Mutarotation is the term given to the change in the
specific rotationThe specific rotation of a chemical compound [α] is defined as the observed angle of optical rotation α when plane-polarized light is passed through a sample with a path length of 1 decimeter and a sample concentration of 1 gram per 1 millilitre. The specific rotation of a pure material is an...
of a cyclic
monosaccharideMonosaccharides are the most basic unit of carbohydrates. They are the simplest form of sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose , fructose , galactose, xylose and ribose...
as it reaches an equilibrium between its α and β anomeric forms. It was discovered by Dubrunfaut in 1846, when he noticed that the specific rotation of aqueous sugar solution changes with time. The optical rotation of the solution depends on the optical rotation of each anomer and their ratio in the solution.
For example if a solution of β-D-glucopyranose is dissolved in water, its specific optical rotation will be +18.7.
Mutarotation is the term given to the change in the
specific rotationThe specific rotation of a chemical compound [α] is defined as the observed angle of optical rotation α when plane-polarized light is passed through a sample with a path length of 1 decimeter and a sample concentration of 1 gram per 1 millilitre. The specific rotation of a pure material is an...
of a cyclic
monosaccharideMonosaccharides are the most basic unit of carbohydrates. They are the simplest form of sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose , fructose , galactose, xylose and ribose...
as it reaches an equilibrium between its α and β anomeric forms. It was discovered by Dubrunfaut in 1846, when he noticed that the specific rotation of aqueous sugar solution changes with time. The optical rotation of the solution depends on the optical rotation of each anomer and their ratio in the solution.
For example if a solution of β-D-glucopyranose is dissolved in water, its specific optical rotation will be +18.7. Over time, some of the β-D-glucopyranose will undergo mutarotation to become α-D-glucopyranose, which has an optical rotation of +112.2. Thus the rotation of the solution will increase from +18.7 to an equilibrium value of +52.5 as some of the β form is converted to the α form. The equilibrium mixture is actually about 64% of β-D-glucopyranose and about 36% of α-D-glucopyranose, though there are also with traces of the other forms including furanoses and open chained form.
Mechanism
Though the cyclic forms are usually heavily favoured, liquid monosaccharides (or monosaccharaides in aqueous solution) are always in equilibrium with their straight-chain forms. This equilibrium is established as the
hemiacetalHemiacetals and hemiketals are compounds of the general formula R1R'1COR2, where R2 is not hydrogen. These types of compounds are formed from carbonyl compounds and alcohols, i.e. hemiacetals from aldehydes, hemiketals from ketones...
bond between C1 (the only carbon bound to two oxygens) and C5 is cleaved (forming the straight-chain compound) and reformed (forming the cyclic compound). When the
hemiacetalHemiacetals and hemiketals are compounds of the general formula R1R'1COR2, where R2 is not hydrogen. These types of compounds are formed from carbonyl compounds and alcohols, i.e. hemiacetals from aldehydes, hemiketals from ketones...
bond is reformed, the OH group on C5 may attack either of the two stereochemically distinct sides of the
aldehydeAn aldehyde is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom bonded to a hydrogen atom and double-bonded to an oxygen atom , is called the aldehyde group...
group that contains C1. Which side it actually does attack on decides whether the α or β
anomerIn carbohydrate chemistry, an anomer is a special type of epimer. It is a stereoisomer of a cyclic saccharide that differs only in its configuration at the hemiacetal carbon, also called the anomeric carbon....
is formed.
If the reaction took place in amphoteric solution such as 2-pyridone, the rate of mutarotation would be much faster
Observed effects
These α and β
anomerIn carbohydrate chemistry, an anomer is a special type of epimer. It is a stereoisomer of a cyclic saccharide that differs only in its configuration at the hemiacetal carbon, also called the anomeric carbon....
s are diastereomers of each other and usually have different
specific rotationThe specific rotation of a chemical compound [α] is defined as the observed angle of optical rotation α when plane-polarized light is passed through a sample with a path length of 1 decimeter and a sample concentration of 1 gram per 1 millilitre. The specific rotation of a pure material is an...
s: a solution or liquid sample of a pure α anomer will rotate plane polarised light by a different amount and/or in the opposite direction than the pure β
anomerIn carbohydrate chemistry, an anomer is a special type of epimer. It is a stereoisomer of a cyclic saccharide that differs only in its configuration at the hemiacetal carbon, also called the anomeric carbon....
of that compound.
in a different direction or by a different amount number of degrees than a solution of pure β
anomerIn carbohydrate chemistry, an anomer is a special type of epimer. It is a stereoisomer of a cyclic saccharide that differs only in its configuration at the hemiacetal carbon, also called the anomeric carbon....
will when such light is passed through them. In a mixture, each anomer affects the light in its own way, so the net effect is the total affect of each component.
Most importantly, the optical rotation of the sample is an average of the optical rotation of each anomer weighted by the amount of that anomer present. Therefore one can use a
polarimeterA polarimeter is a laboratory instrument used to determine the angle of optical rotation of plane-polarized light passing through a sample of material.A typical polarimeter consists of a light source, a polarizer, a sample holder, and some form of viewer...
to measure the rotation of a sample and then calculate the ratio of the two anomers present, as long as one knows the rotation of each pure anomer. One can monitor the mutarotation process over time or determine the equilibrium mixture by observing the optical rotation and how it changes.
See also
- Anomer
In carbohydrate chemistry, an anomer is a special type of epimer. It is a stereoisomer of a cyclic saccharide that differs only in its configuration at the hemiacetal carbon, also called the anomeric carbon....
- Monosaccharide
Monosaccharides are the most basic unit of carbohydrates. They are the simplest form of sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose , fructose , galactose, xylose and ribose...
- Carbohydrate
Carbohydrates
[Means "hydrates of carbon"] or saccharides
[The word comes from the Greek σάκχαρον, sákcharon, meaning "sugar").] are the most abundant of the four major classes of biomolecules...
- Polysaccharide
Polysaccharides are polymeric carbohydrate structures, formed of repeating units joined together by glycosidic bonds. These structures are often linear, but may contain various degrees of branching. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit...
- Stereochemistry
Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms within molecules. An important branch of stereochemistry is the study of chiral molecules....