Mutarotation reaction rate

To accommodate these facts, the earliest mechanisms proposed for degradation of D-fructose assumed that it was present in the furanose form, and that the ring remained intact. It was assumed that the initial reaction was the elimination of water, to form the 1,2-enolic form of 2,5-anhydro-D-mannose, and that further dehydration resulted in 2-furaldehyde. The necessity for D-glucose to isomerize to D-fructose was assumed to account for the much lower reaction-rate of D-glucose. This mechanism does not account for the observation that 2,5-anhydro-D-mannose is less reactive than D-fructose, nor is there any evidence that 2,5-anhydro-D-mannose is present in reacting D-fructose solutions. Nevertheless, similar mechanisms have since been proposed.13-16 Because of the ease of mutarotation of D-fructose... 

The mutarotation reaction exhibits general acid-and-base catalysis, the rate depending on the concentrations of all the acids and bases present. 

The kinetics of the optical rotatory changes of poly-L-proline in various solvents and at various temperatures have also been studied by Steinberg et al. (1960a). In acetic acid the course of the forward mutarotation reaction was found to be independent of concentration (over the range 0.25 to 2.0 gm/KK) ml) but, as observed by Downie and Randall, the rate constant depends on the degree of mutarotation. An activation enthalpy, AH = 21 kcal/mole, was determined for both the forward mutarotation of form I in acetic acid and the reverse mutarotation of form II in acetic acid-w-pro-panol. 

The rate constants for the mutarotation reaction have also been determined under a variety of conditions, catalysis being more marked... 

Differences in the rate of mutarotation of sugars in water and in deuterium oxide provide a valuable means for studying mutarotation reactions.135,224,233 237,238 The difference in rates arises from a combination of kinetic and solvent isotope-effects, and is usually expressed as a ratio,knlkD, called the isotope effect. Kinetic isotope-effects are caused by differences in the energy required for alteration of the normal and the isotopic bonds in the corresponding transition states solvent isotope-effects can exist when the isotopic compound is used both as a reactant and as a solvent. 

For elucidation of the course of mutarotation reactions, the timing of the addition and elimination of the proton is important. If the proton transfer occurs after the rate-controlling step, it will have no primary kinetic consequence. If the proton transfer occurs during the rate-... 

Many of the early studies were conducted with enzymes from fermentation, particularly invertase, which catalyzes the hydrolysis of sucrose to monosaccharides D-glucose and D-fmctose. With the introduction of the concept of hydrogen ion concentration, expressed by the logarithmic scale of pH (Sorensen, 1909), Michaelis and Menten (1913) realized the necessity for carrying out definitive experiments with invertase. They controlled the pH of the reaction medium by using acetate buffer, allowed for the mutarotation of the product and measured initial reaction rates at different substrate concentrations. Michaelis and Menten described their experiments by a simple kinetic law which afforded a foundation for a subsequent rapid development of numerous kinetic models for enzyme-catalyzed reactions. Although the contribution of previous workers, especially Henri (1902, 1903), was substantial, Michaelis and Menten are regarded as the founders of modern enzyme kinetics due to the definitive nature of their experiments and the viability of their kinetic theory. 

The mutarotation reactions are catalyzed by both hydrogen and hydroxyl ions. The rate of mutarotation of glucose and galactose is at a minimum between the pH limits 3.0 to 7.0. At pH values greater than 7.0 and less than 3.0, the velocity increases rapidly. The curve for mutarotation velocity vs. pH is represented by a catenary. The influence of hydrogen and hydroxyl ions on the rate was found by Hudson to be expressible by equations of the type ... 

Investigations into the anomeric specificity of the enzyme suggested that the a-xylopyranose was utilised exclusively, a fact demonstrated by comparison of initial reaction rates with pure and mixed anomers assuming that the rate of spontaneous mutarotation of each sugar is slow [30]. 

We saw in Chapter 15 that ester hydrolysis can be catalyzed by either acids or bases (Figure 23.2). In the acid-catalyzed process, all the steps are reversible, and the direction of the reaction is effectively controlled by mass action. The rate-determining step is the attack of water on the pro-tonated ester. In the base-catalyzed process, the hydroxyl ion attacks the neutral ester in the rate-determining step. The reaction is rendered irreversible by the final step in which methoxide deprotonates the acid (pK MeOH = 15.5, pK of a typical acid is 5), and the hydroxyl ion is not regenerated at the end of the reaction. Thus, ester hydrolysis is possible in either acidic or basic solution if the reaction rate was plotted against pH, it would show a minimum at neutral pH. We have met other reactions of this type, for example, the mutarotation of glucose. 

PentaGalU-ol was used as a substrate for PGI and PGII and triGalU-ol was used for PGX. Substrates were used at 20 mM final concentration in 0.5 ml 100 mM D2O buffer pD 4.5. The amount of enzyme used was such that the rate of hydrolysis was much higher than the rate of mutarotation. Time courses of the reaction mixtures were recorded on a Brucker AM 400 spectrometer at 25 °C. The assignments of relevant resonances was based on data published by Hricovini et al. [5]. 

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... 

Further experiments by Brown and particularly Henri were made with invertase. At that time the pH of the reactions was not controlled, mutarotation did not proceed to completion, and it is no longer possible to identify how much enzyme was used (Segal, 1959). Nevertheless, in a critical review of kinetic studies with invertase, Henri concluded (1903) that the rate of reaction was proportional to the amount of enzyme. He also stated that the equilibrium of the enzyme-catalyzed reaction was unaffected by the presence of the catalyst, whose concentration remained unchanged even after 10 hours of activity. When the concentration of the substrate [S] was sufficiently high the velocity became independent of [S]. Henri derived an equation relating the observed initial velocity of the reaction, Vq, to the initial concentration of the substrate, [S0], the equilibrium constant for the formation of an enzyme-substrate complex, Ks, and the rate of formation of the products, ky... 

Zeffren and Hall (1973) have commented that, since reactions with polar transition states in nonpolar solvents can be accelerated by several orders of magnitude by the presence of low concentrations of salts (Winstein et al., 1959), the rate enhancement of tetramethylglucose mutarotation provided by the presence of acid-base pairs such as phenol and pyridine may be due to formation of ion pairs in benzene solution. Salts which do not act as acids and bases catalyse mutarotation of tetramethylglucose in aprotic solvents (Eastham et al., 1955 Blackall and Eastham, 1955 Pocker, 1960). The efficiency of enzymatic catalysis could arise largely from electrostatic catalysis... 

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