Mutarotation effect

When Emil Fischer, some 70 years ago, prepared sugar phenylhydrazones, he represented them as open-chain compounds. The conception of ring structure arose later to account for mutarotational effects in solution. The question is still a moot one, although many workers have attempted to clarify the point. 

In 1899, Lowry discovered the change in the rotatory power over time of a solution of nitrocamphor in benzene, an effect previously encountered only with aqueous solution of sugars. He named this effect "mutarotation," and its discovery was taken as a prominent achievement for Armstrong s laboratory research group. 50 Lowry ascribed the phenomenon to tautomeric conversion (from a CH-N02 form to a C = NO-OH form), that is, the shift of a hydrogen atom and the shift of a double bond. In 1909, he and Desch concluded that this reversible transformation occurs very quickly because they could not find an ultraviolet absorption spectral band characteristic of either isomer. 51 But what triggered this reversible transformation ... 

Faulkner (1923 1927) reported that mutarotation occurred much more readily in a mixture of pyridine and cresol than in either by itself. Swain and Brown (1952) extended this work and found that a-pyridone, where an acidic and a basic group are incorporated in the same molecule, is an excellent catalyst, 0 05 M concentration giving rise to a reaction 50 times faster in benzene solution than a solution of 0 05 M phenol and 0-05 M pyridine. A 0-001 M solution of a-pyridone was 7000 times more effective than equivalent concentrations of phenol and pyridine. A concerted mechanism was proposed [4]. 

Acetals and ketals are also called glycosides. Acetals and ketals (glycosides) are not in equilibrium with any open chain form. Only hemi-acetals and hemiketal s can exist in equilibrium with an open chain form. Acetals and ketals do not undergo mutarotation or show any of the reactions specific to the aldehyde or ketone groups. For example, they cannot be oxidized easily to form sugar acids. As an acetal, the carbonyl group is effectively protected. 

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

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

Reduction of 10 with lithium aluminum hydride (LAH) in ether furnished19 an intermediate, presumably the phosphine derivative 11, which was treated with acid to effect ring enlargement, giving the 5-phosphino-D-xylopyranose derivative 14. This compound was immediately converted by air oxidation19 into the stable crystalline compounds, 5-deoxy-3-0-methyl-5-C-(phosphinyl)-D-xylopyranose (15) and the 5-C-(hydroxyphosphinyl) derivative 16 in overall yields of 15 and 3.5%, respectively, from 10. Compound 16 was obtained in 90% yield from 15 by oxidation with bromine.19 No mutarotation was observed19 for compounds 15 and 16 in water during 48 h. 

The presence of sugars and salts can also affect the rate of mutarotation. Although the effect is small in dilute solutions, a combination of salts equal to that found in solution in milk nearly doubles the rate of mutarotation (Haase and Nickerson 1966). This catalytic effect is attributed primarily to the citrates and phosphates of milk. The presence of high levels of sucrose, on the other hand, has the opposite ef-... 

A third possible type of catalysis requires that a base and an acid act synchronously to effect the breaking and formation of bonds in a single step. Thus, tetramethyl-glucose mutarotates very slowly in benzene containing either pyridine (a base) or phenol (an acid). However, when both pyridine and phenol are present, mutarota-tion is rapid. This suggested to Swain and Brown132 a concerted mechanism (Eq. 9-92) in which both an acid and a base participate. 

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... 

The majority of the many methods used to study the composition of equilibrium solutions of carbohydrates examine the mixture without separating the individual components. With the discovery that the anomeric forms of sugars could be readily separated by gas chromatography of their tri-methylsilyl ethers, a new approach to the problem was found. A protocol was developed for the direct gas chromatographic analysis of the amount of each anomer present in an aqueous solution. The protocol can be used on the micro scale and can be used in enzyme assays such as that for mutarotase. The method has been made more effective by combining gas chromatography with mass spectrometry. It is shown how mass spectral intensity ratios can be used to discriminate anomers one from another. The application of these methods to the study of complex mutarotations is discussed. 

---