Forms of Sugars in Solution

Exchange of Hydrogen and Oxygen Atoms with Solvent 

Upon dissolution in water at room temperature, reducing sugars undergo spontaneous transformations which, in the absence of strong acids or bases, usually involve anomerization at the reducing carbon 

Because of rapid, reversible reactions, direct chemical methods for measuring the concentration of the acyclic form of a sugar in solution are not generally satisfactory. Attempts have been made to estimate the proportion of the acyclic form by study of the reactions of sugEirs with chlorous acid and with cyanide. The results clearly show the absence of a large concentration of the acyclic modification in the solution, but the methods do not give satisfactory values for the concentration, because the rate of formation of the acyclic modification is fast in comparison with the time required for the analysis. 

Determination of the concentration of the acyclic form by physical means is more satisfactory. Measurement of ultraviolet absorption clearly shows that, under normal conditions, the concentration of the free carbonyl form must be low, but that, in strongly alkaline solutions, detectable amounts of carbonyl compounds may be present.  

In the course of time, in alkaline solution, a series of enolization, foeto-elimination, and rearrangement reactions take place. These reactions are not considered in this article, but they indicate that the acyclic modification is present in the equilibria. 

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Although most of these forms of sugars have never been isolated, they can be detected in the n.m.r. spectra of the sugars, and their proportion in the equilibrium mixture can be measured. Before the advent of n.m.r. spectroscopy, only a rudimentary knowledge of the composition of sugars in solution was available, but, since 1961, an extensive collection of data has been built up, mainly by the use of n.m.r. spectroscopy. These data are the subject matter of this article. The composition of sugars in solution was reviewed3 in 1969, but since then, much new information has been accumulated. 

Knowledge of the composition of sugars in solution is fundamental to carbohydrate chemistry. The physical and chemical properties of the sugars in solution depend on the proportions of their various forms and their biological properties may also show such dependence. Enzymes that utilize these sugars as substrates may not be able to use each of the forms. Where only a single form is utilized, the other forms may either be converted into the reactive form or may function as inhibitors. The latter is especially important if the reactive form is present in very low proportion at equilibrium. It is possible that the substrate form is utilized faster than it is generated from the other forms the observed rate of the reaction is then that of the tautomeric interconversion.4... 

Those caramels which are prepared from plain sugars without any catalyst present the relatively simplest chemical features. In order the better to understand the complexity of caramelization specific interconversions of sugars in solution have to be kept in mind. It is well known that pyranoid and furanoid sugars are considerably more stable than the acyclic forms. Mu-tarotation of sugars is autocatalyzed by protons, " and, with D-glucose, the equilibrium slightly favors )5-D-glucopyranose over the a anomer. It has... 

Fig. 1. Formulas of sugars which have been identified in the urine. The monosaccharides are represented in the straight chain form. The disaccharides are illustrated in the ring form. All sugars in solution are chiefly in the ring form.

It takes 20 to 40 L of sap to yield one liter of maple syrup. To remove the water, you could freeze the sap, as the Native Americans did 300 years ago. The ice that forms is pure water by discarding it, you increase the concentration of sugar in the remaining solution. Large-scale operators today use reverse osmosis to remove about half of the water. The remainder must be boiled off. The characteristic flavor of maple syrup is caused by compounds formed on heating, such as... 

Sports drinks provide water to the body in the form of an isotonic solution (one having the same total molar concentration of solutes as human blood). These drinks contain electrolytes such as NaCI and KCI as well as sugar and... 

One of the more difficult problems encountered in obtaining a valid assay of formic acid is that of formate ester formation. The formate is derived from the cyclic, hemi-acetal structure which is an equilibrium form of many free sugars in solution. For example, the oxidation of one of the cyclic forms of D-glucose can readily be seen to give a formate ester (as well as a C-formyl group) on the atom originally denoted as C5. It... 

The enzymatic synthesis of sucrose also throws light on the formation of the furanose form of fructose in the sucrose molecule. The fact that sucrose is directly formed from D-glucose-l-phosphate and D-fructose supports Isbell and Pigman s34 and Gottschalk s85 evidence that the latter monosaccharide occurs in solution in an equilibrium mixture of furanose and pyranose forms. This makes it unnecessary to postulate a special mechanism of stabilization of a five membered (furanose) ring before the formation of compound sugars containing the D-fructose molecule.86... 

Yeast cells are able to metabolize many types of sugars. In this experiment, you will observe the fermentation of sugar by baker s yeast. When yeast cells are mixed with a sucrose solution, they must first hydrolyze the sucrose to glucose and fructose. Then the glucose is broken down in the absence of oxygen to form ethanol and carbon dioxide. You can test for the production of carbon dioxide by using a CBL pressure sensor to measure an increase in pressure. 

Cooling solutions to below their freezing point results in the formation of ice. If solutions of sugars are cooled rapidly, non-equilibrium ice formation occurs. This is the most common form of ice in frozen dairy products (e.g. ice-cream). Rapid freezing of ice-cream mixes results in the freeze concentration of lactose and other sugars, resulting in supersaturated solutions if the temperature is too low to permit crystallization. The rapid cooling of lactose results in the formation of a supersaturated, freeze-concentrated amorphous matrix. 

Cane-sugar combines directly with the oxides of the alkalies and of the alkaline earths, forming with them sugarates—improperly called sometimes sacckarates and also with certain salts, particularly with chloride of sodium. The potassa and soda compounds, which are both unstable and indefinite, are obtained as sirupy liquids on adding a strong aqueous solution of the alkali to a solution of sugar in alcohol. Their compositions are—... 

The two aldotetroses, erythrose and threose, differ from the other aldoses in their behavior.23 Ring formation, to give furanoses, can occur only through the primary hydroxyl group, and is therefore less favored than with the higher sugars. Consequently, considerable proportions of the aldehydo and aldehydrol forms are found in solution. Like all a- and /J-hydroxyaldehydes, the aldehydo form of the aldotetroses readily forms dimers in concentrated solutions of the tetroses, the signals of the dimers are readily visible in their n.m.r. spectra. In the syrupy state, the tetroses consist mainly of dimers, rather than of furanoses they have never been crystallized. 

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