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Carbohydrates: History, Definitions, and Concepts
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Objectives:
• To understand history of carbohydrate & its nomenclature
• To be familiar with selected definitions and concepts
• To be able to manipulate stereochemistry models of hexose
rings
Chronological Summary of Carbohydrate Uses & Knowledge
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10,000 BCE
6000 BCE
6000 BCE
4000 BCE
1500 BCE
1000 BCE
100 CE
700 CE
1600 CE
1700 CE
1792 CE
1802 CE
Primitive sugarcane processing in New Guinea
Sugarcane culture developed in India
Cellulose, as cotton, is used by many cultures
Starch is used as an adhesive for papyrus in Egypt
Cotton cloth from India spreads to Persia and China
Use of sucrose in candies, confections, and medicines in Egypt
Paper is made for writing in China
Paper is coated with starch paste to retain ink and provide strength
Many sugar refineries developed in Europe
Sugar beet is developed in Europe for obtaining sucrose
A sugar is isolated from honey that is different from sucrose
A sugar is found in grapes that is also different from sucrose
1808 CE
Malus developed plane polarized light and observes optical rotation by
carbohydrates
Acid-hydrolyzed starch produces a sweet crystalline sugar
Acid-hydrolyzed cellulose also produces a sweet crystalline sugar
1811 CE
1820 CE
1821 CE
Production of dextrins by heating starch is discovered and leads to British
Gums
Adapted from John Robyt, Essentials of Carbohydrate Chemistry
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Chronological Summary of Carbohydrate Uses & Knowledge
1838 CE
The sugar from honey, grapes, starch, and cellulose is found to be identical
and called glucose
1866 CE
Kekule changes the name of glucose to dextrose because it rotates plane
polarized light to the right
1870 CE
Bayer and Fittig propose that the formula for glucose is HO-CH2-CH(OH)CH(OH)-CH(OH)-CH(OH)-CHO
1874 CE
Le Bel and van’t Hoff independently develop the theory of optical rotation
by asymmetric tetrahedral carbon atoms with four different groups around
each carbon
1881 CE
A sugar is crystallized from honey that is different from sucrose and
glucose, It is named levulose because it rotates plane polarized light to the left
1886 CE
Fructose, galactose, sorbose, and mannose are shown to have the same
empirical formula C6H12 O6
1888 CE
Glucose is shown to be a six-carbon polyhydroxy aldehyde
1888 CE
Fischer embarks on the determination of the structure of carbohydrates
1891 CE
Fischer reports the structure of glucose, mannose, fructose, and arabinose
1892-1900 CE
Controversies and discussions about intramolecular hemiacetals forming two
isomeric acetals (glycosides)
1920-1930 CE
The carbohydrate group led by Haworth demonstrated the size of
carbohydrate rings to be primarily six-membered pyranoses and propose a
six-membered hexagon to represent the carbohydrates
Adapted from John Robyt, Essentials of Carbohydrate Chemistry
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Development of Carbohydrate Nomenclature – Earlier Approach
In the early nineteenth century, individual sugars were often named after
their source, e.g. grape sugar for glucose, cane sugar for saccharose (sucrose)
“Glucose” was coined in 1838; Kekulé in 1866 proposed the name “dextrose”
because glucose is dextrorotatory*, and the levorotatory* “fruit sugar”
(fructose) was for some time named “levulose”
Very early, consensus was reached that sugars should be named with the
ending “-ose”, and by combination with the French word “cellule” for cell the
term cellulose was coined, long before the structure was known
The term “carbohydrate” (French “hydrate de carbone”) was applied
originally to monosaccharides, in recognition of the fact that their empirical
composition can be expressed as Cn(H2O)n. However the term is now used
generically in a wider sense
* Dextrorotatory (Levorotatory): Of or relating to an optically active chemical that
rotates the plane of polarized light to the right (left) , or clockwise (counterclockwise)
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
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Development of Carbohydrate Nomenclature – Emil Fischer
Emil Fischer began carbohydrate studies in 1880, assigned the relative configurations of
most known sugars and had also synthesized many sugars. He and others laid the
foundations of a terminology still in use, based on the terms triose, tetrose, pentose, and
hexose.
The concept of stereochemistry, developed since 1874 by van't Hoff and Le Bel, had a
great impact on carbohydrate chemistry. Emil Fischer introduced the classical projection
formula for sugars, with a standard orientation (Fischer projection: carbon chain vertical,
carbonyl group at the top).
He assigned to the dextrorotatory glucose the projection with the OH group at C-5
pointing to the right, well knowing that there was a 50% chance that this was wrong.
Much later (Bijvoet, 1951), it was proved CORRECT!!.
Rosanoff in 1906 selected the enantiomeric glyceraldehydes as the point of reference. Any
sugar derivable by chain lengthening from what is now known as D-glyceraldehyde
belongs to the D series, a convention still in use.
