Mutarotation

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As mutarotation (from latin mutare , dt, change '.) Is the spontaneous change in the rotation angle of a solution of an optically active substance from time to adhesion of the solution to reach a fixed value. At this point the thermodynamic equilibrium is reached. The cause of the mutarotation can be epimerization or some other asymmetrical transformation.

Mutarotation in carbohydrates

Mutarotation: D-glucose molecules exist as cyclic hemiacetals, which are epimeric (= diastereomeric) to one another. The epimer ratio α: β is 36:64. In α-D-glucopyranose (left) the blue- marked hydroxyl group at the anomeric center is in the axial position, while in β-D-glucopyranose (right) the blue- marked hydroxyl group at the anomeric center is in the equatorial position. The open-chain form of the aldose with the free aldehyde group is in the middle. Only 0.02% of the D-glucose molecules are present in this way.

Many carbohydrates are optically active; That is, they rotate linearly polarized light through a rotation angle specific for each structure . If you solve z. B. crystalline D- glucose in water , a continuous change in the angle of rotation is observed in the polarimeter in the freshly prepared solution until a constant value is finally reached. The speed at which equilibrium is established is temperature and pH dependent.

The reason for this is that aldopentoses, aldohexoses and longer-chain carbohydrates (sugar molecules with one aldehyde group and five, six or more carbon atoms) in aqueous solution are essentially not open-chain, but rather ring-shaped as a hemiacetal . that is, the aldehyde group bonds with the penultimate hydroxy group . A new center of chirality and two possible isomers (more precisely: diastereomers ), the α or β anomer , which also differ in their optical angle of rotation, are created here. Now the open-chain shape is in a dynamic equilibrium with the ring shape , which is very much on the side of the ring shape. Because the α- or β-shape can arise with every ring formation, these two are also in equilibrium with a very specific constant. Such a chemical transformation, in which the configuration of only one stereogenic center of a diastereomer is changed, is known as epimerization.

For example, glucose is in equilibrium in its open-chain form (0.02% in neutral solution), α-D- glucopyranose and β-D-glucopyranose. A solution of α-D-glucopyranose (angle of rotation + 109 °) thus becomes a mixture with β-D-glucopyranose (angle of rotation + 20 °) through continuous ring cleavage, and the angle of rotation of the solution reaches after some time when equilibrium prevails , the value of + 52 °. Obviously there is no 1: 1 mixture, the epimer ratio α: β is rather 36:64.

Reaction mechanism for the mutarotation of α and β anomers.

Other examples

D - gluconic acid ([α] 20 D  = −6.72 °) is in equilibrium with γ-glucono-1,5-lactone ([α] 20 D  = + 67.8 °) and δ-glucono-1,6 -lacton before ([α] 20 D  = + 63.5 °). In equilibrium, the optical rotation is + 12 °. In this case, equilibrium is established through lactone formation.

Individual evidence

  1. ^ S. Ebel, HJ Roth (editor): Lexikon der Pharmazie , Georg Thieme Verlag, 1987, ISBN 3-13-672201-9 , p. 450.
  2. ↑ Albert Gossauer: Structure and reactivity of biomolecules , Verlag Helvetica Chimica Acta, Zurich, 2006, ISBN 978-3-906390-29-1 , pp. 322-324.
  3. ↑ Reinhard Mattisek, Gabriele Steiner, Markus Fischer: Food Analysis . 4th edition. Springer, Berlin 2010, ISBN 978-3-540-92205-6 , pp. 400-401 .
  4. ↑ K. Rehorst: On the knowledge of some oxy acids of the sugar group, I .: d-sugar acid and d-glyconic acid . In: Reports of the German Chemical Society (A and B Series) . tape 61 , no. 1 , 1928, pp. 163-171 , doi : 10.1002 / cber.19280610126 .
  5. ↑ W. Bähr, H. Theobald: Organic Stereochemistry. Springer-Verlag, 1973.