Effect of Inhibitors

Separate the metal from the environment with a physical barrier. Many corrosion inhibitors make use of this principal to protect metals. Proper use of an appropriate inhibitor may reduce or eliminate pitting. Pits are frequently initiation sites for corrosion-fatigue cracks. The effectiveness of inhibitors depends upon their application to clean metal surfaces. An example of this method is the use of zinc coatings on steel to stifle pit formation. 

The corrosion rig has been used to study the effect of inhibitors e.g. silicate and phosphate commonly used to overcome problems with iron. This has revealed that these inhibitors hardly affect the long-term corrosion rate, indeed in certain circumstances they may actually increase it. They produce their effect by stabilising the corrosion product developed, thereby preventing the water quality deterioration which is the real complaint... 

THE MECHANISM OF CORROSION PREVENTION BY INHIBITORS Effects of Inhibitors on Corrosion Processes... 

The effects of adsorbed inhibitors on the individual electrode reactions of corrosion may be determined from the effects on the anodic and cathodic polarisation curves of the corroding metaP . A displacement of the polarisation curve without a change in the Tafel slope in the presence of the inhibitor indicates that the adsorbed inhibitor acts by blocking active sites so that reaction cannot occur, rather than by affecting the mechanism of the reaction. An increase in the Tafel slope of the polarisation curve due to the inhibitor indicates that the inhibitor acts by affecting the mechanism of the reaction. However, the determination of the Tafel slope will often require the metal to be polarised under conditions of current density and potential which are far removed from those of normal corrosion. This may result in differences in the adsorption and mechanistic effects of inhibitors at polarised metals compared to naturally corroding metals . Thus the interpretation of the effects of inhibitors at the corrosion potential from applied current-potential polarisation curves, as usually measured, may not be conclusive. This difficulty can be overcome in part by the use of rapid polarisation methods . A better procedure is the determination of true polarisation curves near the corrosion potential by simultaneous measurements of applied current, corrosion rate (equivalent to the true anodic current) and potential. However, this method is rather laborious and has been little used. 

Temperature effects may also be used in test methods and notably for assessing the effects of inhibitors in acid solutions. The technique is based on that first proposed by Mylius which records the temperature-time behaviour associated with the exothermic reaction resulting from the initial contact of a metal with a corrosive acid solution. The effectiveness of inhibitors may then be determined from their effects on the temperaturetime behaviour. ... 

The effectiveness of inhibitors is measured in terms of the rate constant ratio kz/kp and the stoichiometric coefficient. The stoichiometric coefficient is the moles of radicals consumed per mole of inhibitor. These parameters may be determined by various methods. A brief description of the classical kinetic treatment for evaluating k7/kp follows. Consider the reaction scheme shown which describes ideal inhibition and retardation (Scheme 5.11). 

The simple two-reactor series shown in Figure 1 will be analyzed to demonstrate the effect of inhibitor on the performance of continuous systems. Since inhibitor will be present in the continuously added feed stream, it will serve to reduce the effective initiation rate in the first reactor. Since inhibitor is very reactive with free radicals, all inhibitor fed must be destroyed before significant reaction can take place. Thus the effective rate of initiation in the first reactor is given by Equation 1. 

Lie TJ, W Godchaux, JR Leadbetter (1999) Sulfonates as terminal electron acceptors for growth of sulfite-reducing bacteria (Desulfitobacterium spp.) and sulfate-reducing bacteria effects of inhibitors of sulfidogenesis. A / Environ Microbiol 65 4611-4617. 

Stimulation fluids to minimize this corrosion. The effectiveness of inhibitors can be estimated with laboratory screening methods [279]. 

Receptor Expression Ligand PI3K activation Effect of inhibitors on T-cell migration... 

Table 3.3 Effects of inhibitors of different modalities on the apparent values of steady state kinetic constants and on specific steps in catalysis... 

Mutual exclusivity can also be tested for by the effects of combinations of two inhibitors on the activity of a target enzyme. The advantage of this approach is that it does not require any special labeling of either compound, and only catalytic quantities of enzyme are required for the studies. There are a number of graphical methods that can be used to determine the effects of inhibitor combinations on enzyme velocity (see Copeland, 2000). The most popular of these was introduced by Yonetani and Theorell (1964) and is based on the following reciprocal equation ... 

Thus, by analysis of the effects of inhibitor concentration on kobs, we can obtain estimates of the affinity of the inhibitor for both the initial and final conformational states of the enzyme. 

Petersen LC. 1977. The effect of inhibitors on the oxygen kinetics of cytochrome c oxidase. Biochem Biophys Acta 460 299-307. 

Figure 9. Effect of inhibitor concentration on wznmax f°r various inhibitors in 15% HC1 at 65 C.

