Carbohydrates

Carbohydrates

• Monosaccharides have many stereoisomers due to the present of several chiral centers.
• Carbohydrates can be modified adding new functionality
• Monosaccharides are linked to form polysaccharides via different types of O-glycosidic linkages
• Carbohydrates are the most abundant material on earth and cellulose is most abundant biological polymer.
• Polysaccharides can take various structures including long linear, branched, highly-branches structures depending on the function of the particular polysaccharide. Examples include starch, cellulose, glycogen, etc.
• Monosacchardies can be either cyclic or linear, they can contain either aldheyde or ketones. D-Glucose is the most common monosaccharide all others are variations of D-glucose.
• Each carbon except one in a monosaccharide contains a -OH group except one. The other carbon contains either a aldehyde or ketone group.
• The simplest sugars are three carbon sugar: Glyceraldehyde (aldose) and dihydroxyacetone (ketose). The use of the words aldose and ketose suggest whether the sugar contains an aldehyde (carbonyl at end of the chain) or a ketone (carbonyl anywhere else). These two can be interconvered between either by an isomeriation reaction.. >
• The most abundant are the hexose family of sugars. There are potentially 5 stereocenters in glucose, which means there are 2ⁿ stereoisomers or 32 different stereoisomers of gluclose. Some common stereoisomers of glucose found are galactose, mannose, fructuose, etc.
• Carboyhdrates are very polar and soluble in water and are 'sweet' in taste.
  

Stereochemistry and Structual Significance of Carbohydrates:

• The simplest monosaccharide is Glyceraldehye. It has one chiral center and as a result 2 enantiomers.
• The two enantiomers are called D- and L-glyceraldehyde. These two compound are special in that they are used as reference in naming absolute configuration of stereoisomers.
• For sugars with more than 2 chiral carbons, the prefix D or L refers to the chiral center that is furthest away from the carbonyl carbon. On this carbon if the -OH is on the left it is a L- sugar if the -OH is on the right it is a D sugar.
• Chiral centers other than the one furthest away from the carbonyl group are distinguish between each other by giving them different names. For example glucose and mannose.
• Two sugars that differ in configuration on only 1 carbon are called epimers. D-glucose and D-mannose differ in th configuration around C-2.

• All six carbon straight-chain aldose sugars have 4 stereocenters and hence 16 stereoisomers. While six carbon straight-chain ketose only contain three stereocenters and hence ony 8 stereoisomers.
• Monosaccahrides are optically active, and in solution usually exist as a ring rather than a linear structure

• Glucose in solution forms a six membered ring called a pyranose. There are several distinguishing features. What used to be C-1 (where the carbonyl used to be) is called the anomeric position.

• There are two unique position Alpha (a) and Beta (b). a is down and b is up on the anomeric position.
• Ketoses also form cyclic structures which are referred to as Furanose (For example Fructose).
• Haworth projections are used to analyze pyranoses and furanose.
• The rings in solution are not planar but rather exist as a chair configuration.
• You must be able to go between Fischer and Haworth Projections: "Fischer left it up to Haworth" Which signified that anything that is drawn to the left on a Fischer Projection will be 'up' in the corresponding Haworth projection.

MORE PRACTICE HERE: FISCHER PROJECTIONS going back and forth etc

How to project a 3D image into a Fischer Projection

Monosaccharides can be modified in many ways:

1. amino group
2. methylation
3. phosphorylation
4. deoxy
5. the carboyl group can be oxidized or reduced to carboxylic acids or alchols respectively

Disaccharides

Lactose-

Sucrose- Glucose + Fructose

• O-Glycosidic linkages connect the anomeric carbon to a second alcohol to form di-, oligo-, and poly saccharides.
• Most glycosidic linkages take place between the anomeric carbon of one monosaccharide and C-4 of the second monosaccharide.

Polysaccharides

Cellulose-

Homopolysaccharides- composed of a single type of monomer

• Starch- two kinds: Amylose- unbranched a -1-4 linkages of glucose. Amylopectin- branched a-1-4 linkages with 1-6 branches
• Glycogen- used for storage in animals. Like amylopectin but more branches
• Cellulose- component of plant cell walls. Very few organisms can digest cellulose. b-1-4 glucose linkages, the glucose residues are flipped 180° relative to each other
• Peptidoglycan- major component of bacterial cell wall. Alternating N-acetylglucosamine and N-acetylmuramic acid in 1-4 linkages
• Glycoproteins- carbohydrate-protein conjugages in which the carbohydrate moiety is short
• Glycolipids- membrane lipids containing covalently attached oligosaccharide chains. Very critical for cell recognition (cell-cell recognition, etc).