Seminar 5 Carbohydrates
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Seminar 5
Carbohydrates
2
Saccharides (glycids)
are polyhydroxyaldehydes, polyhydroxyketones, or substances that give such compounds on hydrolysis
Definition
3
POLYSACCHARIDES
polymeric
Give monosaccharides when hydrolyzed
GLYCANS
Basal units
MONOSACCHARIDES polyhydroxyaldehydes polyhydroxyketones
OLIGOSACCHARIDES
2 – 10 basal units
GLYCOSES (sugars) water-soluble, sweet taste
The historical misleading term carbohydrates. It was primarily derived from the empirical formula Cn(H2O)n and currently is taken as incorrect, not recommended in the IUPAC nomenclature (even though it can be found in numerous textbooks till now)
Classification
4
• occur widely in the nature, present in all types of cells
– the major nutrient for heterotrophs
– energy stores (glycogen, starch)
– components of structural materials (glycosaminoglycans)
– parts of important molecules (nucleic acids, nucleotides, glycoproteins, glycolipids)
– signalling function (recognition of molecules and cells, antigenic determinants)
Saccharides
5
are simple sugars that cannot be hydrolyzed to simpler compounds
Aldoses Ketoses Simple derivatives (polyhydroxyaldehydes) (polyhydroxyketones) modified monosaccharides
are further classified according to the number of carbon atoms in their chains:
glyceraldehyde (a triose) dihydroxyacetone tetroses tetruloses pentoses pentuloses hexoses hexuloses heptoses … heptuloses …
deoxysugars amino sugars uronic acids
other simple derivatives
alditols glyconic acids glycaric acids
Trivial names for stereoisomers
glucose (i.e. D-glucose) fructose (i.e. D-fructose) L-idose L-xylulose, etc.
Systematic names (not used in biochemistry) comprise trivial prefixes according to the configuration: e.g., for glucose D-gluco-hexose, for fructose D-arabino-hexulose
Monosaccharides
6
Secondary alcoholic groups CH-OH in monosaccharides are stereogenic centres. Monosaccharides are chiral compounds and, therefore, most of them are optically active
Stereogenic centres are mostly carbon atoms that bind four different groups; those atoms are often called "asymmetric" carbon atoms
If there are more (n) stereogenic centres in the given molecule,
the maximal number of stereoisomers equals 2n
Each of those stereoisomers has its enantiomer (mirror image) so that there will be a maximum of 2n / 2 pairs of enantiomers
Stereoisomers that differ from the particular pair of enantiomers are diastereomers of the pair
In contrast to enantiomers, diastereomers differ in their properties and exhibit different values of specific optical rotation
Stereoisomerism in monosaccharides
7
are structural formulas that describe the configuration of particular stereoisomers When a plane formula of an aldose with four stereogenic centres is drawn anywhere
an hexose
it is necessary to see a spatial arrangement of the atoms and assess it according to the established rules:
• the least number carbon (carbonyl group in monosaccharides) is drawn upwards
• the carbon chain is directed downwards then on each stereogenic centre
• the bonds to neighbouring carbon atoms written above and below are projected from beneath the plane of drawing (the carbons are behind the plane)
• the horizontal bonds written to the left and right are projected from above the plane of drawing, they are in front of plane
Fischer projections formulas
8
Assigning configurations D- and L- (from Latin dexter and laevus) at stereogenic centres is carried out
by comparison with the configurations of D- and L-glyceraldehyde
Without changing the configuration,
Fischer formulas may only be turned 180° in the plane of the paper.
Monosaccharides are classified as D- or L-sugars according to
configuration at the configurational carbon atom – the chiral
carbon with the highest numerical locant (i.e. the asymmetric
carbon farthest from the aldehyde or ketone group):
D-aldose L-ketose
9
D-allose D-glucose
What is that?
D-mannose L-glucose
Enantiomers, diatereomers, epimers
• L-glucose is enantiomer of D-glucose because of
having opposite configuration at all centres of chirality
• Are there, among the following sugars, some diastereomers
of D-allose that are not epimers of it?
• Is there any epimer of D-mannose?
10
Stereogenic centres in molecules of monosaccharides are the cause of their optical activity Solutions of mono- and oligosaccharides turn the plane of polarized light Optical activity is measured by using polarimeters and usually expressed as specific optical rotation [α]D
20.
Dextrorotatory substances are marked (+), laevorotatory (–)
Configurations at stereogenic centres other than configurational carbon cannot be deduced from the assignment to D- or L-sugars.
