Mutarotation

4 Key excerpts on "Mutarotation"

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  eBook - PDF

  The Carbohydrates Volume 1A

  Chemistry and Biochemistry

  • W.W. Pigman(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  One oxygen atom, presum-ably that of the carbonyl or hemiacetal hydroxyl group is much more active than the others. Thus, when glucose is kept in 18 0-labeled water, one oxygen atom is exchanged after 100 hr at 55°. 3 Interconversions between a and ß anomers, and between ring tautomers, take place under the mildest possible condition of acidity and temperature. Such changes are manifested by the change of optical rotation with time which may be observed for freshly prepared sugar solutions. This change of rotation is known as Mutarotation. Mutarotations may arise from changes other than interconversions between a and ß anomers and between ring tautomers, but for neutral or slightly acid or slightly alkaline solutions of the sugars they arise most often from such changes. The phenomenon was observed first by Dubrunfaut (1846), who noted that the optical rotation of freshly dissolved D-glucose changes with time and that after a number of hours the rotation becomes constant. The ordinary form of glucose (a-D-glucopyranose) muta-rotates downward, and the β-Ό anomer mutarotates upward; in both cases the same equilibrium value is reached. a-D-Glucopyranose ^ equilibrium ^ ß-D-Glucopyranose +112° -> +52.7° «- +18.7° As mentioned in Chapter 1, the Mutarotation of glucose and other sugars showed that the original aldehyde structure for glucose was not adequate for explaining the properties of the sugar. The separation of isomers (anomers) of lactose (Erdmann, 1880), and later of D-glucose (Tanret, 1896), which mutarotated to the same equilibrium value, provided good evidence that the observed Mutarotations result from an interconversion of the various modi-fications. B. KINETICS The Mutarotation of a-D-glucose may be represented by the equation for a first-order reversible reaction.

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  eBook - ePub

  Carbohydrate Chemistry

  Fundamentals and Applications

  • Raimo Alén(Author)
  • 2018(Publication Date)
  • WSPC

    (Publisher)

  In addition to the spatial structure, the pH of the solution and the temperature, among others, also influence the mutual equilibrium between the tautomeric forms. Because of the amphoteric nature of water, Mutarotation of monosaccharides takes place in a solution in pure water, but it is catalyzed already by small amounts of acid or base. On the other hand, as the pH of the solution becomes increasingly acid or basic, the tautomeric equilibriums increase and, due to a momentary increase of the open-chain aldehyde, various further reactions, such as enolization or dehydration become more probable. Increasing the temperature of the solution accelerates the Mutarotation of the monosaccharides and generally increases the proportion of the thermodynamically less stable furanoid and open-chain aldehyde forms. The speed rate of Mutarotation increases on the average by a factor of about 2.5 for each 10°C rise in temperature but, as the temperature approaches 100°C, the general stability of tautomeric forms decreases substantially.

  Mutarotation is markedly influenced by the use of solvents other than water (e.g., dimethyl sulfoxide) or their various aqueous mixtures. It is also possible that when a “typical” solvent (such as

  N,N-

  dimethylformamide or pyridine) is employed, the Mutarotation is very slight without an acid or basic catalyst. Likewise, the other components of an aqueous solution (“impurities”) affect the stability of the monosaccharides and thus, their Mutarotation. For example, certain metal cations can have an effect on the equilibrium of the tautomeric forms if the spatial structure of the monosaccharides (i.e., the spatial orientation of the substituents) favors the formation of a metal complex, especially if the anomeric hydroxyl group can participate in the formation of the complex. In such cases, the equilibrium shifts towards the tautomeric form that is able to form the metal-monosaccharide complex. However, in certain cases, the metal ions can also form coordination bonds with water molecules more strongly than with hydroxyl groups of monosaccharides, leading to the weakening of the stabilizing effect of water on monosaccharides.

  6.2.Planar Formulas

  Open-chain structural formulas are illustrative for examining the different configurations of monosaccharides, and such formulas are used as the foundation for naming compounds. However, as the monosaccharides tend to cyclize and be present in nature mainly as ring structures, it is also meaningful to be able to represent the corresponding three-dimensional ring structures unambiguously as planar formulas.

