Mutarotation lactose

The solubility characteristics of the a- and /J-isomers are distinctly different. When a-lactose is added in excess to water at 20°C, about 7 g per 100 g water dissolve immediately. Some a-lactose mutarotates to the fS anomer to establish the equilibrium ratio 62.7/J 37.3a therefore, the solution becomes unsaturated with respect to a and more a-lactose dissolves. These two processes (mutarotation and solubilization of a-lactose) continue until two criteria are met 7g a-lactose in solution and a / /a ratio of 1.6 1.0. Since the /J/a ratio at equilibrium is about 1.6 at 20°C, the final solubility is 7 g + (1.6 x 7) g = 18.2 g per 100 g water. 

When /J-lactose is dissolved in water, the initial solubility is 50 g per 100 g water at 20°C. Some /J-lactose mutarotates to a to establish a ratio of 1.6 1. At equilibrium, the solution would contain 30.8 g / and 19.2 g a/lOOml therefore, the solution is supersaturated with a-lactose, some of which crystallizes, upsetting the equilibrium and leading to further mutarotation of p - a. These two events, i.e. crystallization of a-lactose and mutarotation of / , continue until the same two criteria are met, i.e. 7 g a-lactose in solution and a / /a ratio of 1.6 1. Again, the final solubility is 18.2g lactose per 100 g water. Since /J-lactose is much more soluble than a and mutarotation is slow, it is possible to form more highly concentrated solutions by dissolving / - rather than a-lactose. In either case, the final solubility is the same. 

Does lactose mutarotate Is it a reducing sugar Explain. Draw the two anomeric forms of lactose. 

Lactose, mutarotation of, 12,16,23 —, 6,l, 6 -tri-0-p-tolylsulfonyl-, 244 Lactulose, conformational equilibria, 32 Levoglucosan... 

The mutarotation rate of lactose (as with other carbohydrates) depends on pH. The minimum rate of lactose mutarotation is in solutions of pH 4-5. In solutions of pH <2 and pH >7, the mutarotation rate increases signihcantly. Conformation of lactose in aqueous solutions (4-123) resembles the conformation of ceUobiose. 

With a-lactose the negative influence was less than with the jS-lactose and during the mutarotation reaction the conductivity increased in the case of the former, while it became less in the case of /3-lactose. The two isomers were very carefully purifled. Table VIII gives a review of the measurements. ... 

There is no doubt that for (8-lactose the conductivity in boric acid increases and for a-lactose it decreases. Thus, in a-lactose two hydroxyls are situated more favorably than in /3-lactose. On the strength of this investigation we may attribute the cis configuration to a-lactose and the trans to the /3-lactose. However, the observations were not accurate enough to serve as a basis for the calculation of a constant for the reversible change of the conductivity at most we can say that this change is of the same order of magnitude as that of the mutarotation. 

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-... 

Mutarotation has been shown to be a first-order reaction, the velocity constant being independent of reaction time and concentration of reactants. The rate of mutarotation increases 2.8 times with a 10°C rise in temperature. By applying the law of mass action, equations have been developed to measure the rate of the reversible reaction between the a and (3 forms of lactose. If a dilute lactose solution at constant temperature contain a moles of a and b moles of /3, then the amount of (3 formed (x) per unit of time is... 

Mutarotation also manifests itself in the solubility behavior of lactose. When a-lactose hydrate is added in excess to water, with agitation a definite amount dissolves rapidly, after which an additional amount dissolves slowly until final solubility is attained. 

Equations similar to those for mutarotation have been derived, expressing the relationship between the solubility behavior of the two forms of lactose and the equilibrium or rate constants (Hudson 1904). The constants derived by both mutarotation and solubility methods are in agreement. The solubility equations have been used to develop procedures for measuring a- and /3-lactose in dry milk (Roetman 1981). 

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-... 

If mutarotation (step 1) is slower than crystallization (step 2), it will determine the overall reaction, and the a-lactose level will be lower than the mutarotatory equilibrium value (37.3% a at 20°C). Conversely, if crystallization is slower, the a- and /3-lactose isomers in solution will be close to their equilibrium value. It has been shown that mutarotation occurs more rapidly under conditions normally found in milk products thus crystallization becomes the rate-determining step (Haase and Nickerson 1966). However, under conditions of very rapid crystallization where the supersaturated a-lactose is being deposited on a large surface area of nuclei, the percentage of a in solution will drop below its equilibrium value. Under these conditions, neither mutarotation nor surface orientation appears to be completely rate-limiting (Twieg and Nickerson 1968). 

Fig. 2.54 presents a two-dimensional carbon-proton shift correlation of D-lactose after mutarotational equilibration (40% a-, 60% / -D-lactose in deuterium oxide), demonstrating the good resolution of overlapping proton resonances between 3.6 and 4 ppm by means of the larger frequency dispersion of carbon-13 shifts in the second dimension. The assignment known for one nucleus - carbon-13 in this case - can be used to analyze the crowded resonances of the other nucleus. This is the significance of the two-dimensional CH shift correlation, in addition to the identification of CH bonds. For practical evaluation, the contour plot shown in Fig. 2.54(b) proves to be more useful than the stacked representation (Fig. 2.54(a)). In the case of D-lactose, selective proton decoupling between 3.6 and 4 ppm would not afford results of similiar quality. 

Fig. 2.54. Two-dimensional carbon-proton shift correlation of mutarotated D-lactose (1 mol/L in deuterium oxide , 3C 100.6 MHz H 400.1 MHz measuring time 90min transform lime 2.5 min) (a) stacked plot [64] (b) contour plot. 

In polarimetric determinations, the mutarotation must always be taken into account, this being greatest in glucose and less marked in levulose, invert sugar, maltose and lactose. Solutions of these sugars do not assume constant rotations until they have been prepared about 24 hours or kept at a high temperature for a short time. 

Carbon-1 of the glucose unit in lactose is a hemiacetal carbon and will be in equilibrium with the open-chain aldehyde form. Therefore, lactose will be oxidized by Fehling s solution and will mutarotate. 

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. 

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