Lecture 2
Transcript
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1. Reduction• On reduction with sodium amalgam glucose is converted into polyhydric
alcohol-sorbitol• Fructose into 2 isomeric products sorbitol and mannitol Sorbitol
Mannitol
2. Reaction of glucose with weak alkali In the presence of weak alkali glucose in
converted into fructose and vice versa
3. Reaction of carbonyl groupGlucose and fructose condense with hydroxyl amine and
phenyl hydrazine to produce oximes and osazones
4. Reaction with non-reducing agents• Pentoses on heating with HCl/H2SO4
producefurfural-aldehyde of furan, whereas hexosesproduce hydroxy methyl furfural•
This provides the basis of tests such asMolisch’s test, Anthrone test, Salivonoff test etc.
5. Cont’d
6. Reaction with calcium hydroxideOn reacting with calcium hydroxide, glucose
produces calciumglucosate + Ca (OH)2
7. FermentationBoth glucose and fructose are attacked by enzymes of yeast to produce
ethyl alcohol and carbon dioxide C6H12O6 → 2C2H5OH + 2CO2
8. Reaction with acetone 2 acetone -2 H2O
9. MethylationOn methylation glucose produces glucosides orglycosides
10. Configuration of monosaccharides• Due to presence of asymmetric carbon
monosaccharides rotate the plane polarized light either toward right or left• Those
rotating towards right are dextrorotatory (d or +) and those towards left are levorotatory
(l, -)• (d and l) isomers are mirror image of each other when substituents are arranged in
space
11. • D and L are used in place of + and – with reference to glyceraldehyde- the farthest
asymmetric or penultimate carbon• Aldoses with at least three carbons and ketoses with
at least four carbons contain chiral centres
12. Cyclic Structures and Anomeric Forms• Although Fischer projections are useful for
presenting the structures of particular monosaccharides and their stereoisomers , they
ignore one of the most interesting facts of sugar structure —the ability to form cyclic
structures with formation of an additional asymmetric centre
13. • Alcohols react readily with aldehydes to form hemiacetals – A British Chemist Sir
Norman Haworth showed that the linear form of glucose and other aldohexoses could
undergo a similar intramolecular reaction to form a cyclic hemiacetal – The resulting sixmembered, oxygen-containing ring is similar to pyran and is designated as pyranoseglucopyranose. The reaction is catalysed by acid or base and is readily reversible Pyran
14. • An analogous intramolecular reaction of a ketose sugar such as fructose yields a
cyclic hemiketal• The five-membered ring thus formed is reminiscent of furan and is
referred to as a furanose• The cyclic pyranose and furanose forms are the preferred
structures for monosaccharides in aqueous solution• At equilibrium, the linear aldehyde
or ketone structure is only a minor component of the mixture (generally much less than
1%)
15. • When hemiacetals and hemiketals are formed, the carbon atom that carried the
carbonyl function becomes an asymmetric carbon atom – Isomers of monosaccharides
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that differ only in their configuration about that carbon atom are called anomers,
designated as α or β• When the hydroxyl group at the anomeric carbon is on the same
side of a Fischer projection as the oxygen atom at the highest numbered carbon, the
configuration at the anomeric carbon is α, as in α-D-glucopyranose
16. • When the anomeric hydroxyl is on the opposite side of oxygen in the Fischer
projection, the configuration is β, as in β-D- glucopyranose
17. Muta rotation• The addition of this asymmetric centre upon hemiacetal and hemiketal
formation alters the optical rotation properties of monosaccharides Early carbohydrate
chemists frequently observed that the optical rotation of glucose (and other sugar)
solutions could change with the time, a process called muta rotation. This indicated that a
structural change was occurring
18. Muta rotation• A freshly prepared D-glucose solution shows specific rotation of
+111.5o which changes to +52.5o and becomes constant• It was eventually found that αD-glucose has a specific optical rotation, +111.5°, and that β -D-glucose has a specific
optical rotation of +19.2°. When both of these are mixed in 38: 62, specific rotation of