Unfortunately, configurations of several most important monosaccharides have to be remembered
There is no obvious relation between the assignment D- or L- and either the values or direction of optical activity
11
D-glyceraldehyde
D-erythrose D-threose
D-ribose D-arabinose D-xylose D-lyxose
D-allose D-altrose D-glucose D-mannose D-gulose D-idose D-galactose D-talose
D- Aldoses stereochemical relations
12
D-(–)-erythrose D-(–)-threose
D-(–)- arabinose D-(+)-xylose D-(–)-lyxose
D-(+)-allose D-(+)-altrose D-(–)-gulose D-(–)-idose D-(+)-talose
D- Aldoses optical rotation
D-(+)-glyceraldehyde
D-(–)-ribose
D-(+)-glucose D-(+)-mannose D-(+)-galactose
(+) dextrorotatory
(–) laevorotatory
13
D-(–)-erythrulose
D-(+)-xylulose
D-(+)-psicose D-(+)-sorbose D-(+)-tagatose
D- Ketoses stereochemical relations
dihydroxyacetone
D-(–)-fructose
D-(–)-ribulose
14
Cyclic forms of monosaccharides
Monosaccharides (polyhydroxyaldehydes and polyhydroxy- ketones) undergo rapid and reversible intramolecular addition of some properly located alcoholic group to carbonyl group
so that they form cyclic hemiacetals
Monosaccharides exist mainly in cyclic hemiacetal forms, in solutions the acyclic aldehydo- or keto-forms are in minority.
al-D-glucose a hemiacetal, pyranose ring
15
In this way, six- or five-membered rings can originate.
In pyranoses, there is the tetrahydropyran (oxane) ring, tetrahydrofuran (oxolane) ring
in furanoses.
In the acyclic forms, carbon of the carbonyl group is achiral, but this carbon becomes chiral in the cyclic forms. Two configurations are possible on this new stereogenic centre
called anomeric (or hemiacetal) carbon so that the
cyclization results in two epimers called α or β anomers:
α-anomer β-anomer
16
• the configuration of - anomer is the same as the configuration at anomeric reference carbon
• in monosaccharides comprising five and six carbon atoms (pentoses and hexoses, pentuloses and hexuloses), the anomeric reference carbon is the configurational carbon α-anomers in Fischer formulas of D-sugars have the anomeric hydroxyl localized on the right
• the configuration of β-anomers is opposite, the
anomeric hydroxyl is written on the left in Fischer formulas of D-sugars
The hemiacetal hydroxyl group is called the anomeric hydroxyl
17
In solutions, all five forms of a hexose or hexulose occur; the cyclic forms usually prevail
E.g., in the aqueous solution of D-glucose equilibrated at 20 °C, there is approximately 62 % -D-glucopyranose, 36 % -D-glucopyranose, < 0.5 % -D-glucofuranose, < 0.5 % -D-glucofuranose, and < 0.003 % aldehydo-D-glucose.
If D-glucose is crystallized from methanol or water, the pure α-D-glucopyranose is obtained; crystallization of D-glucose from acetic acid or pyridine gives the β-D-glucopyranose. These pure forms exhibit mutarotation, when dissolved:
α-D-Glucopyranose just after dissolution exhibits [α]D20 = + 112°, the β-form
[α]D20 = + 19°. After certain time period, [α]D
20 of both solutions will settle at the same equilibrium value of + 52°. This change can be explained by opening of the cyclic homicidal to the acyclic aldehyde. which can then recyclize to give either the α or the β form till an equilibrium is established.
18
Epimers – are those diastereomers that differ in configuration at only one centre of chirality, they have the same configuration at all stereogenic centres except one.
Don't confuse:
Enantiomers (optical antipodes) – stereoisomers that are not superimposable mirror images of each other, the configurations at all stereogenic centres are exactly opposite. All their chemical and physical properties are the same but the direction of optical rotation.
Anomers (α or β) represent a special kind of epimers, they have identical configuration at every stereogenic centre but they differ only in configuration at anomeric carbon atom.
Diastereomers – stereoisomers that are not enantiomers of one another. They have different physical properties (melting points, solubility, different specific optical rotations) so that they are viewed as different chemical substances.
