Carbohydrates

1
Carbohydrates

• Compounds containing C,H, and O
• General formula Cx(H2O)y
• Contain two functional groups CO -OH
• Classification based on
• size of base carbon chain
• number of sugar units
• location of CO
• Stereochemistry

2
Types of Carbohydrates

• Classification based on the number of sugar units
  in total chain
• Monosaccharides single sugar unit
• Disaccharides two sugar units
• Oligosaccharides 3 to 10 sugar units
• Polysaccarides more than 10 units
• Chaining is done through bridging oxygen atoms
  which are called glycoside bonds or glycosidic
  bonds

3
Types of Carbohydrates

• Based on location of CO
• Monosaccharides with six carbons (hexoses) They
  cannot be hydrolyzed into smaller units, unlike
  disaccharides and polysaccharides.
• Aldoses (aldehyde group)
• Ketoses (ketone group)
• C6H12O6 Glucose aldehyde group (blood
  
• sugar, grape sugar or
  dextrose)
• Galactose aldehyde group
• Fructose ketone group

4
Types of Carbohydrates

• Based on the number of carbons in the chain
  triose, tetrose, pentose, hexose, etc

5
Types of Carbohydrates

• Disaccharides 2 monosaccharides from an a or ß
  1,4- glycosidic bond with the loss of water.
• Maltose (glucose glucose)(germinating grains)
• Lactose (glucose galactose)(milk, yogurt, ice
  cream)
• Sucrose ( glucose fructose) (table sugar,it is
  found in sugar cane, sugar beets)

6
Types of Carbohydrates

• Polysaccharides long-chain polymers
• Starch rice, wheat, beans, potatoes and cereals.
• Amylose 20 of starch
• a-D-glucose molecules connected by
  a1,4-glycosidic bonds in a continuous (straight)
  chain.
• 250-4000 glucose units
• Amylopectin 80 of starch
• a-1,4-glycosidic bonds connect most of the
  glucose molecules
• Every 25 glucose units there are branches of
  glucose attached by a-1,6-glycosidic bonds
  between carbon 1 of the branch and carbon 6 in
  the main chain.
• Glycogen is found in liver and muscle and is made
  up
• of glucose units. a-1,4-glycosidic bonds and
  a-1,6-
• glycosidic bonds

7
Types of Carbohydrates

• Polysaccharides long-chain polymers
• Cellulose wood and plants
• ß-1,4-glycosidic bonds connect the glucose
  molecules to form unbranched chains that align in
  parallel rows held in place by hydrogen bonds
  which makes it more resistant to hydrolysis.

8
Stereochemistry and Stereoisomers

• Stereochemistry
• Study of the spatial arrangement of the molecules
• Stereoisomers
• Different spatial arrangements
• Different properties
• Same order and types of bonds
• Enantiomers
• They are mirror images of each other but they
  cannot be overlapped
• Designated by D- or L- at the start of the name

9
Stereoisomers enantiomers

• Chiral carbon (asymmetric carbon)
• a carbon that has 4 different groups
  attached to it
• You must have at least one asymmetric (chiral)
  carbon in order to have stereoisomers

10
Physical Properties

• Optical activity
• ability to rotate plane polarized light
• Dextrorotatory
• Rotate to the right
• Usually D isomers
• Use symbol
• Levorotatory
• Rotate to the left
• Usually L isomers
• Use - symbol

11
D-Sugars

• All monosaccharides have more than one chiral
  carbon, they are classified as
• D-sugars if the chiral carbon furthest away
  from the carbonyl group has the same
  configuration as D-glyceraldehyde. If they are
  dextrorotatory this is shown by a () after the
  D if laevorotatory by
• a (-).

12
Fischer Projections

• A tetrahedral carbon represented by two crossed
  lines
• A horizontal bond to an asymmetric (chiral)
  carbon designates bonds in the front plane of the
  page
• Vertical bonds are behind the plane of the page
• Places the most oxidized group at the top

13
Important Monosaccharides
14
Intramolecular Cyclization

• In glucose the OH on carbon 5 forms a hemiacetal
  bond with the aldehyde group. The new OH group on
  carbon 1 may be drawn below carbon 1 ---- a form
• above carbon 1-----ß form
• Haworth projections can be used to help see the a
  and ß orientations

15
Haworth Structures

• ß a
• Change in rotation of solution is called
  mutarotation.
• The two forms are called anomers and the switch
  is called epimerization. The cyclic structures
  are hemiacetals.

16
Haworth Structures
ß-()-ribose D-()-galactose
a-D-(-)-fructose
17
Reducing Sugars

• Monosaccharides and most of disaccharides are
  hemiacetals so they can be oxidized to carboxylic
  acids, so they are reducing sugarsglucose,
  galactose, fructose
• Cu2 from the Benedict reagent is getting reduced
  to Cu(Cu2O a green to red precipitate), so it is
  the oxidizing agent
• They react with alcohols at C1 forming an ether
  linkage known as a glycosidic bond to form
  disaccharides.

18
Reducing Sugars

• Ketoses are also reducing sugars, because the
  ketone group on carbon 2 isomerizes to give an
  aldehyde group on carbon 1 this process is
  called keto-enol tautomerism
• Under basic conditions fructose changes to an
  aldehyde by keto-enol tautomerism
• Polysaccharides are NOT reducing agents.

19
Disaccharides
Cellobiose ß(1-4)
maltose a(1-4)
Lactose ß(1-4) sucrose ß(1-2) Sucrose has both
sugars as acetals, no Benedicts reaction is not a
reducing sugar.