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
Emil Fischer (1852-1919)
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Fischer began his career as student of
Adoph v. Baeyer *, in 1879 he became
professor in München (1879), Erlangen
(1881), Würzburg (1885), and Berlin (1892).
In 1883 Fischer found a synthesis for indol,
later he did research on purines (1880-84),
on sugars, on barbituric acid, and on aminoacids. In 1902 he was awarded the Nobel
Prize for chemistry, in recognition of the
extraordinary services he has rendered by
his work on sugar and purine syntheses .
Affiliation: Berlin University
* Adoph v. Baeyer was awarded Nobel Prize for Chemistry in 1905
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Development of Carbohydrate Nomenclature – Commissions
In 1939 American Chemical Society formed a committee and laid the foundations
of modern systematic nomenclature for carbohydrates and derivatives:
numbering of the sugar chain, the use of D and L and α and β, and the
designation of stereochemistry by italicized prefixes.
In 1952, a joint British-American committee published “Rules for Carbohydrate
Nomenclature”. Its revised version was endorsed in 1963 by the American
Chemical Society and by the Chemical Society in Britain.
This report led the IUPAC Commission on Nomenclature of Organic Chemistry
to consider the preparation of a set of IUPAC Rules for Carbohydrate
Nomenclature.
This was done jointly with the IUPAC-IUB Commission on Biochemical
Nomenclature, and resulted in the “Tentative Rules for Carbohydrate
Nomenclature, Part I, 1969”, published in 1971/72.
Currently used is a revision of the 1971 document
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
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Development of Carbohydrate Nomenclature – Commissions
For more details about IUPAC carbohydrate
nomenclature, please go to:
http://www.chem.qmul.ac.uk/iupac/2carb/
Carbohydrate Definitions and Concepts: the List
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Carbohydrates
Aldoses and ketoses
Configurational atom
Monosaccharides
Chiral carbon atom
Isomerization
Cyclic form
Anomeric forms
Conformation of cyclic forms
Haworth representation
Ring equilibrium
Deoxy sugars
Alditols
Uronic acids
Amino sugars
Aldonic acids
Aldaric acids
Glycosides
Polysaccharides
Oligosaccharides
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Carbohydrate Definitions and Concepts
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Carbohydrates
The generic term “carbohydrate” includes monosaccharides, oligosaccharides and
polysaccharides as well as substances derived from monosaccharides by:
•
Reduction of the carbonyl group (alditols)
•
Oxidation of one or more terminal groups to carboxylic acids
•
Replacement of one or more hydroxy group(s) by a hydrogen atom, an amino
group, or a thiol group
“Carbohydrate” also includes derivatives of these compounds
The term “sugar” is frequently applied to monosaccharides and lower
oligosaccharides
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
Carbohydrate Definitions and Concepts
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Monosaccharides
Parent monosaccharides are
•
Polyhydroxy aldehydes H-[CHOH]n-CHO, or
•
Polyhydroxy ketones H-[CHOH]n-CO-[CHOH]m-H
•
With three or more carbon atoms
The generic term “monosaccharide” (as opposed to oligosaccharide or
polysaccharide) denotes a single unit, without glycosidic connection to other such
units
“Monosaccharide” includes aldoses, dialdoses, aldoketoses, ketoses and diketoses,
as well as deoxy sugars and amino sugars, and their derivatives, provided that
the parent compound has a carbonyl group.
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
Carbohydrate Definitions and Concepts
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Aldoses and ketoses
Monosaccharides with an aldehydic carbonyl or potential
aldehydic carbonyl group* are called aldoses
Monosaccharides with a ketonic carbonyl or potential ketonic
carbonyl group* are called ketoses
* “Potential aldehydic (ketonic) carbonyl group” refers to the hemiacetal
(hemiketal) group arising from ring closure
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
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Carbohydrate Definitions and Concepts
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Chiral carbon atom
One of the distinguishing features of carbohydrates is that they
contain one or more chiral carbon atoms
A chiral carbon atom has four different groups attached to it
A structure containing a chiral carbon atom and its mirror image
are non-superimposable
CHO
HO
CHO
H
H
OH
HO
L-glyceraldehyde
Mirror
OH
D-glyceraldehyde
Carbohydrate Definitions and Concepts
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Configurational atom
A monosaccharide is assigned to the D or the L series according to the
configuration at the highest-numbered centre of chirality. This asymmetrically
substituted carbon atom is called the “configurational atom”. If the hydroxy
group projects to the right in the Fischer projection, the sugar belongs to the D
series and receives the prefix D-. If the hydroxy group projects to the left, the
sugar belongs to the L series and receives the prefix L-.
