Mutarotation, of glucose

In equation 8.2-6a, the slope of -1 with respect to pH refers to specific hydrogen-ion catalysis (type B, below) and the slope of + 1 refers to specific hydroxyl-ion catalysis (Q if k0 predominates, the slope is 0 (A). Various possible cases are represented schematically in Figure 8.5 (after Wilkinson, 1980, p. 151). In case (a), all three types are evident B at low pH, A at intermediate pH, and C at high pH an example is the mutarotation of glucose. Cases (b), (c), and (d) have corresponding interpretations involving two types in each case examples are, respectively, the hydrolysis of ethyl orthoacetate, of P -lactones, and of y-lactones. Cases (e) and (f) involve only one type each examples are, respectively, the depolymerization of diacetone alcohol, and the inversion of various sugars. 

The following results refer to the mutarotation of glucose in aqueous hydrochloric acid solutions at 20 C ... 

Various possibilities that may arise in the reactions have been classified by Skrabal who plotted log k against the pH of the solution as shown in Fig. 6.5. Curve (a) represents a most general type of behavior for catalysis by H+ and OH ions. The velocity in the intermediate region is equal to H2o [S] so that h2o can also be determined directly from the rate in this region. A curve of the type a is given by the mutarotation of glucose, if k 2o is sufficiently... 

Slightly basic pH to enhance mutarotation of glucose, i.e., the interconversion of a- and /i-glucopyranose forms, which becomes rate limiting as the enzyme only converts the a-form. 

By considering the reversibility of the acetalforming reactions, it is apparent that treatment of either of the two methyl pyranosides with acidic methanol will prodnce the same eqnilibrium mixture. A related equilibration occurs with the anomers of glucose, as seen earlier (see Box 7.1, mutarotation of glucose). 

Br0nsted plot for base catalysis of the mutarotation of glucose. 

The idea that simultaneous (i.e., concerted) acid and base catalysis could strongly accelerate reactions was probably introduced by T. M. Lowry in his considerations of general acid-general base catalysis of the mutarotation of glucose (Lowry and Faulkner, 1925). The concept was provided with strong experimental support by C. G. Swain and J. F. Brown (1952a,b), who used... 

Effects of Metal Ions on Imidazole Catalysis of the Mutarotation of Glucose... 

The rates of mutarotation of glucose, in the presence of 0 to 0.238M imidazole and of 0 to 0.230M benzimidazole at 25°, have been measured the rate constant data are given in Table I. Linear plots of the observed rate constants for glucose vs. the molarity of imidazole or benzimidazole (Im or BIm) are obtained. The straight lines follow the equations... 

Bruice and Schmir (3) have shown that for a series of imidazole derivatives, klm depends on the base strength of the catalyst and since pKA is an approximate measure of base strength, the value of klm should increase with increase in pKA. Table I shows that this is indeed the case. Imidazole, pKA = 7.08, has a catalytic constant eight times larger than that of benzimidazole, pKA = 5.53. Bronsted and Guggenheim (2) have obtained a linear relationship between log k/ and pKA for a series of carboxylic acids in the pKA range of 2 to 5, where kB is the carboxvlate anion basic catalytic constant for the mutarotation of glucose and Ka is the acid dissociation constant of the acid. Our results for imidazole and benzimidazole fit fairly well into the Bronsted plot. 

A scheme for the imidazole catalysis of the mutarotation of glucose, similar to the concerted mechanism proposed by Swain and Brown (13), is shown below, in which a proton is transferred from the D-glucose to imidazole or benzimidazole and from the H20 (represented as an acid) to the D-glucose in the rate-determining step. 

Table III. Effect of Ni(ll) on Benzimidazole Catalysis of Mutarotation of Glucose at 25°...

The rapid spontaneous mutarotation of glucose-6-phosphate has been shown to result from an intramolecular catalysis of the reaction by the phosphate group at carbon 6 (81). The cleavage of glucose into three carbon fragments, which is essentially a reversal of the aldol condensation reaction, requires the ketohexose as substrate. The necessary isomerization reaction to form the ketohexose then uses the open-chain form intermediate of the mutarotation reaction. Salas et al. (80) have speculated that the enhanced mutarotation of glucose-6-phosphate may thus have been the key requirement which led to the evolution of the phosphorolytic pathway for glucose metabolism. 

Hudson10 showed that the mutarotation of fructose in water at 30° is eleven times faster than that of glucose. He therefore assumed that in a sucrose solution which is undergoing very rapid inversion with invertase at that temperature, practically all of the fructose has reached equilibrium and exists as a mixture of its a and 0 forms, while the glucose is being liberated in only one form which, however, slowly passes to its a, 0 equilibrium mixture. The drop in rotation between the apparent and real curves of inversion by invertase must therefore be due almost entirely to the mutarotation of glucose. Hudson thus showed that the D-glucose liberated from sucrose by invertase had a specific rotation between [< ]d +100° and +125° and is thus most likely the a-form. 

Polarimetry is extremely useful for monitoring reactions of optically active natural products such as carbohydrates which do not have a useful UV chromophore, and samples for study do not need to be enantiomerically pure. Nevertheless, compared with spectrophotometry, the technique has been applied to relatively few reactions. It was, however, the first technique used for monitoring a chemical reaction by measuring a physical property when Wilhemy investigated the mutarotation of sucrose in acidic solution and established the proportionality between the rate of reaction and the amount of remaining reactant [50]. The study of a similar process, the mutarotation of glucose, served to establish the well-known Bronsted relationship, a fundamental catalysis law in mechanistic organic chemistry. 

This visit to Princeton (to receive the Ph. D. degree) gave me the opportunity of working a few days in Professor Hulett s laboratory, during which I measured the speed of mutarotation of glucose at various pH values, and from the data inferred the formula that has come into general use. ... 

Figure 1.13 The mutarotation of glucose anomers. The specific rotation of the aqueous equilibrium mixture is +52°...

It was generally believed that the characteristic non-linear dependence of the rate constant on n was associated with the existence of the preequilibrium (Bell, 1941). For the mutarotation of glucose a linear variation of rate constant with n had been reported (Hamill and La Mer, 1936c) and cited as an example of a mechanism of acid catalysis without pre-equilibrium. 

The solid chosen for their work on reaction (XLVIII) was alumina whose surface possesses several types of Bronsted and Lewis acidic and basic functional groups (-Al+-, -OHa+, -O - H, -0 , and defect sites) [175, 219] which could be potential catalysts for the mutarotation of glucose. Woelm... 

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