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  The Organic Chemistry of Sugars

  • Daniel E. Levy, Péter Fügedi, Daniel E. Levy, Peter Fügedi(Authors)
  • 2005(Publication Date)
  • CRC Press

    (Publisher)

  The Organic Chemistry of Sugars 34 the L -isomers. Consequently, the mirror image (called enantiomer or antipode ) of any a -D -stereoisomer is the corresponding a -L -isomer (and not the b -L ). When using the abbreviated names of the carbohydrates, the furanose and pyranose forms can be indicated by adding an italic f or p respectively, to the three-letter symbol of the name. 2.3.2 M UTAROTATION When a crystalline optically active compound is dissolved in a solvent, and this solution shows a time-dependent change of the optical rotation approaching a certain equilibrium value, this phenomenon is called Mutarotation . Mutarotation is typical for crystalline free sugars, a freshly prepared solution of which (in water or other solvents) illustrates this phenomenon. In crystalline state, each compound is a discrete stereoisomer in which both the ring size and the configuration at the anomeric center are fixed. In solution, however, the hemiacetal ring opens and an equilibration between anomers, and eventually between furanose and pyranose forms, takes place. This is accompanied by a change in optical rotation, because each component of this equilibrium has its own specific rotation. The final value of the optical rotation of the solution will therefore be a weighted average sum of the individual specific rotations. In the case of glucose, the a -D -glucopyranose (which can be obtained on crystallization from acetic acid) has a specific rotation of þ 112 8 , while the crystalline b -D -glucopyranose (which can be obtained on crystallization from aqueous ethanol) has a specific rotation of þ 19 8 . When either of them is dissolved in water, at 20 8 C an equilibrium value of þ 52.7 8 is reached after about 3 h. Because in this equilibrium only the two pyranose anomers are appreciably present, their ratio can be calculated from the equilibrium rotation and corresponds to a : b ¼ 38:62.

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  Organic Reaction Mechanisms 1971

  An annual survey covering the literature dated December 1970 through November 1971

  • B. Capon, C. W. Rees, B. Capon, C. W. Rees(Authors)
  • 2008(Publication Date)
  • Wiley-Interscience

    (Publisher)

  Bowden and 98 R. P. Bell and J. E. Critchlow, Proc. Roy. SOC., A , 325, 35 (1971); of. Org. Reaction Mech., 1968, 99 F. E. Rogers and R. J. Rapiejko, J. Am. Chem. SOC., 93,4596 (1971). 100 E. B. Whipple, J. Am. Chem. SOC., 92, 7183 (1970). 101 G. C. S. Collins and W. 0. George, J. Chem. SOC. (B), 1971, 1352. 102 F. Pod0 and V. Viti, Org. Mag. Res., 3, 259 (1971). G. R. Taylor, J . Chem. SOC. (B), 1971, 1390, 1395. 356-357. Reactions of Aldehydes and Ketones and their Derivatives 403 The pH-rate profile for the Mutarotation of 6-deoxyglucohepturonic acid (19) is sigmoid and the form with the carboxyl group ionized reacts about 8 times faster than expected from a linear free-energy relationship for the Mutarotation of other 6-substi- tuted glucoses. The value of the catalytic constant for catalysis by hydroxyl ions for the Mutarotation of 6-0-(o-hydroxyphenyl)-~-glucose (20) is 2800 times greater than that for the Mutarotation of 6-0-phenyl-~-ghcose, probably because the ionized form reacts very rapidly. It seems likely that the Mutarotation of both (19) and (20) are intra- molecularly catalysed, the mechanisms possibly being those symbolized by (21) and (22) .lo3 CHzCOzH HO 7xY LOOH un CHZ-0 A The Mutarotation of tetra-0-methyl-D-glucose in benzene is enhanced by micellar dodecylammonium propionate and benzoate. The rate increases proportionally to the concentration of the long-chain salt at low concentrations, 10-5 to 1 0 -3 ~ but reaches a plateau at higher concentrations >lO-2~.104 Mutarotatase from E. coli K12 catalyses the Mutarotation of D-galactose, n-glucose, D-fucose, and D-xylose, but not that of n-mannose or 2-deoxy-~-glucose, although the last two sugars are inhibitors. The V,,, depends on the concentration of one functional group, pK, 5.5, in its basic form. K , depends on the concentration of a functional group of pKa 7.6 present in its acidic form.105 The kinetics of the complex Mutarotation of 2-deoxy-/&~-ribose have been studied.

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