19
Haworth projection formulas
α-D-glucopyranose
Fischer projection Haworth projetion (the usual basal position)
– the rings are projected as planes perpendicular to the plane of drawing,
– carbon atoms of the rings and hydrogens attached to them are not shown,
– each of the formulas can be drawn in four positions, one of which is taken as the basal position (used preferentially)
20
Rules for drawing Haworth projection formulas (the basal position):
C
1
OH
pyranose ring of a hexose
C
1
OH
furanose ring of a pentose
C C
2
OH
furanose ring of a hexulose
– The anomeric carbon atom (C-1, in ketoses C-2) on the right;
– oxygen atom in the ring is "behind", i.e. carbon atoms are numbered in the clockwise sense;
Then, – hydroxyl groups and hydrogens on the right in the Fischer projection are down in the Haworth projection (below the plane of the ring), and conversely, hydroxyls on the left in Fischer formulas means up in Haworth formulas;
– the terminal –CH2OH group is up for D-sugars (for L-sugars, it is down).
21
α-D-glucopyranose can be drawn in four different positions:
The basal position: Position obtained by rotation of the "model" round a vertical axis
O
Positions obtained by tilting the „model” over: because the numbering of carbons is then counter-clockwise, the groups on the right in Fischer projection as well as the terminal –CH2OH are up in those Haworth formulas:
or
22
al-D-glucose α-D-glucopyranose β-D-glucopyranose
β-D-glucofuranose α-D-glucofuranose
Four different cyclic forms of glucose
(all are depicted in the basal position)
23
Four different cyclic fructose forms
α-D-fructofuranose β-D-fructofuranose
keto-D-fructose
β-D-fructopyranose α-D-fructopyranose
(all are depicted in the basal position)
24
Conformation of pyranoses
α-D-glucopyranose-4C1 β-D-glucopyranose-4C1
The chair conformation of six-membered rings is more stable than the boat one. From two possible chair conformations, that one prevails, in which most of the voluminous groups (-OH, -CH2OH) are attached in equatorial positions.
steric hindrance
boat conformation 4C1-chair conformation 1C4-chair conformation
E.g., conformations of β-D-glucopyranose:
25
D-fructose
Reduction of monosaccharides results in formation of
D-glucose D-glucitol
D-mannitol
alditols (sugar alcohols):
26
Oxidation of monosaccharides
a glyconic acid (aldonic)
an aldose
a glycaric acid (aldaric)
a glycuronic acid (uronic acid)
32
Deoxysugars
Deoxyribose (2-deoxy-β-D-ribose) is a constituent of nucleotides in DNA
L-Fucose (6-deoxy-L-galactose) is, e.g., present in some determinants of blood group antigens, and in numerous glycoproteins
33
Amino sugars
are important constituents of saccharidic components of glyco- proteins and glycosaminoglycans.
N-acetylgalactosamine α-D-glucosamine N-acetylglucosamine
glucosamine (2-amino-2-deoxy-D-glucose)
fructose
CH–
CH=O
NH2
CH–OH
CH2–OH
HO–CH
CH–OH
CH–OH
CH2–OH
HO–CH
CH–OH
C=O
CH2–OH
The basic amino groups –NH2 of amino sugars are nearly always "neutralized“ by acetylation in the reaction with acetyl-coenzyme A, so that they exist as N-acetyl-hexosamines. Unlike amines, amides (acetamido groups) are not basic.
39
+ HO-CH3 – H2O
glycosidic bond
Glycosides
Cyclic forms of saccharides, relatively unstable hemiacetals, can
react with alcohols or phenols to form acetals called glycosides.
The hemiacetal hydroxyl group (the anomeric hydroxyl) on the anomeric carbon is replaced by an alkoxy (or aryloxy) group.
The bond between the anomeric carbon and the alkoxy group is called the
glycosidic bond or O-glycosidic bond, at need.
Similarly, glycosidic bonds can be formed by reaction with an amino group, N-
glycosidic bonds, or with a sulfanyl group, S-glycosidic bonds
Example:
α-D-glucopyranose methanol methyl-α-D-glucopyranoside
41
Disaccharides are the most common disaccharides, in which two monosaccharides are linked through glycosidic bond. There are two types of these sugars – reducing and nonreducing disaccharides.
Reducing disaccharides are formed by a reaction between the anomeric hydroxyl of one monosaccharide and a alcoholic hydroxyl group of another, so that this second monosaccharide unit retains its anomeric hydroxyl, the reducing properties, it may anomerize and exhibits mutarotation.
Their names take the form D-glycosyl-D-glycose (with specification of the glycoside bond).
Nonreducing disaccharides Both anomeric hydroxyl are linked in the glycosidic bond (called anomeric bond), neither unit has its anomeric hydroxyl. They cannot reduce Benedict's reagent and cannot mutarotate.
Their names have the form D-glycosyl-D-glycoside.