1
2
3
4
5
6
D-Glucose
L-Glucose
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
Carbohydrate Definitions and Concepts
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Aldoses
Trivial names (with three-letter abbreviations in parentheses) and structures (in the aldehydic, acyclic form)
of the aldoses with three to six carbon atoms. Only the D-forms are shown; the L-forms are the mirror images
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
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Carbohydrate Definitions and Concepts
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Ketoses
Structures, with systematic and trivial names, of the 2-ketoses with three to six carbon atoms
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
Carbohydrate Definitions and Concepts
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Isomerization
CHO
HO
OH
HO
H
H
OH
OH
D-glucose
CHO
OH
O
OH
HO
H
HO
HO
HO
HO
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
trans-enediol
OH
D-fructose
cis-enediol
D-mannose
Industry uses glucose isomerase to convert glucose (dextrose) to fructose to produce
high fructose corn syrup (HFCS). When fructose is hydrogenated, it produces both
sorbitol ($0.2/lb) and mannitol ($2.0/lb)
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Carbohydrate Definitions and Concepts
Monosaccharides Browser
http://www.beechtreecommon.org/biochemistry/monosaccharide/
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Carbohydrate Definitions and Concepts
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Cyclic forms
The Fischer projection of cyclic form
α-D-Glucooxirose
α-D-Glucopyranose
α-D-Glucooxetose
Modified Fischer projection
of α-D-Glucopyranose
α-D-Glucofuranose
α-D-Glucoseptanose
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
Carbohydrate Definitions and Concepts
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Haworth representation
A perspective drawing of a simplified model. The ring is orientated almost
perpendicular to the plane of the paper, but viewed from slightly above so that
the edge closer to the viewer is drawn below the more distant edge, with the
oxygen behind and C-1 at the right-hand end. To define the perspective, the ring
bonds closer to the viewer are often thickened
The new centre of chirality generated by hemiacetal
ring closure is called the anomeric centre
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
Carbohydrate Definitions and Concepts
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Anomeric forms; use of α and β
1.
The anomeric centre: the new centre of chirality generated by hemiacetal
ring closure
2.
The two stereoisomers are referred to as anomers, designated α or β
according to the configurational relationship between the anomeric centre
and the anomeric reference atom
3.
In the α anomer, the exocyclic oxygen atom at the anomeric centre is
formally cis, in the Fischer projection, to the oxygen attached to the
anomeric reference atom; in the β anomer these oxygen atoms are formally
trans
trans
cis
Anomeric
reference atom
Anomeric
reference atom
α-D-Glucopyranose
β-D-Glucopyranose
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
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Carbohydrate Definitions and Concepts
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Mixtures of anomers
In solution, most simple sugars and many of their derivatives occur as equilibrium
mixtures
The presence of a mixture of two anomers of the same ring size may be indicated by
the notation α,β-, e.g. α,β-D-glucose
In formula, mixture of anomers is expressed by separating the representation of the
ligands at the anomeric centre from the α and β bonds [see (a) and (c)], or by use of a
wavy line [see (b) and (d)] (particularly if hydrogen atoms are omitted).
α,β-D-glucopyranose
α,β-D-fructofuranose 6-phosphate
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
Carbohydrate Definitions and Concepts
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Ring equilibrium
β-D-glucopyranose, 62%
α-D-glucopyranose, 37%
Open
chain
α-D-glucofuranose, 0.5%
β-D-glucofuranose, 0.5%
Carbohydrate Definitions and Concepts
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Conformation of cyclic forms
The conformation, i.e. the (approximate) spatial arrangement of the ring atoms of a
monosaccharide in the cyclic form, may be indicated by an italic capital letter
designating the type of ring shape, and numerals, distinguishing the variants. The
conformational descriptor is joined to the end of the name of the monosaccharide
by a hyphen.
Conformation of six-member rings
Chairs Boats
Skews Half-chairs
α-D-Glucopyranose-4C1
Let’s look at the stereochemistry models!
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
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Carbohydrate Definitions and Concepts
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Deoxy sugars
Monosaccharides in which an alcoholic hydroxy group has been replaced by a
hydrogen atom are called deoxy sugars
Several deoxy sugars have trivial names established by long usage, e.g. fucose (Fuc),
quinovose (Qui) and rhamnose (Rha). They are illustrated below in the pyranose form
α-L-Fucopyranose
or 6-Deoxy-α-L-galactopyranose
β-D-Quinovopyranose
or 6-Deoxy-β-D-glucopyranose
L-Rhamnopyranose
or 6-Deoxy-L-mannopyranose
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
Carbohydrate Definitions and Concepts
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Ribose & Deoxyribose
Ribose is a pentose that is critical to living creatures. It is a component of the RNA
(Ribonucleic acid) that is used for genetic transcription, and is related to deoxyribose.
