BIOC 2300 Lecture Notes - Lecture 15: Anomer, Haworth Projection, Fischer Projection
3 May 2018
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Carbohydrates
February 12th, 2016
• May form branched as well as linear polymers
Introduction to Carbohydrates
• (CH2O)n *n is atleast 3
• major roles include energy from the diet, structural support and molecular recognition
• they can occur as monosaccharides or as polymers linked by glycosidic bonds which may
be linear or branches
• classified by:
o nature of carbonyl group: aldoses (aldehyde) or ketoses (ketone)
o number of C (triose, tetrose, pentose…)
o ring size: most sugards form closed rings with five (furanose) or six (pyranose)
atoms including one oxygen
o stereochemistry – most carbohydrates have multiple chiral centres
• ex. Glucose – aldohexase
• ex. Ribose – aldopentose
Stereochemistry
• many sugars are isomers – same formula and chemical properties (different structures and
biological properties)
• simplest sugar (glyceraldehyde) has one chiral centre: its D and L forms are enanitomers
(non-super imposable mirror images)
• larger sugars are diastereomers, and the D/L designation is reserved for the chiral carbon
furthest from the carbonyl group
• most natural carbohydrates are in the D configuration (OH to the right)
• two isomers with a different configuration around any one carbon except the anomeric
carbon are called epimers
• Fischer projection – 2D representation of stereochemistry with the carbonyl groups at the
top, horizontal above the plane of the page with vertical ones behind it
• Ex. D-Glucose and D-Galactose are epimers
• Ex. D-Glyceraldehyde and L-Glyceraldehyde are mirror images
Anomers and Ring Closure
• Sugars can interconvert between linear and cyclic forms: attack of an OH group on the
anomeric carbon produces two possible configurations (alpha and beta)
• Cyclic structures are normally shown in Haworth projection (Fisher projection tilted
90degrees to the right) with the anomeric OH group below (alpha) or above (beta) of the
plane of ring
• If the anomeric OH is unattached, the alpha and beta forms can freely interconvert with
the latter predomination 2:1 and very little linear form
• If the preferred chair conformation of beta-D-glucose has all the bulky OH groups
extending outward (equatorial) and the H groups are vertical (axial)
• Beat-D-glucose in chair conformation – least steric hinderance **most abundant
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