42
Maltose
Reducing disaccharides
(4-O--D-glucopyranosyl-D-glucopyranose, malt sugar) is obtained by the partial hydrolysis of starch or glycogen. Two molecules of glucose are linked through (1→4) glycosidic bond, further hydrolysis results in only glucose. Maltose is laevorotatory. Crystalline maltose is the β-anomer and exhibits mutarotation, when dissolved..
β-maltose 4-O--D-glucopyranosyl-β-D-glucopyranose
43
Isomaltose
may be viewed as a constituent of glycogen and amylopectin placed
at branching points of the long chains connected through α(1→4) bonds.
α-isomaltose 6-O--D-glucopyranosyl-α-D-glucopyranose
(1→6) glycosidic bond
6
44
Cellobiose
(4-O-β-D-glucopyranosyl-D-glucopyranose) is obtained by the partial hydrolysis of cellulose. Two molecules of glucose are linked through β(1→4) glycosidic bond, further hydrolysis results in only glucose. Cellobiose is dextrorotatory.
4
β-cellobiose 4-O--D-glucopyranosyl-β-D-glucopyranose
45
Lactose
(4-O-β-D-galactopyranosyl-D-glucopyranose, milk sugar) is the major sugar in human and cow's milk. Equimolar mixture of glucose and galactose is obtained by hydrolysis of β(1→4) glycosidic bonds. Lactose is dextrorotatory. Crystalline lactose is the α-anomer and exhibits mutarotation, when dissolved.
α-lactose 4-O--D-galactopyranosyl-α-D-glucopyranose
β
4
46
1
2
β
α
Nonreducing disaccharides
Sucrose (saccharose)
(-D-fructofuranosyl--D-glucopyranoside, beet or cane sugar) is the ordinary table sugar. Both hemiacetal hydroxyl groups of fructose and glucose are involved in the (β2↔α1) glycosidic bond (called occasionally anomeric glycosidic bond).
Sucrose is dextrorotatory and cannot mutarotate. When hydrolyzed, an equimolar mixture of glucose and fructose results that is laevorotatory (invert sugar), because the anomers of fructose are stronger levorotatory than the dextrorotatory anomers of glucose.
sucrose -D-fructofuranosyl--D-glucopyranoside
Seminar problems (answers)
Q1. For each of followings structure decide whether the carbohydrates is: an aldose or ketose and tetrose, pentose or hexose.
(a) (b) (c)
Aldose Hexose
Ketose Hexose
Aldose Tetrose
Q2. Draw the open-chain and the ring formulas of glucose.
Q3. Draw the open-chain and the ring formulas of fructose.
Q4. The followings questions apply to the sugar A.
a) Ketose
b) Anomeric carbon
c) OH C4
d) This is β (D)
Q5. Use the following structures in answering the next questions.
a) L-sugars: B and G – last asymatric atom (the highest locant) is 4 (OH up means it is on the left)
b) α-anomers: D, E c) Reducing sugars: A, C, D, F, G d) Derived from the same sugar: A and F, E and H e) Deoxy sugars: Only C f) Names of „D” sugar: α- and β-D-fructofuranose
Q6. What is maltose? Write the formula.
Maltose (malt sugar), is a disaccharide formed from two units of glucose joined with an α(1→4) bond
* the isomer isomaltose – the two glucose molecules are joined with an α(1→6) bond
Q7. What is sucrose? Write the formula.
Sucrose is a disaccharide, a molecule composed of two monosaccharides: glucose and fructose. Sucrose is produced naturally in plants, from which table sugar is refined
Q8. Which one of the followings would show mutarotation when dissolved in water? the following structures in answering the next questions.
Mutarotation: Only C (it has „free” anomeric carbon)
Q9. What are reducing sugars? Explain and give examples of reducing monosaccharides and disaccharides.
A reducing sugar is any sugar that is capable of acting as a reducing agent because it has a free aldehyde group or a free ketone group
All monosaccharides are reducing sugars, along with some disaccharides, oligosaccharides, and polysaccharides.
Q10. Which of the following sugars are reducing sugars?
Reducing sugars
Q11. Describe shortly physical properties of mono, di and polysaccharides.
Q13. A 4 g sugar cube (Sucrose: C12H22O11) is dissolved in a 350 ml teacup of 80°C water. What is the percent composition by mass of the sugar solution?
Given: Density of water at 80 °C = 0.975 g/ml
Q12. Easy – see the lecture
𝐶𝑝 =4 𝑔
4 𝑔 + 350 𝑚𝑙 ∙ 0,975 𝑔
𝑚𝑙
∙ 100%
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