PH profiling

Sodium dithionite solution can be produced on-site utilizing a mixed sodium borohydride—sodium hydroxide solution to reduce sodium bisulfite. This process has developed, in part, because of the availabiHty of low cost sulfur dioxide or bisulfite at some paper mills. Improved yields, above 90% dithionite based on borohydride, can be obtained by the use of a specific mixing sequence and an optimized pH profile (360,361). Electrochemical technology is also being offered for on-site production of sodium hydrosulfite solution (362). 

The pH dependency of enzyme-catalyzed reactions also exhibits an optimum. The pH optima for enzyme-catalyzed reactions cover a wide range of pH values. Eor instance, the subtihsins have a broad pH optima in the alkaline range. Other enzymes have a narrow pH optimum. The nature of the pH profile often gives clues to the elucidation of the reaction mechanism of the enzyme-catalyzed reaction. The temperature at which an experiment is performed may affect the pH profile and vice versa. 

Fig. 8. Protease washing performance in a U.S. liquid detergent. Grass soiling in a 10 min wash at 30°C with one enzyme dosage, (a) pH profile of commercial proteases A and B. (b) Effect of increasing ionic strength, adjusted with Na2S04, of commercial protease B at (—°—) pH 8 and (- pH 11.

Bell-shaped activity versus pH profiles arise from two separate active-site ionizations, (a) Enzyme activity increases upon deprotonation of (b) Enzyme activity decreases upon deprotonation of A-H. (c) Enzyme activity is maximal in the pH range where one ionizable group is deprotonated (as B ) and the odier group is protonated (as A-H). 

This expression has the same form as Eqs. (6-81) and (6-84). Here, of course, the substrate is not protonated to an appreciable extent. With other suitable experiments and some luck, the steady-state situation can be distinguished from substrate titration. For example, is the pKa value deduced under the assumption of a titration reasonable for the molecule in question That is, is it reasonable for one of the functional groups of A to have a pKa near the pH of the bend Can one detect significant amounts of two species, AH+ and A, at a pH near the presumed pKal Can one modify the substrate, eliminating the site of protonation If so, and if a titration mechanism operates, then (as the reader should show) the pH profile should become linear. Obviously, were substrate titration and a steady-state intermediate situation to coexist in the same system, a more complicated but not intractable situation would result. 

The pH profile for the decomposition of peroxybenzoic acid in aqueous solution at 25°C. Data are from Ref. 15. 

The reaction follows second-order kinetics and is characterized by a bell-shaped pH profile, as shown in Fig. 6-2. This suggests a bimolecular reaction of one molecule each of conjugate base and acid ... 

Each term in this equation represents an independent pathway. The low-pH arm in the figure is equivalent to reaction (6-57), or one similar to it, in which the proton attacks the substrate directly. The high-pH pathway represents the unimolecular reaction of the substrate or else its reaction with water. As this discussion illustrates, a reaction whose pH profile shows upward bends can be analyzed in terms of separate pathways. A complex profile can be separated into regions at each upward bend each region is a distinct pathway. 

To conclude this section, we shall consider a more complex example, the pH effects on the hydrolysis of aspirin, acetylsalicylic acid.14,16 The pH profile is given in Fig. 6-4 for the reaction and rate law... 

The pH profile for the hydrolysis of aspirin. The numbers designate four regions for the respective terms in the rate law, Eq. (6-104). 

To summarize the analysis of pH profiles, even complex ones, is not an arcane or difficult art. Systematic analysis in terms of ionic equilibria, predominant species, and the reaction orders with respect to [H+] provides the solution. Kinetically indistinguishable alternatives can never, by definition, be distinguished from the kinetic data contained in the pH profile. Other measurements, including some alluded to earlier and others given in Chapter 10, may, however, allow these distinctions. 

It was pointed out in Section 6.5 on pH profiles that substrate titrations and certain steady-state mechanisms take the same algebraic form. This ambiguity also prevails when association equilibria can be established. This is illustrated by the reaction17... 

The reaction is pseudo-first-order in the total concentration of reactant, and shows a pH profile like that depicted in Fig. 6-1 b. Present an interpretation. 

Substrate titration. Devise two schemes for the conversion of substrate to product that match the pH profile in Fig. 6-1 b. These two are to feature substrate titration. Devise a third in which a steady-state intermediate intervenes in a two-step sequence. 

