mechanism inside (activation). 

Narcotics suppress many activities of living cells and they also inhibit the action of some enzymes.1 Their action may be an adsorption on the active patches, or possibly they may, through their solubility in lipoid substances, enter into the structure underlying the active patches and distort these so that they can no longer perform their specific functions. 

And finally, a certain amount of information is being obtained as to the growth of these active patches or enzymes if bacteria are grown in the presence of certain substances, they tend to develop the power of acting on these substances.2... 

The actions of these poisons can be diminished by using large amounts of the substrate, showing that the poisons and the substrates compete for adsorption on the active patches. 

The original view that the mechanism of activation is mainly due to electrostatic fields between the active patches and the specifically fitting adsorbed molecules, appears rather too simple at the present time the reader should consult the original papers (a, f) for details. Whatever be the mechanism, an electrical activation of a simple nature, or an elaborate quantum mechanical deformation, it is certainly fairly specific, and its further investigation should yield a rich reward, and be perhaps no more difficult than the investigation of the mechanism of activation by inorganic catalysts. In the case of these more complex catalysts, one has the weapon of a controlled variation of chemical constitution, which is not always readily available for researches on inorganic catalysts. 

McKinnie J. Transdermal market the future of transdermal drug delivery relies on active patch technology. Drug DeUv. Tech. 2006 6 54-58. 

In a theoretical paper, Loffler and SchmidL have considered the effect of surface heterogeneity on catalyst effectiveness and selectivity under external mass transfer limiting conditions. Their model assumes that only part of the surface is active, active patches being separated by completely inactive surface. They show that the effect of such heterogeneity on selectivity depends on the reaction scheme. A consecutive reaction scheme can yield higher concentrations of... 

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