It is also a component of ATP, NADH, and several other chemicals that are critical to
the metabolic process
Deoxyribose (more precisely 2-deoxyribose) is a pentose derived from ribose by the
replacement of the hydroxyl group at the 2 position with hydrogen, leading to the net
loss of an oxygen. It is a component of DNA (Deoxyribonucleic acid)
Ribose
Deoxyribose
Carbohydrate Definitions and Concepts
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Amino sugars
Monosaccharides in which an alcoholic hydroxy group has been replaced by an
amino group are called amino sugars. When the hemiacetal hydroxy group is
replaced, the compounds are called glycosylamines
2-Amino-2-deoxy-D-glucopyranose (D-glucosamine)
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
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Carbohydrate Definitions and Concepts
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Alditols
The polyhydric alcohols arising formally from the replacement of a carbonyl
group in a monosaccharide with a CHOH group are termed alditols.
Alditols are named by changing the suffix “-ose” in the name of the
corresponding aldose into “-itol”
D-Glucitol (Sorbitol)
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
Carbohydrate Definitions and Concepts
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Aldonic acids
Monocarboxylic acids formally derived from aldoses by replacement of the
aldehydic group by a carboxy group are termed aldonic acids
Aldonic acids are divided into aldotrionic acid, aldotetronic acids, aldopentonic
acids, aldohexonic acids, etc., according to the number of carbon atoms in the
chain. The names of individual compounds of this type are formed by replacing
the ending “-ose” of the systematic or trivial name of the aldose by “-onic acid”
D-Gluconic acid
D-Gluconate
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
Carbohydrate Definitions and Concepts
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Uronic acids
Monocarboxylic acids formally derived from aldoses by replacement of the
CH2OH group with a carboxy group are termed uronic acids
The names of these compounds are formed by replacing
(a) the “-ose” of the name of the aldose by “-uronic acid”
(b) the “-oside” of the name of the glycoside by “-osiduronic acid”
(c) the “-osyl” of the name of the glycosyl group by “-osyluronic acid”
D-Glucuronic acid
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
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Carbohydrate Definitions and Concepts
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Aldaric acids
The dicarboxylic acids formed from aldoses by replacement of both terminal
groups (CHO and CH2OH) by carboxy groups are called aldaric acids
Names of individual aldaric acids are formed by replacing the ending “-ose” of
the name of the parent aldose by “-aric acid”
L-Altraric acid
D-Glucaric acid
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
Carbohydrate Definitions and Concepts
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Glycosides
Glycosides are acetals arising by elimination of water between the hemiacetal or
hemiketal hydroxy group of the cyclic form of sugar and a hydroxy group of a second
compound. The bond between the two components is called a glycosidic bond
(linkage)
The term “glycoside” may be used to cover compounds in which the anomeric
hydroxy group is replaced not only by a group -OR, but also a group -SR
(thioglycosides)
The compound ROH from which the carbohydrate residue has been removed is often
termed the aglycone, and the carbohydrate residue itself is sometimes referred to as
the “glycone”
Usually, glycosides can be named by replacing the terminal “-e” of the name of the
corresponding cyclic form of the monosaccharide by “-ide” and preceding this, as a
separate word, the name of the group R
Methyl α-D-glucopyranoside
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
Carbohydrate Definitions and Concepts
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Oligosaccharides
Oligosaccharides are compounds in which monosaccharide units are joined by
glycosidic linkages
According to the number of units, they are called disaccharides (DP2), trisaccharides
(DP3), tetrasaccharides (DP4), pentasaccharides (DP5) etc. DP means “degree of
polymerization”
The term “oligosaccharide” is commonly used to refer to a defined structure as
opposed to a polymer of unspecified length or a homologous mixture
α-D-Glucopyranosyl-(1→6)-α-D-glucopyranosyl-(1→4)-D-glucopyranose (panose)
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
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Carbohydrate Definitions and Concepts
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Polysaccharides
“Polysaccharide” is the name given to a macromolecule consisting of a large
number of monosaccharide residues joined to each other by glycosidic linkages.
Polysaccharides composed of only one kind of monosaccharide are described as
homopolysaccharides. Similarly, if two or more different kinds of monomeric
unit are present, the class name heteropolysaccharide may be used.
A general term for a homopolysaccharide is obtained by replacing the ending “ose” of the sugar name by “-an”. For example: xylan for polymers of xylose,
mannan for polymers of mannose, and galactan for polymers of galactose.
Cellulose and starch are both glucans, as they are composed of glucose residues
[3)-β-D-Glcp-(1→4)-βL-GlcpA-(1→4)-β-D-Glcp-(1→4)-α-L-Rhap-(1 →]n
Gellan (a bacterial polysaccharide)
Adapted from IUPAC Nomenclature of Carbohydrates (1996) http://www.chem.qmul.ac.uk/iupac/2carb/
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