The starting material undergoes acid dissociation with pKa = 8.9 the reaction proceeds by parallel hydrolysis of acid (k = 4X 10-6 s l) and anion kj = 8x 10-3 s l). Sketch the expected pH profile with numerical scales on the axes. 

Assume that the reaction occurs between the two uncharged species, MNNG and Am, with a rate constant kN. Express ks as a function of tN, K m, Kan, and [H. Sketch the anticipated pH profile. Actually, this situation is further complicated because kN, although constant over some pH range, shows a further variation that can be attributed to an acidic intermediate. Derive an expression for kN as a function of [H + ] from the scheme shown, denoting the acid ionization constant of the steady-state intermediate as Knl. 

The pH profile for the hydrolysis of methyl aspirin, which shows specific acid-base catalysis. The solid line shows the fit according to Eq. (10-21), and the dashed one where ko = 0. Data are from Ref. 16. 

The rate of a reaction that shows specific acid (or base, or acid-base) catalysis does not depend on the buffer chosen to adjust the pH. Of course, an inert salt must be used to maintain constant ionic strength so that kinetic salt effects do not distort the pH profile. 

Specific acid catalysis. Derive an expression for the minimum rate constant along the pH profile in terms of kw, oh> o> and Kw. 

Parallel reactions, 58-64, 129 Partitioning ratios, 79 Perturbation (see Chemical relaxation) pH profiles, 139-145 bell-shaped, 141-142 Phosphorous acid, oxidation of, 186-187 Physical methods for kinetics, 22-25 end point reading unknown, 25-28 sample calculation for, first-order,... 

A new chapter (5) on reaction intermediates develops a number of methods for trapping them and characterizing their reactivity. The use of kinetic probes is also presented. The same chapter presents the Runge-Kutta and Gear methods for simulating concentration-time profiles for complex reaction schemes. Numerical methods now assume greater importance, since useful computer programs are available. The treatment of pH profiles in Chapter 6 is much more detailed. 

A detailed physical examination of the purple complex formed in alkaline solution between Fe(III), ethylenediaminetetraacetic acid (EDTA) and peroxide shows it to have a composition [Fe "(EDTA)02] (togA, 3 c =4.33). This complex catalyses decomposition of peroxide, the rate-pH profile going through a maximum at pH 9-10 . 

Wohnsland, F., Faller, B. High-throughput permeability pH profile and high-throughput alkane/water log P with artificial membranes. J. Med. 

Fig. 3.1 Octanol-water distribution (log Don) versus pH profile for pindolol, based on data reported by Barbato et al. [70]. The pCEL-X computer program (plON) was used to...

The diff3-4 Approximation in logDod-pH Profiles for Monoprotic Molecules... 

When a compound forms a dimer or a higher-order oligomer in aqueous solution, the characteristic solubility-pH profile takes on a shape not predicted by the Henderson-Hasselbalch equation and often indicates an apparent pJCj that is different from the true piQ. Figure 3.3 shows several examples of sparingly-soluble... 

Permeability-pH profiles, log Pe - pH curves in arhficial membrane models (log Pjpp - pH in cehular models), generally have sigmoidal shape, similar to that of log Dod - pH cf. Fig. 3.1). However, one feature is unique to permeabihty profiles the upper horizontal part of the sigmoidal curves may be verhcally depressed, due to the drug transport resistance arising from the aqueous boundary layer (ABL) adjacent to the two sides of the membrane barrier. Hence, the true membrane contribution to transport may be obscured when water is the rate-limiting resistance to transport. This is especially true if sparingly soluble molecules are considered and if the solutions on either or both sides of the membrane barrier are poorly stirred (often a problem with 96-well microhter plate formats). 

This chapter considered ionizable drug-like molecules. Absorption properties that are influenced by the pKj were explored. The impact of the pKj-absorption relationship on key physicochemical profiling underlying absorption (solubihty, per-meabihty and ionization) was examined in detail and several simpUfying equations were discussed. The various diff relationships considered in the chapter are systematized in Table 3.2. Table 3.3 summarizes the apparent pfQ shift method for detecting aggregates in solubility profiles, when the apparent pff value derived from Henderson-Hasselbalch analysis of log S pH profile does not agree with the... 

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