Mutarotation kinetics

FISCHER PROJECTION ALDOSE 1-EPIM ERASE MUTAROTATION KINETICS MYELOBLASTIN MYELOPEROXIDASE MYOGLOBIN OXYGENATION MYOSIN ATPase... 

Table I. Mutarotation Kinetics by Gas Chromatography of TMSi Derivatives0...

These specific mixed solvents were chosen for this study, since solvent systems of this type have been used in similar studies such as the study of glucose mutarotation kinetics and equilibria in 50 mass % THF-H20... 

D-, mutarotation kinetics, 57 Gelatin, mutarotation of, 13 j3-Gentiobiose, octaacetate, 264 Glucal, 2-acetoxytri-O-acetyl-D-, oxo reaction with, 92... 

The type of mutarotation kinetics found experimentally is strongly dependent on the solvent. Forward mutarotation in acetic acid yields a plot of log d[a /dt versus log ([a]( — [ ] ) which is linear over 97% of the reaction with a slope of 1.33. On the other hand the rate is virtually invariant over two-thirds of the reaction in a solvent of 30% water-70 % acetic acid. Reverse mutarotation in acetic acid-n-propanol appears to follow first-order kinetics over 90% of the transformation. From these experiments and the fact that the enthalpies of form I and form II are essentially identical (Steinberg et al., 1960a) it seems likely that solvation is decisive not only in determining the kinetic pathway of mutarotation but also the structural pattern which is stabilized in a given solution. 

We have seen that the form I —> form II interconversion may take place along different pathways depending on the solvent system, with differing over-all mutarotation kinetics. In acetic acid the apparent order of the forward mutarotation reaction is 1.33 (0). It will be seen from Eq. (8) that in this case K t) should decrease with time, in agreement with the experimental observations. When 0 = 1, the mutarotation proceeds with first-order kinetics. This behavior is seen in the reverse mutarotation of poly-L-proline II in acetic acid-propanol. When /3 < 1, K (t) increases with time. This situation obtains in the initial stage of the forward mutarotation in acetic acid-water solvent which proceeds with zero-order kinetics. 

The observed rate constant initially rises steeply during freeze-concentration it increases to a maximum at -8°C to decline with further decrease in temperature. The study also shows how any so-called inert additives, e.g. salt, significantly affect the mutarotation kinetics during freezing. 

The tautomeric equilibria of aldopentoses, aldohexoses and hexuloses in DMSO have been examined by C-n.m.r. spectroscopy the mutarotation kinetics of aqueous D-glucose have been investigated by i.r. methods, and Raman spectroscopic data on the tautomeric transformations of glucose in aqueous solution over a wide pH range confirmed that the ring-chain tautomerism depends on pH." The ring shapes of the two fiiranose and two pyranose forms of psicose in aqueous solution, riiown by n.m.r. spectroscopy to be present in almost equal proportions, have been analyzed by MM3."... 

A simple mutarotation exists in this example, unlike complex mutarotations of other sugars, e. g., idose which, in addition to pyranose, is also largely in the furanose form. Hence, the order of its mutarotation kinetics is more complex. 

However, the pattern is complicated by several factors. The sugar molecules to be hydrogenated mutarotate in aqueous solutions thus coexisting as acyclic aldehydes and ketoses and as cyclic pyranoses and furanoses and reaction kinetics are complicated and involve side reactions, such as isomerization, hydrolysis, and oxidative dehydrogenation reactions. Moreover, catalysts deactivate and external and internal mass transfer limitations interfere with the kinetics, particularly under industrial circumstances. 

By application of first-order, kinetic equations, B. Anderson and Degn claimed that an equilibrated (25°) aqueous solution of D-fructose contains 31.56% of jS-D-fructofuranose and 68.44% of -D-fructopyranose. N.m.r. studies, however, showed that, at equilibrium, a solution of D-fructose contains /3-D-fructopyranose, -D-fructofuranose, a-D-fructofuranose, and a trace of a-D-fructopyranose the distribution of these isomers was shown by gas-liquid chromatography to be 76,19.5, and 4%, respectively. Based on Anderson and Degn s result, Shallenberger reasoned that, as 0.68 X 1.8 = 1.22 (which approximates the reported sweetness of mutarotated D-fructose ), the furanose form(s) must possess very little sweetness. 

The data recorded below are typical of a study of the mutarotation of 7i-bromonitrocam-phor in chloroform solution at 14 °C. The reaction obeys the kinetics indicated by... 

Fig. 13.9 Facsimile of a molecular and kinetic scheme of the one-pot glucose into mannitol bio-chemo cascade, showing the many different species that undergo interconversion during the overall process, involving enzymatic isomerization, homogeneous mutarotation and heterogeneous hydrogenation. For simplicity, the various sugar-borate species have been omitted [23, 24],...

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... 

The original experimental evidence for concerted acid-base catalysis of the mutarotation in benzene is now considered unsound133 134 and concerted acid-base catalysis has been difficult to prove for nonenzy-matic reactions in aqueous solution. However, measurements of kinetic isotope effects seem to support Swain and Brown s interpretation.135 Concerted acid-base catalysis by acetic acid and acetate ions may have been observed for the enolization of acetone136 and it may be employed by enzymes.1363... 

Computer Modeling of the Kinetics of Tautomerization (Mutarotation) of Aldoses Implications for the Mechanism of the Process... 

In the region between pH 7 and 4 the mutarotation constants are relatively unaffected by pH. Figure 3 shows the kinetic data obtained for the change in hydrogen ion concentration of a 0.01M boric acid solution adjusted to pH 7 with sodium hydroxide when different amounts of... 

Carbodiimide Polymers An optically active carbodiimide, (/ )-152 ([a]365 +7.6°), gives a polymer by polymerization using a titanium (IV) isopropoxide catalyst (Scheme 11.9) [203], The polymer showed optical activity essentially identical to the monomer however, on heating, the polymer indicated mutarotation and specific rotation reached a plateau value of [a]363 -157.5°, which is considered to be based on excess helical sense of the main chain. The mutarotation has been ascribed to a conformational transition from a kinetically controlled one to a thermodynamically controlled one. Excess single-handed helical conformation can be induced for polyfdi-/ -hexyl carbodiimide) by protonating the polymer with chiral camphorsul-fonic acid. 

In other studies, analysis of the products of reaction between formaldehyde and guanosine at moderate pH shows a new adduct—formed by condensing two molecules of each reactant—which has implications for the mechanism of DNA cross-linking by formaldehyde,17 while the kinetics of the mutarotation of N-(/ -chlorophcnyl)-//-D-glucopyranosylamine have been measured in methanolic benzoate buffers.18 For a stereoselective aldol reaction of a ketene acetal, see the next section. 

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