D. H. Rich, and E. T. Sun, Mechanism of inhibition of pepsin by pepstatin. Effect of inhibitor structure on dissociation constant and time-dependent inhibition, Biochem. Pharmacol. 29 2205 (1980). 

It is not yet known whether the increase of these activities in the course of fruit ripening is due to a direct synthesis of the enzymes, or whether conversion of inactive precursors into the active enzymes is involved. There is also the possibility of (a) an effect of inhibitors present in unripe fruit on the activity of endo-D-galacturonanase, and (b) disappearance of such inhibitors in the course of ripening.184 For pectinesterase, it has been speculated that plants may be able to regulate the activity of this enzyme by metabolic control of the amount and proportion of certain, naturally occurring, fatty acids.82... 

T. C. Friedman, S. Wilk, The Effect of Inhibitors of Prolyl Endopeptidase and Pyro-glutamyl Pephde Hydrolase on TRH Degradation in Rat Serum , Biochem. Biophys. Res. Commun. 1985, 132, 787-794. 

That derivatization may increase rather than decrease peptidase-catalyzed degradation is illustrated with aspartame (6.79, R = MeO), the C-terminal methyl ester of the dipeptide Asp-Phe. The metabolism of this artificial sweetener was compared to that of the underivatized dipeptide (6.79, R = H) and of the corresponding amide Asp-Phe-NH2 (6.79, R = NH2) in microvillar membranes obtained from human duodenum, jejunum, and ileum [189]. The activities monitored were clearly those of peptidases as shown by the effects of inhibitors. Whereas the peptide bond in Asp-Phe and Asp-Phe-NH2 was hydrolyzed at a comparable rate, that in aspartame was hydrolyzed approximately twice as fast. This is an interesting and favorable situation, given that aspartame is expected to be degraded once it has elicited its effect in the buccal cavity. 

Often, demonstration of potency and selectivity in vitro is assumed to extrapolate to effects of inhibitors administered in vivo. Unfortunately, such assumptions are often invalid, and selectivity and potency must also be assessed in vivo or ex vivo. There are several reasons for such discrepancies between in vitro and in vivo data. 

However, rates of precipitation in soil systems may be quite different from those in solutions because precipitation is catalysed by adsorption of the reacting solutes onto soil surfaces the nature of the solid phases formed may be different and sorption may also alter the effects of inhibitors. There are very few data in the literature on these effects actually measured in soils. Figure 3.15 shows data of Huang (1990) for calcite precipitation in three soils incubated with urea. Precipitation was induced as the pH increased during urea hydrolysis ... 

Kornilova, A.Y., Das, C., Wolfe, M.S. (2003) Differential effects of inhibitors on they-secre-tase complex. Mechanistic implications. J. Biol. Chem., 278, 16470-16473. 

To help clarify the effect of inhibitors, it is useful to divide inhibition into four classes reversible, allosteric, irreversible covalent, enzyme-catalysed covalent. 

Figure 17.5 The dual vasodilatoiy effect of inhibitors of the angiotensin-converting enzyme (ACE). The ACE inhibitors not only inhibit ACE but also the kininase which degrades bradykinin. (See also Chapter 22).

The enhanced catalatic activity could arise from more facile exhaust of products just as easily as from enhanced substrate accessibility. The effect of inhibitors is a largely static process that is complete once the inhibitor has become bound in the active site. The catalatic process, on the other hand, requires a constant influx of substrate peroxide and efflux of product oxygen and water. As a result, the inlet channels for inhibitors and substrate may be different. 

Effect of Inhibitors on Arrhenius Plots. The presence of inhibitors can lead to nonlinear Arrhenius plots as well as an increase in E. For example, jack bean urease exhibits a linear Arrhenius plot in the absence of an inhibitor, but a curved plot in the presence of 34 mM sodium sulfite . 

Rotating culture vessels such as simulated microgravity systems are primarily used to study 3-D tumor growth and differentiation. However, mixed cell populations combined with matrix proteins can be used to generate a complex microenvironment in which cell-cell interactions and invasion can be measured (95). A similar system has also been described for the coculture of endothelial cells, myofibroblasts, and tumor cell clusters embedded in Matrigel . Differential labeling of the cell populations enables their invasion and the effects of inhibitors to be measured (96). 

The free-radical nature of ATRP is well established through a number of studies [Matyjaszewski et al., 2001], The effects of inhibitors and retarders, solvents, and chain-transfer agents are the same in ATRP as in conventional radical polymerization. The regio-selectivity, stereoselectivity, and copolymerization behaviors are also the same. 

Bickel M, Baader E, Brocks DG, et al. 1991. Beneficial effects of inhibitors of prolyl 4-hydroxylase in CCh-induced fibrosis of the liver in rats. J Flepatology 13 S26-S34. 

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