+52.5 is obtained
19. Muta rotation• Hence, naturally occurring glucose contains 38% α and 62% β isomer•
Therefore, Muta rotation involves inter-conversion of α into β forms of the
monosaccharide with intermediate formation of the linear aldehyde or ketone
20. Derivatives of Monosaccharides• Sugar Acids Sugars with free anomeric carbon
atoms are reasonably good reducing agents and reduce hydrogen peroxide, ferricyanide ,
certain metals (Cu2+ and Ag+), and other oxidizing agents. Such reactions convert the
sugar to a sugar acid
21. • Sugar Alcohols – Prepared by the mild reduction of the carbonyl groups of aldoses
and ketoses. Sugar alcohols are linear molecules that cannot cyclize in the manner of
aldoses – Alditols are characteristically sweet tasting, and are widely used as sweetening
agents – Sorbitol build-up in the eyes of diabetics is implicated in cataract formation
22. Deoxy SugarsPhosphate esters
23. Amino Sugars• D-glucosamine and D-galactosamine contain an amino group (instead
of a hydroxyl group) at the C-2 position. They are found in many oligo- and
polysaccharides, including chitin, a polysaccharide in the exoskeletons of insects
24. Storage Polysaccharides• Storage polysaccharides are important carbohydrate forms
in plants and animals• It seems likely that organisms store carbohydrates in the form of
polysaccharides rather than as monosaccharaides to lower the osmotic pressure of the
sugar reserves• Because, osmotic pressure depends only on numbers of molecules –
Hence, the osmotic pressure is greatly reduced by formation of a few polysaccharide
molecules out of thousands (or even millions) of monosaccharide units
25. Starch• The most common storage polysaccharide in plants is starch, which has two
components: – α-amylose – amylopectin• Most forms of the starch in nature contain 1030% α- amylose and 70-90% amylopectin• α -Amylose is composed of linear chains of
D-glucose in α(1-4) linkages. The chains are of varying lengths, having molecular
weights from several thousand to half a million
26. • The chain has a reducing end and a non- reducing end• Although poorly soluble in
water, α -amylose forms micelles in which the polysaccharide chain adopts a helical
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conformation. Iodine reacts with α-amylose to give a characteristic blue colour, which
arises from the insertion of iodine into the middle of the hydrophobic amylose helix
27. CH2OH 6CH OH CH2OH CH2OH CH2OH 2 O 5 O H O H O H H O HH H H H H
H H H H H OH H 1 4 OH H 1 OH H OH H OH H O O O O OHOH 2 3 H OH H OH H
OH H OH H OH amyloseNon-Reducing end Reducing end
28. • In contrast to α-amylose, amylopectin, the other component of typical starches, is a
highly branched chain of glucose units• Branches occur in these chains every 12 to 30
residues• The average branch length is between 24 and 30 residues, and molecular
weights of amylopectin molecules can range up to 100 million• The linear linkages in
amylopectin are α(1-4), whereas the branch linkages are α(1-6)• As is the case for αamylose , amylopectin forms micellar suspensions in water. Iodine reacts with such
suspensions to produce a red-violet colour
29. Starch CH2OH 6CH OH CH2OH CH2OH CH2OH 2 O 5 O H O H O H H O HH H
H H H H H H H H OH H 1 4 OH H 1 OH H OH H OH H O O O O OHOH 2 3 H OH H
OH H OH H OH H OH amylose
30. Glycogen• The major form of storage polysaccharide in animals is glycogen•
Glycogen is found mainly in the liver (where it may amount to as much as 10% of the
liver mass) and skeletal muscle (where it accounts form 1-2% of the muscle mass)• Liver
glycogen consists of granules containing highly branched molecules, with α(1-4) linkage
in linear structure and α(1-6) linkage at branching, which occurrs every 8-12 glucose
units
31. • Like amylopectin, glycogen yields a red-violet colour with iodine• Glycogen can be
hydrolyzed by α-amylase, yielding glucose• It can also be hydrolyzed by glycogen
phosphorylase, an enzyme present in liver and muscle tissue, to release glucose-1phosphate
32. Structural polysaccharides• Cellulose• Chitin• Alginates• Agarose•
Glycosaminoglycans
33. Cellulose• The most abundant natural polymer found in the world• Found in the cell
