C2131 Carbohydrate Chemistry

Chemistry 2131:
Organic Chemistry for the Life Sciences (3)

1. Carbohydrate Structure and Nomenclature:

sugars, or carbohydrates, have the general formula C_n(H₂O)_n, hence the name carbohydrates
carbohydrates are very important molecules. They are the main immediate and stored energy forms for plants and animals, they form the structural backbone of nucleic acids, they are attached to proteins and lipids (often serving as identifiers on cells)
all of the sugars are polyhydroxy aldehydes and ketones. One can name them using the IUPAC system, but normally one uses common names such as glucose...
simpler sugars are called saccharides. These can be monosaccharides, disaccharides, oligosaccharides, polysaccharides... depending on how many molecules are chemically joined together.
there are two major categories of monosaccharides: the aldoses and the ketoses. As the name suggests, the aldoses have a terminal carbonyl (hence aldehyde), whereas the ketoses have an internal carbonyl group.

2. Stereochemistry:

there are many stereocentres in the larger sugars, making for many stereoisomers. One can designate the centres as R or S.
one convention that is often used with sugars is the D L system.
Emil Fischer observed in the 1890's that only one form of the 3 carbon sugar glyceraldehyde was present in organisms. The molecule was dextrorotatory, so he called it D-glyceraldehyde. It turns out in this case that it is actually R-glyceraldehyde.
all sugars are named relative to D-glyceraldehyde. If they have the same orientation as D-glyceraldehyde at the penultimate carbon, that is the second to last one, then they are D sugars. Most sugars are D sugars
if one looks at the aldoses, one sees that there are 2 D tetroses (four carbon sugars), 4 D pentoses and 8 D hexoses. These sugars all have common names
the easiest way to represent sugars is with a Fischer projection.

3. Hemi-acetals and Mutarotation:

we have already seen that sugars do not exist as striaght chains most of the time. They form cyclic hemi-acetals.
6 membered sugar rings are called pyranoses, whereas 5 membered rings are called furanoses
formation of the hemi-acetal can be either acid or base catalysed (see notes for nucleophilic addition reactions for mechanism).
the carbon atom which was the carbonyl carbon in the straight chain form is called the anomeric carbon. It can be easily identified in a ring as the carbon bonded to 2 oxygen atoms (see how it maintains its oxidation state?)
when a hemi-acetal forms, a new stereocentre is created, so there are two possible stereoisomers for each sugar when it is in the ring form
in the beta-form, the anomeric hydroxyl group is on the same face of the molecule as the group on carbon 5. Or, in other words, if you draw the sugar in the chair position with the oxygen atom to the right and away from you, then the hydroxyl should be up for beta and down for alpha.
since the hemi-acetal formation is fully reversible, there is free isomerization around the anomeric carbon.

4. Glycoside Bonds:

simple sugars can be chemically joined together by acetal formation or glycoside bonds.
the "alcohol" involved is a hydroxyl group on another simple sugar
as in any acetal formation, water is lost during this reaction.
many different glycoside bonds are possible. By convention, the first sugar is named, then the bond is described. To do this the orientation of the anomeric carbon is defined then the carbon atom on the first sugar and the carbon atom on the second sugar which are attached to the hydroxyls involved in the reaction are named.
for example lactose is D-galactose in the Beta form with a glycoside bond between the hydroxyl on carbon 1 to the hydroxyl on carbon 4 of glucose.
sugars can be classified as reducing or non-reducing sugars. A reducing sugar has at least one anomeric carbon not involved in a glycoside bond. Non-reducing sugars have all of their anomeric carbons in glycoside bonds.