Carbohydrates Flashcards
What are the 3 key roles of carbohydrates?
structure
- form the cell walls in bacteria, plants and the exoskeletons of arthropods
metabolism
- energy carriers = ATP and coenzymes carry phosphorylated sugars
- energy storage = starch and glycogen are broken down into glucose
information
- framework for DNA/RNA
- carbohydrates on cell surface allow for cell-cell recognition
What are carbohydrates?
group of natural products related to simple sugars
- are hydrated carbons
What is the structure of sugars?
sugars (saccharides) are the simplest types of carbohydrates
there are two types
1 - polyhydroxyaldehydes = aldoses = aldehyde group
2 - polyhydroxyketones = ketoses = ketone group
How is the stereochemistry of carbohydrates decided/labelled?
stereochemistry is labelled with a D or L
- D = R, meaning it is clockwise
- L = S, meaning it is anti-clockwise
labelled with respect to glyceraldehyde
- if last OH next to the carbon furthest from the carbonyl is to the left = L
- if last OH next to the carbon furthest from the carbonyl is to the right = D
What are Fischer projections?
can be used to represent the carbohydrate structure in a simpler form
- shows the carbonyl group at the top
can be used to represent the sugar rings
- groups on the LEFT of a Fischer projection go ABOVE. the ring
- groups on the RIGHT of a Fischer projection go BELOW the ring
What re the cyclic isomers that cyclised sugars can form?
beta anomer
- cis groups in the aldoses/ketoses
- OH groups both point up in the equatorial position
alpha anomer
- trans groups on the aldoses and ketoses
- OH groups where one points up and one points down in the axial position
What is the stereocentre of a cyclised sugar called?
anomeric carbon
- has 4 constituent groups
- has two isomers = beta and alpha
What is a Hayworth projection?
can be used to represent cyclised sugars
- groups on the LEFT of a Fischer projection go ABOVE. the ring
- groups on the RIGHT of a Fischer projection go BELOW the ring
CH2OH on the top left of the ring
O on the top right of the ring
What is mutarotation?
process of reaching an equilibrium between the beta and alpha anomers of a cyclised ring in aqueous solution
- occurs when pure crystalline anomer is dissolved in water
- process can be catalysed by an acid or base = switches the one in excess to the opposite
What are the different conformations of rings?
chair
- cyclohexane prefer chair conformation
boat
envelope
- furanose (5C) rings prefer envelope conformation = 4C in the ring and 1C is raised
- furanose rings can rapidly interconvert between
How does the cyclisation of glucose occur?
O on carbonyl group reacts with water first - lone pair attacks H atom can form the furanose form - 4C attacks C1 can form the pyranose form - 5C attacks C1
What are the reactions that carbohydrates can go through?
due to the sugar functional groups - aldehydes, ketones and hydroxyls
oxidation
reduction
formation of glycosides
What is acetylation?
reaction of glucose with acetic anhydride to form a pentagram-acetate product
- all the H atoms of the hydroxyl group are replaced with an acetyl group
Why and how are sugars oxidised?
sugars are highly oxygenated so undergo a variety of oxidation reactions
these reactions can be used to identify structural information
- use metal ions to oxidise aldehyde groups in aldoses into carboxylic acids (aldonic acids)
What are the three tests for oxidising aldoses?
Benedict’s test
Fehling’s test
Tollen’s test
What is the Bendict’s test?
- Reagents
- Products
- Observations
Reagents
- Copper (2+) in aqueous sodium citrate
Products
- Carboxylic acid and copper oxide
Observations
- Red precipitate
What is Fehling’s Test?
- Reagents
- Products
- Observations
Reagents - Copper (2+) in aqueous sodium tartrate Products - Carboxylic acid and copper oxide Observations - Red precipitate
What is Tollen’s Test?
- Reagents
- Products
- Observations
Reagents - Silver (Ag+) in aqueous ammonia (NH3) Products - Carboxylic acid and silver Observations - Silver mirror on glassware
Why are sugars reducing agents?
they are oxidised by the Benedict’s and Fehling’s reagent and reduce the metal ions (Cu and Ag)
How can it be determined whether or not a sugar is a reducing sugar or not?
any aldehyde containing sugar that can remain in the open chain form is a reducing sugar
- allows the aldehyde group to be available for oxidation
any any aldehyde containing sugar that cannot remain in the open chain form is not a reducing sugar
sucrose is not a reducing sugar
Why can’t oxidation reaction be used preparatively for sugars? When can they be used preparatively?
they cannot be used preparatively because these conditions lead to the eventual decomposition of the sugar
aqueous bromine can be used for preparative oxidation
Why can some ketoses be used as reducing sugars?
some ketoses can be used as reducing sugars due to their rearrangement under basic conditions
What are the effects of stronger oxidising agents?
example - nitric acid
can result in diacids = oxidation at both ends of the chain
What are monosaccharides reduced to?
monosaccharides - polyaldoses = ribose, deoxyribose, glucose, fructose, galactose
can be reduced to give polyalcohols = alditols
reducing agent is sodium borohydride
What is an example of an alditol (polyalcohol)?
D-mannitol
- naturally occurring in mushrooms, olives and onions
What is a glycoside?
cyclic acetal = glycoside
- formed when sugars are mixed/combined with alcohols in the presence of an acid
- forms a glycosidic bond = covalent bond that joins a sugar to another molecule
contains hemi-acetal group
- chiral carbon with 2 R groups and 2 groups with an oxygen (carbon is directly bonded to one O and not the other)
How is a glycoside hydrolysed?
glycoside is in equilibrium
if a glycoside is mixed with water in the presence of catalytic amount of mineral acid
- hydrolysis favours the formation of a mixture of anomer of the parent sugar and alcohol
What happens to a glycoside in neutral aqueous solution?
glycoside is in equilibrium
in neutral aqueous solution
- no hydrolysis
- no mutarotation
under neutral conditions the glycosides are stable and exist as rings
Why are glycosides not reducing sugars?
they do not exist as/cannot remain in open chain form
- under neutral conditions they are stable and exists as rings
How is glycoside hydrolysis affected by enzymes?
hydrolysis can be catalysed by certain enzymes
- they selectively cleave the alpha or beta glycosidic bonds
alpha - OH substituted group is down
beta - OH substituted group is up
