Carbohydrates

What are carbohydrates?

Organic molecules that have a general formula of C$_{x}$(H$_{2}$O)$_{y}$.

What are the general categories of carbohydrates?

Table 54.1: Categories of monosaccharides.

Monosaccharides	$-$
Disaccharides	2 monosaccharides
Oligosaccharides	2 to 10 monosaccharides
Polysaccharides	$>$10 monosaccharides

How are monosaccharides classified?

1. Based on the number of carbons they have:

   
   

   Table 54.2: Carbon number of common monosaccharides.

   Triose	3 carbons
   Tetrose	4 carbons
   Pentose	5 carbons
   Hexose*	6 carbons

   
   

   *Hexoses can form five-membered rings
   
   
   

2. Based on their functional groups:

   Figure 54.1: Aldoses and ketoses: aldoses contain an aldehyde (left) and ketose contains a ketone (right).

   

What are some common monosaccharides?

Figure 54.2: Common monosaccharides: (a) $L$-glyceraldehyde, (b) $L$-glucose, (c) $D$-fructose, (d) $D$-glucose, (e) $D$-ribose, (f) $D$-galactose.

What are the $D$ and $L$ designations for monosaccharides?

A system of distinguishing the enantiomers of a carbohydrate with a stereocenter. This system was created before the $R$ and $S$ classification in the early 1900Õs using glyceraldehyde.

Figure 54.3: $D$ (left) and $L$ (right) designations for glyceraldehyde.

How are the $D$ and $L$ designations made?

1. Remember how glyceraldehyde looks:
   • Note that the the dextro classification puts the $-OH$ on the right and the levatory classification puts the $-OH$ on the left.
     

2. Draw your monosaccharide with the same orientation as the glyceraldehydes, i.e. with the highest stereocenter at the bottom such that the aldehyde (ÐCHO) is at the top:
   

   Figure 54.4: Stereocenter orientation. The highest stereocenter is highlighted.

   

What are epimers and anomers?

Epimers are diastereomers (stereoisomers that are not mirror images of each other) that differ in the configuration of one stereoisomer:

Figure 54.5: Epimers: $D$-glucose (left) and $D$-mannose (right).

Anomers are special types of epimers that occur after cyclization and reflect a change in configuration at the hemiacetal or hemiketal carbon. This carbon is also called the anomeric carbon, and the two variants are identified with an $\alpha$ or $\beta$:

Figure 54.6: Anomers: $\alpha$-D-(+)-glucopyranose (left) and $\alpha$-D-(+)-glucopyranose (right).

What is meant by the absolute configuration of a molecule?

Absolute configurations: When the arrangement around a stereocenter is precisely determined by experiment, e.g. X-ray crystallography, to be $R$ or $S$.

Relative configurations: When the arrangement in an optically active molecule is determined (by experiment) to be ``+'' $(D)$ or ``-'' $(L)$ but it is unknown to what $R$ or $S$ configuration they are matched to.

How do straight-chain monosaccharides create rings?

1. The aldehyde or ketone group reacts with the hydroxyl group on the carbon chain
2. This creates a hemiacetal (from aldehydes) or a hemiketal (from ketones), resulting in an oxygen bridge between the two carbon atoms
3. A heterocyclic ring is formed
   • Five-membered rings are called furanose rings
   • Six-membered rings are called pyranose rings

What are acetals?

The product of reacting glucose with a small amount of methanol. Acetals are also called glycosides.

Figure 54.7: Acetals (glycosides).

What are glycosidic linkages?

Connections between glycosides, i.e. carbohydrate acetals or ketals.

What is the product of glycoside hydrolysis?

A sugar and an alcohol:

Figure 54.8: The product of glycoside hydrolysis: a sugar and an alochol.