Question

Can anyone explain to me how to identify epimers and anomers in carbohydrates (sugars)? Also, state examples.

Answer 1

Epimers and anomers are both optical isomers that differ in the configuration at a single carbon atom, but there is a difference in their definitions.

Explanation:

Epimers

Epimers are optical isomers that differ in the configuration of a single carbon atom

For example, D-galactose and D-mannose are epimers of D-glucose.

biochemnoob.files.wordpress.com
(from biochemnoob.wordpress.com)

D-Galactose is an epimer of D-glucose because the two sugars differ only in the configuration at `"C-4"`.

D-Mannose is an epimer of D-glucose because the two sugars differ only in the configuration at `"C-2"`.

Anomers

When a molecule such as glucose converts to a cyclic form, it generates a new chiral centre at `"C-1"`.

www.chem.ucalgary.ca
(from www.chem.ucalgary.ca)

The carbon atom that generates the new chiral centre (`"C-1"`) is called the anomeric carbon.

Anomers are special cases — they are epimers that differ in configuration only at the anomeric carbon.

For example, α-D-glucose and β-D-glucose are anomers.

wikispaces.psu.edu
(from wikispaces.psu.edu)

The α form has the anomeric `"OH"` group at `"C-1"` on the opposite side of the ring from the `"CH"_2"OH"` group at `"C-5"`.

The β form has the anomeric `"OH"` group on the same side as the `"CH"_2"OH"`.

In D-fructose, the carbonyl group is at `"C-2"`.

web.pdx.edu
(fromweb.pdx.edu)

Here, `"C-2"` is the anomeric carbon.

Fructose
(From pinterest.com)

α-D-Fructofuranose and β-D-fructofuranose are anomers.