walls of nearly all plants• One of the principal components providing physical structural
and strength• Cotton is almost pure cellulose• Cellulose is a linear homopolymer of Dglucose units, just as in α-amylose. • The structural difference, which completely alters
the properties of the polymer, is that in cellulose the glucose units are linked by β(1-4)glycosidic bonds, whereas in α - amylose the linkage is α(1-4)
34. CH2OH 6CH OH CH2OH CH2OH CH2OH 2 O 5 O O H O H O OHH H H H H H H
H OH H 1 O 4 OH H 1 O OH H O OH H O OH HOH H H H H 2 H 3 H OH H OH H OH
H OH H OH cellulose β-linkages promote intra-chain and inter-chain H-bonds and van
der Waals interactions, that cause cellulose chains to be straight & rigid, and pack with a
Schematic arrangement of crystalline arrangement in thick cellulose chains in a
microfibril. bundles - microfibrils
35. Glycosaminoglycans• Previously called mucopolysaccharides•Linear polymers of
repeating disaccharides –The constituent monosaccharides tend to be modified, with
acidic groups, amino groups, sulfated hydroxyl and amino groups, etc.•Such compounds
tend to be negativelycharged, because of the prevalence of acidicgroups
36. Hyaluronate (Hyaluronic acid)•Hyaluronate is a glycosaminoglycan with arepeating
disaccharide consisting of 2 glucosederivatives, D-glucuronic acid & N-acetyl-D-
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glucosamine•These monosaccharides are linkedthrough β(1→3) linkages•Disaccharides
are linked through β(1→4).
37. CH2OH D-glucuronate 6 − H 5 O 6COO H 4 1 O O H H 5 H OH H4 H 1 3 2 OH H
H NHCOCH3 3 2 O H OH N-acetyl-D-glucosaminehyaluronate
38. Proteoglycans• These are also glycosaminoglycans that are covalentlylinked to serine
residues of specific core proteins•The glycosaminoglycan chain is synthesized
bysequential addition of sugar residues to the coreprotein
39. Heparan sulfate is initially synthesized on a membrane-embedded core protein as a
polymer of alternating N-acetylglucosamine and glucuronate residues.Later, in segments
of the polymer, glucuronateresidues may be converted to the sulfated sugariduronate 2sulfate, while N-acetylglucosamineresidues may be sulfated- N-sulfo-glucosamine-6sulfate iduronate-2-sulfate N-sulfo-glucosamine-6-sulfate H CH2OSO3− H O H O H −
COO H OH H O OH H H O H OSO3− H NHSO3− heparin or heparan sulfate - examples
of residues
40. Heparin, a soluble glycosaminoglycan found in granulesof mast cells, has a structure
similar to that of heparansulfates, but is more highly sulfatedWhen released into the
blood, it inhibits clot formationby interacting with the protein antithrombin.Heparin has
an extended helical conformationCharge repulsion by the many negatively chargedgroups
may contribute to this conformation
41. Some proteoglycans of the extracellular matrix bindnon-covalently to hyaluronate via
protein domains calledlink modules. E.g. • Multiple copies of the aggrecan proteoglycan
associate with hyaluronate in cartilage to form large complexes • Versican, another
proteoglycan, binds hyaluronate in the extracellular matrix of loose connective tissues.
CH2OH D-glucuronate 6 − H 5 O 6COO H 4 1 O O H H 5 H OH H 4 H 1 3 2 OH H H
NHCOCH3 3 2 O H OH N-acetyl-D-glucosamine hyaluronate
42. Chitin• A polysaccharide that is similar to cellulose, both in its biological function
and its primary, secondary, and tertiary structure• The structure of chitin is identical to
cellulose, except that the -OH group on each C-2 is replaced by -NHCOCH3, so that the
repeating units are N-acetyl-D-glucosamines in β-(1-4) linkage
43. • An other, significant difference between cellulose and chitin is that the chains are
arranged in parallel (all the reducing ends together at one end of a packed bundle and all
the non-reducing ends together at the other end) or antiparallel (each sheet of chains
having the chains arranged oppositely from the sheets above and below)
44. • Natural cellulose seems to occur only in parallel arrangements. Chitin, however, can
occur in three forms, sometimes all in the same organism – alpha-Chitin is an all-parallel
arrangement of the chains – β-chitin is an antiparallel arrangement – d-chitin, the
structure is thought to involve pairs of parallel sheets separated by single antiparallel
sheets