Mutarotation of glucose

D-Glucose can crystallize in two forms: α-D-glucopyranose, with a specific rotation of +112°, and β-D-glucopyranose, +19°. If either of them is dissolved in water, the rotation changes slowly until it settles at +53°. This change is called mutarotation.

What happens is that the cyclic hemiacetal opens, with acid or base catalysis, and gives the open-chain form, with the free aldehyde. This form is very minor, but it keeps closing again: the C5 OH attacks the C=O on one face or the other and gives the α anomer (axial anomeric OH) or the β anomer (equatorial). After a few hours equilibrium is reached, with 36 % α and 64 % β, plus traces of the open form.

OOHOHOHHOCH2OHOOHHHOHHHOHHOHCH2OH
α-D-Glucopyranose as a Haworth projection and in the chair conformation: the anomeric OH (C1) is axial. In the chair, axial bonds are blue and equatorial bonds green.
OOHOHOHHOCH2OHOHOHHOHHHOHHOHCH2OH
β-D-Glucopyranose as a Haworth projection and in the chair conformation: all the substituents, including the anomeric OH, are equatorial. In the chair, axial bonds are blue and equatorial bonds green.

The β anomer predominates because all the ring substituents are equatorial in it. The final ratio is decided by stability, not by rate: it is an equilibrium under thermodynamic control.

The video "Hurto rápido o robo profesional: una historia de glucosa" (in Spanish: "Quick theft or professional robbery: a glucose story") tells this process with the same comparison used on the kinetic and thermodynamic control page: a quick job versus a slow one that leaves a much more stable situation.

All sugars with a free hemiacetal (or hemiketal) mutarotate in the same way, such as galactose, mannose, fructose, lactose or maltose. Sucrose does not mutarotate: its two anomeric carbons are bonded to each other and cannot open.

OOHOHOHHOCH2OH
β-D-Mannopyranose: the C2 epimer of glucose
OOHCH2OHOHOHHOH2C
β-D-Fructofuranose, the form of fructose found in sucrose
OOHOHHOCH2OHOOHOHOHCH2OHO
Maltose: glucose-α(1→4)-glucose; the right-hand glucose keeps its hemiacetal and mutarotates
OOHOHHOCH2OHOOHOHOHCH2OHO
Lactose: galactose-β(1→4)-glucose; the glucose, drawn flipped, keeps its hemiacetal
OOHOHHOCH2OHOHOH2COHOHCH2OHO
Sucrose: glucose-α(1→2)-β-fructose; both anomeric carbons are linked and it does not mutarotate

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