Mechanisms of mutarotation

Figure 1.19 Mechanism of mutarotation and reaction with thiol reagents of 5-thioglucose.

Molecular orbital calculations have been carried out in an investigation of the mechanism of mutarotation of oC-D-glucopyranose. The CNDO/2 results were in agreement with experiment for acid- and base-... 

In the crystalline state, reducing monosaccharides exist exclusively in cyclic structures (a- or P-anomers). In solutions, a balance between the a- and P-anomers and acyclic forms is established as the individual forms interconvert over time. This process, schematically shown in Figure 4.2, is called mutarotation. The mechanism of mutarotation assumes cleavage of the saccharide cyclic forms... 

Muscalure, structure of, 287 Mutarotation, 985-986 glucose and, 985-986 mechanism of. 986 Mycomycin, stereochemistry of, 330 Mylar, structure of, 819 n yo-Inositol, structure of, 135 Myrcene. structure of, 202 Mvristic acid, catabolism of, 1137 structure of. 1062... 

The reversible reactions are initiated by an equilibrium between neutral and ionized forms of the monosaccharides (see Fig. 6). The oxyanion at the anomeric carbon weakens the ring C-O bond and allows mutarotation and isomerization via an acyclic enediol intermediate. This reaction is responsible for the sometimes reported occurrence of D-mannose in alkaline mixtures of sucrose and invert sugar, the three reducing sugars are in equilibrium via the enediol intermediate. The mechanism of isomerization, known as the Lobry de Bruyn-... 

Lowry is best known to chemistry students through the tradition of eponymony, since the proton theory of acidity is known as the "Bronsted/Lowry theory" of proton donors. His most important experimental investigation likely was a long series of studies on optical rotatory dispersion.49 For our purposes, there is special interest in his discovery of mutarotation in camphor derivatives and his theory of dynamic tautomerism, which led him to an ionic theory of organic reaction mechanisms. 

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 catalysis of mutarotation of glucosamine hydrochloride involves an intramolecular mechanism, and so the catalytic coefficient of glucosamine. fcc,iNH2> must have the dimension, min.-1, instead of liters/mole min. The total rate is then... 

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

If this reaction is observed polarimetrically, the equilibrium rate constant obtained is equal to the sum of the tautomerization rate constants (Ki -f- K. i) for the overall reaction, as described above (20). Therefore, no matter what the path or the mechanism of the mutarotation phenomenon, the data obtained polarimetrically are completely explained by the above overall configuration change. 

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. 

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. 

Mutarotation of L-arabinosylamine is acid-catalyzed and therefore differs from that of sugars, which is catalyzed by both acids and bases. Isbell and Frush compared the mechanism of the acid-catalyzed mutarotatioiis, dealing first with that of a sugar. 

A part of the evidence for the mechanisms given in equations (40) and (42) is provided by the work of Lowry and co-workers (Lowry and Richards, 113 Lowry and Faulkner, 24) on the mutarotation of tetra methylglucose. In water the reaction proceeds at a measurable rate, and it is clearly catalyzed by both acids and bases. In aqueous solution pyridine is a powerful catalyst but in pure dry pyridine no reaction occurs, and likewise in pure dry re-cresol there is no reaction. Upon investigating the reaction in a mixture of pyridine and n-cresol, Lowry and Faulkner (24) found it to take place very rapidly. From these experiments Lowry drew the important conclusion that a proton cannot by itself wander from one part of the molecule to another. The transformation can occur only if the medium in which the molecule is placed has both acidic and basic properties, so that a proton can be removed from the molecule at one place and a proton added to the molecule at another place. Now these experiments furnish strong support to the mechanism of reactions (40) and (42) whereby both members of the conjugate acid-base pair play a part in the reaction. Instead of representing this mutual dependence by means of consecutive bimolecular reactions, Lowry chose to represent it by means of one trimolecular reaction... 

The mutarotation of glycosylamines has been studied more in methanolic solutions than in water, because any intermediate from addition of this solvent to the acyclic Schilf base cannot yield aldehydo sugar. As would be expected from the mechanisms of Figure 3.23, increased acid strength results in faster mutarotation (Figure 1.24). 

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