Week 5 (Carbohydrate Monomers) Flashcards
Why are carbohydrates important?
- distributed widely in nature
- key intermediates of metabolism (sugars)
- prove a huge range of cellular markers:
Polysaccharides are centrally involved in cell-cell recognition (how one type of cell distinguishes itself from another)
Small polysaccharide chains, covalently bound by glycosidic links to hydroxyl groups on proteins (glycoproteins) act as biochemical markers on cell surfaces, determining things such as blood type
What is an oligosaccharide?
What is a polysaccharides?
They are made up of several units
They are made up of many units
What is an Aldose?
Contains an aldehyde
What is a ketose?
A carbohydrate containing a ketone
What is a chiral centre?
A carbon atom with 4 different groups attached
What are the root names of sugars with varying numbers of carbon atoms?
3 carbons= triose
4 carbons= tetrose
5 carbons= pentouse
6 carbons= hexose
7 carbons= heptose
What are enantiomers?
Mirror images
Transposing any 2 of the groups on a chiral carbon results in a different compound with different physical properties: How do you work out the number of isomers?
2n Where n=the number of asymmetric carbons
What does ‘D’ (dextrorotatory) mean?
The plane has been rotated to the right
Do enantiomers have identical physical properties (e.g boiling point)?
Yes
What are epimers?
Sugars differing at a single asymmetric centre
What are enantiomers?
Sugars differing at all of their asymmetric (chiral) centres are mirror images
What are diasterioisomers?
Sugars differing at one or more chiral centre and which are not enantiomers (mirror images)
Do Diasterioisomers have the same physical properties?
No
What does an aldehyde and a alcohol react to produce?
A hemiacetal (There is an equilibrium set up)
What does a ketone and an alcohol react to produce?
A hemiketal (There is an equilibrium set up)
What is the 5 membered ring called?
Furan (Furanses)
Why are most sugars optically active?
They can rotate the plane of a beam of polarised light
What does ‘L’ (Levorotatary) mean?
The plane has been rotated to the left
Why does ‘D’ or ‘L’ not define the absolute stereochemistry at the chiral centre?
It is wavelength dependent; material could be dextrorotatory at a certain angle at one wavelegnth but something completely different (other direction or different angle) at a different wavelength
Identify if the sterocenter is ‘R’ or ‘S’ (to define the absolute stereochemistry)
When do D and L-glyceraldehyde (enantiomers) not have identical chemical properties?
When exposed to a chiral environment i.e. the optical rotation instrument/ polarimeter (e.g. they have equal but opposite specific rotations)
Do enantiomers have the same physical properties?
Yes
How are molecules in simplified Fisher projections depicted?
- The oxidised group (CHO- aldehyde/ketone) is at the top
- Vertical substiutents away
- Horizontal substiutuents towards
- CH2OH group at the bottom
The enantiomers for each molecule in the D series are in which series?
The L series
Aldehyde + alcohol →
Hemiacetal
Ketone + alcohol →
Hemiketal
How can carbohydrates form the equivalent of a Hemiketal or a hemiacetal?
Carbohydrates that have aldehyde or ketone functional groups and lots of hydroxy groups can form a hemiketal or a hemiacetal if the ring size is correct because one of the hydroxy group can bite round onto the aldehyde or ketone to produce the cyclic compound.
What the the 6 membered ring called?
Pyran (Pyranoses)
How are hemiacetal rings created?
By intermolecular nucleophilic addition reactions (Linear to cyclic form)
Which hemiacetal rings are the most stable?
Five and six membered rings
(Three, four, seven and eight membered rings provide too much strain)
What does the formation of the cyclic hemiacetal create?
An additional chiral centre giving two diasterioisomeric forms designated alpha and beta ( these diastereomers are called anomers)
If it’s the alpha anomer which direction does the OH (hydroxyl group) point?
Down (New hydroxyl is below the plane of the ring)
If it’s the beta anomer which direction does the OH (hydroxyl group) point?
Up (New hydroxyl above the plane of the ring)
Where is the intramolecular chiral centre formed in a hemiketal?
Descibe the change in shape that occurs
It’s formed at what was the C=O
(From sp2 Hybridized trigonal planar to sp3 hybridized tetrahedral)
What is mutarotation?
Mutarotation is occurs when the hydroxyl group at the anomeric position (C-1) switches configuration between alpha and beta
In water alpha and beta D- glucose interconvert via the open chains form to give an equilibrium mixture
Originally detected by changes in optical rotation
(Cyclisation of Glucose to Glucopyranose and mutarotation of alpha- and beta-D-glucose)
What and where are carbohydrates joined to alcohols and amines?
At the anomeric centre by glycosidic bonds
What kind of glycosidic bonds are in DNA?
N- glycosidic bonds
This is when an anomeric carbon of a sugar is linked to the nitrogen of an amine by an N-glycosidic bond e.g. adenosine
What is sucrose made from?
Sucrose (table sugar): disaccharide from two monosaccharides (glucose linked to fructose)
Describe Fischer Projections
Explain L and D configurations
The configuration is either D- or L depending upon the position of the hydroxyl (left or right) on the penultimate carbon.
The central carbon is asymmetric.
Why are many sugars described as opticaly active?
can rotate the plane of a beam of polarised light
What is Specific alpha rotation?
What needs to be defined?
Specific rotation, [α], is a fundamental property of chiral substances that is expressed as the angle to which the material causes polarized light to rotate at a particular temperature, wavelength, and concentration
rotation of a beam of polarised light by 100 g of solute in 100 mL of solvent with a path length of 10 cm.
(Need to define solvent, wavelength and temperature.)
D- and L-glyceraldehyde have identical physical properties except when:
exposed to a chiral environment eg they have equal but opposite specific rotations
4 asymmetric centers: How many sterioisomers
What do Pentoses and hexoses cyclise to form?
Furanose and pyranose rings
Describe the formation of Hemiacetala and Hemiketal
Describe the cyclisation of fructose to fructofuranose
What are the 4 compounds arising fro cyclisation of D-Fructose
This molecule can form α or β, 5 or 6 membered rings
What kind of bonds are Sugars linked to each other by?
Sugars can be linked to each other by O-glycosidic bonds to form disaccharides and polysaccharides eg sucrose, maltose & lactose.
