Sugars are molecules that have a generic formula of
CnH2nOn. They can be descibed as polyhydroxy
compounds containing either a ketose of aldehyde
group. Most sugars are optically active and this comes
from the presence of asymmetrically substituted
carbons in the sugar. Glyceraldehyde is the simplest
sugar molecule with optical activity. Dglyceraldehyde shifts light to the right (+) whereas Lglyceraldeyde shifts light to the left.
Asymmetry in glyceraldehyde
Different sugar conformations
These flat linear drawings are known as Fisher projections and the relative position of the first
asymmetric carbon on sugar chain (relative to the non reducing end) determines if the sugar is a
D or L sugar. The (+) or (-) classification is the optical activity and is not directly related to the D
and L conformations.
The numbering of carbon atoms in sugar systems is based on the Fisher project starting at the top
as C1. Hence in a glucopyranose structure the numbering runs clockwise starts just below the
ring oxygen with the final C6 carbon froming
the -CH2OH side chain.
However sugars can rotate through a wide
range of structures and the reducing end can
cyclise via one of the hydroxy groups to form
ring structures, usually five or six membered
rings. These ring structures can flip between
different conformations themselves and the
most common forms are known as the chair
conformations. The two different chair
alpha sugar
beta sugar
conformations get their names from the
relative positions of the 1 and 4 carbon
positions to the main sugar plane as defined
by the O-C2-C3-C5 plane. There is also a
conformation known as the boat but in most sugars this is not very favoured energetically.