Mutarotation

As mutarotation (from the Latin mutare, dt, change ' ) refers to the spontaneous change in the rotation angle of a solution of an optically active substance from the time of piecing the solution until it reaches a fixed value. At this point, the thermodynamic equilibrium is reached. Cause of the mutarotation may be epimerization or other asymmetric transformation.

Mutarotation in carbohydrates

Many Carbohydrates are optically active, that is, they rotate plane polarized light at a specific angle of rotation for each structure. Solving eg crystalline D -glucose in water, is observed in the polarimeter in the freshly prepared solution, a continuous change of the angle of rotation, until finally a constant value is reached. The speed at which equilibrium is temperature and pH dependent.

The reason is that aldopentoses, aldohexoses and longer carbohydrates ( sugar molecules with an aldehyde group and five, six or more carbon atoms) than in aqueous solution is substantially open-chain, but present as a ring hemiacetal, that is, the aldehyde group is a bond with the penultimate hydroxy group. Here is a new chiral center and two possible isomers (more precisely, diastereomers), the α - or β -anomer, which differ accordingly in their optical rotation angle. Now is the open-chain form with the ring mold in a dynamic equilibrium, which is however very on the side of the ring shape. Because with each ring forming the α - or β - form may occur, so these two are in equilibrium with a specific constant. Such a chemical conversion in which the configuration of only one diastereomer of a stereogenic center is changed is referred to as epimerisation.

For example, the glucose is in the balance of their open-chain form ( 0.02% in a neutral solution), the α -D-glucopyranose, and the β -D-glucopyranose. A solution of α -D-glucopyranose (angle of rotation 109 °) so by continuous ring-cleavage to a mixture of the β -D-glucopyranose (rotation angle 20 ° ) and the rotational angle of the solution obtained after a certain time, when equilibrium prevails, the value of 52 °. It is obviously not a 1:1 mixture before, the epimer ratio α: β is rather 36:64.

Other examples

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