Sucrose, mutarotation

The most familiar of all the carbohydrates is sucrose—common table sugar. Sucrose is a disacchar ide in which D-glucose and D-fructose are joined at then anomeric carbons by a glycosidic bond (Figure 25.7). Its chemical composition is the same ine-spective of its source sucrose from cane and sucrose from sugar beets are chemically identical. Because sucrose does not have a free anomeric hydroxyl group, it does not undergo mutarotation. 

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-... 

The result, [a] D -f-110° with an error of not more than + 2°, showed that prior to mutarotation the D-glucose was the ordinary a-form of rotation approximately - -109°, now known as a-D-glucopyranose. Sucrose, then, was an a-D-glucoside. Inspection of Fig. 1 also shows that after inversion but before mutarotation the sum of the rotations contributed by the a-D-glucose and the D-fructose remained very close to the specific rotation of 66° possessed by the original sucrose. The relationship ... 

Problem 22.39 (u) Give the structure of sucrose (cane and beet sugar) from the following information (1) It is hydrolyzed by maltase or emulsin to a mixture of d-( )-glucose and D-(-)-fructose. (2) It does not reduce Fehling s solution and does not mutarotate. (3) Methylation and hydrolysis gives 2,3,4.6-tetra-O-methyl-D-glucose and a tetramethyl-D-fructose. (b) What structural features are uncertain ... 

Sucrose is a disaccharide that is composed of a unit of glucose (acetal form) and a unit of fructose (ketal form) linked through C-1 of glucose and C-2 of fructose, i.e. a 1,2 link. In sucrose, neither glucose nor fructose can exist in open chain form because of the formation of acetal and ketal as shown below. As a result, sucrose is not a reducing sugar, and does now exhibit mutarotation. The specific rotation [a]D of sucrose is +66°. 

Some carbohydrates actively inhibit the crystallization of lactose, whereas others do not. Carbohydrates that are active possess either the /3-galactosyl or the 4-substituted-glucose group in common with lactose, so that adsorption can occur specifically at certain crystal faces (Van Krevald 1969). (3-Lactose, which is present in all lactose solutions [see Equilibrium in Solution (Mutarotation )], has been postulated to be principally responsible for the much slower crystallization of lactose compared with that of sucrose, which does not have an isomeric form to interfere with the crystallization process (Van Krevald 1969). Lactose solubility can be decreased substantially by the pres-... 

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-... 

The solvent and the presence of salts or sucrose influence the solubility of lactose, as well as the rate of mutarotation. The solubility of lactose increases with increasing concentrations of several calcium salts—chloride, bromide, or nitrate—and exceedingly stable, concen-... 

Among the most interesting examples of varying sugar sweetness is the fact that crystalline or freshly dissolved / -D-fructopyranose is about twice as sweet as sucrose, but after mutarotation or during thermal mutarotation, sweetness diminishes markedly (3). a-D-Glucopyranose is about two-thirds as sweet as sucrose, but the mutarotated solution is even less... 

When crystalline / -D-fructopyranose is newly dissolved in water, it is twice as sweet as sucrose, but shortly thereafter it is only slightly sweeter. Fructose mutarotates rapidly, and such phenomena have been associated by Isbell (4) with the formation of furanose forms of the sugars. Using a gas chromatographic procedure (5), we have shown (6) that the mutarotation primarily results from the formation of that isomer present in the sucrose molecule or -D-fructofuranose. 

Hudson10 showed that the mutarotation of fructose in water at 30° is eleven times faster than that of glucose. He therefore assumed that in a sucrose solution which is undergoing very rapid inversion with invertase at that temperature, practically all of the fructose has reached equilibrium and exists as a mixture of its a and 0 forms, while the glucose is being liberated in only one form which, however, slowly passes to its a, 0 equilibrium mixture. The drop in rotation between the apparent and real curves of inversion by invertase must therefore be due almost entirely to the mutarotation of glucose. Hudson thus showed that the D-glucose liberated from sucrose by invertase had a specific rotation between [< ]d +100° and +125° and is thus most likely the a-form. 

Polarimetry is extremely useful for monitoring reactions of optically active natural products such as carbohydrates which do not have a useful UV chromophore, and samples for study do not need to be enantiomerically pure. Nevertheless, compared with spectrophotometry, the technique has been applied to relatively few reactions. It was, however, the first technique used for monitoring a chemical reaction by measuring a physical property when Wilhemy investigated the mutarotation of sucrose in acidic solution and established the proportionality between the rate of reaction and the amount of remaining reactant [50]. The study of a similar process, the mutarotation of glucose, served to establish the well-known Bronsted relationship, a fundamental catalysis law in mechanistic organic chemistry. 

Answer Lactose (Gal061— 4)Glc) has a free anomeric carbon (on the glucose residue). In sucrose (Glc(al<- 2 8)Fru), the anomeric carbons of both monosaccharide units are involved in the glycosidic bond, and the disaccharide has no free anomeric carbon to undergo mutarotation. 

Lactose undergoes mutarotation in basic solution but sucrose does not. Explain. 

---