BIOC2000 Lecture Notes - Lecture 17: Anomer, Hemiacetal, Carbohydrate Chemistry
Lecture 17&18: Sweet, strong and
sticky
Look at self-test questions
What are carbohydrates?
• An important class of biological molecules
• Originally defined as “hydrates of carbon”
• Cn(H2O)n (where n is at least 3)
- many do not fit this general formula
• Chemically, carbohydrates are:
- polyhydroxyaldehydes or polyhydroxyketones
• Therefore, carbohydrate chemistry is based on that of:
- Hydroxyl (-OH) and carbonyl (C=O) groups
- Acetal groups (from reaction between OH and C=O groups)
• Other functional groups may also be present (e.g. -NH2)
Monosaccharides
• Monosaccharides contain one carbon chain
- General formula: CnH2nOn (n is typically 3 – 9)
• Suffix “-ose” identifies a carbohydrate
• Precursors “tri-, tetr-, pent-” etc indicate the number of carbon
atoms in the main chain e.g. glucose has 6 carbons = hexose; there
are trioses, tetraoses, pentoses etc. depending on # of C
• Contain one carbonyl (C=O) group
- Aldoses contain an aldehyde group e.g. glucose
- Ketoses contain a ketone group
Sugars are aldoses or ketoses.Their names include “aldo” or “keto” and
a reference to the # of carbons in the sugar backbone.
Generally, aldoses are more common
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Chemistry in terms of functional groups monosaccharides and
sugars is pretty limited – stereochemistry is what is important
Sugars have many chiral centres
- Fischer projections of D-glucose and D-mannose represent that a C atom
with 4 single bonds coming out of it has stereochemistry
- The position of those 4 groups relative to each other is important for the
chemistry of the molecule
- Glyceraldehyde has one chiral center and therefore exists as two
different enantiomers (each pair of molecules that are mirror images of
eachother) with opposite optical rotation: D and L Glyceraldehyde
- Orientation around OH and H around C2 in glucose and mannose is
different, this distinguishes the two monosaccharides – they are both
hexoses but they have different stereochemistry, which makes them
completely different sugars in solution – thus they behave differently and
have different biological properties
With each additional C, there is an additional stereocentre and because
there are 2 possible ways that stereocentre can be organized, there are 2
different monosaccharaides that can be generated with each additional C.
#possible sugars doubles with every C added
Most sugars are actually not linear, they are cyclic because there is a
spontaneous reaction that can occur between an aldehyde and a nearby
alcohol group to form a hemiacetal bond – this happens in hexose, to form
a 6-membered ring
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In linear glucose, the C1 atom has only 3 different substituents (carbonyl
O, H and rest of sugar (R)) – not a chiral centre
When C5 reacts with C1 to form cyclic bond, there is a new chiral centre
around C1; there 2 different configurations that that bond can form: with
the OH group pointing down on left/up above ring on right - forming alpha
D and beta D Glucopyranose
Glucose in solution will be present as an equilibrium of all of these
compounds
The anomeric C is a stereocenter. An important feature is the direction of
the OH group attached to the anomeric carbon. Depending on the
direction of the OH group, the anomeric carbon is either or .
Reactions are reversible (linear structures can cyclize and cyclic structures
can be linear again)
So there is a dynamic equilibrium, where at any given time most of glucose
is in cyclic form (maybe 50% alpha, 50% alpha more or less) but a small
fraction is linear
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
What are carbohydrates: an important class of biological molecules, originally defined as hydrates of carbon , cn(h2o)n (where n is at least 3) Many do not fit this general formula: chemically, carbohydrates are: Polyhydroxyaldehydes or polyhydroxyketones: therefore, carbohydrate chemistry is based on that of: Acetal groups (from reaction between oh and c=o groups: other functional groups may also be present (e. g. -nh2) Aldoses contain an aldehyde group e. g. glucose. Their names include aldo or keto and a reference to the # of carbons in the sugar backbone. Chemistry in terms of functional groups monosaccharides and sugars is pretty limited stereochemistry is what is important. Fischer projections of d-glucose and d-mannose represent that a c atom with 4 single bonds coming out of it has stereochemistry. The position of those 4 groups relative to each other is important for the chemistry of the molecule.
