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Carbohydrates Chapter 27. Larry Emme Chemeketa Community College. Carbohydrates: A First Class of Biochemicals. Carbohydrates are energy-yielding macronutrients in the same class of nutrients as fats and proteins. - PowerPoint PPT Presentation
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Carbohydrates: A First Class of Biochemicals
33
Carbohydrates are energy-yielding macronutrients in the
same class of nutrients as fats and proteins.
These polyhydroxy aldehydes or ketones include simple
carbohydrates like glyceraldehyde and dihydroxyacetone.
What are carbohydrates?
44
Carbohydrates are important in society because they provide;
(1) basic diets in the form of starch and sugar and…….
(2) clothing and shelter.
Many of the chemical properties of carbohydrates are
determined by the chemistry of the hydroxyl and carbonyl
functional groups.
555
Fischer projection formulas are drawn with thesecharacteristics: (1) The keto or aldehyde group is placed at the top of the projection.
(2) Each interior carbon atom is shown as an intersection point between two lines ( )
(3) The H atom and –OH group are written to left or right of the projection .
666
This is an example of a modified structural formula of glucose written as a Fischer projection formula.
D-glucose
(modif ied structural f ormula)
D-glucose(Fischer projection f ormula)
7
Classification of
Carbohydrates
88
The four major types of carbohydrates are……..
1. Monosaccharides
2. Disaccharides
3. Oligosaccharides
4. Polysaccharides
Types of Carbohydrates
9
Monosaccharides
• A monosaccharide is a carbohydrate that cannot be hydrolyzed to simpler carbohydrate units.
• The monosaccharide is the basic carbohydrate unit of cellular metabolism.
1010
Monosaccharides can be classified by the ….
(a) number of carbon atoms in the molecule
( e.g. a pentose versus a hexose)
(b) functional group ( aldoses versus ketoses)
(c) configuration ( D versus L isomers)
(d) optical activity [(+) versus (–) isomers]
(e) ring structure ( furanoses versus pyranoses)
(f) stereochemistry at an anomeric carbon ( versus isomers)
Types of Monosaccharides
1111
Types of Monosaccharides
Number of Carbons
The monosaccharides shown below are classified based on
the number of carbons in the molecule.
1212
Types of Monosaccharides
Functional Group
Mononosaccharides with a –CHO (aldehyde) group are known as aldoses while those with a (keto) group are known as ketoses.
C O
1313
Types of MonosaccharidesConfiguration
Monosaccharides with the –OH group on the right of thecarbon alpha to the terminal ROH carbon are D isomerswhile those with the –OH group on the left are L isomers.
1414
Types of MonosaccharidesOptical Activity
Monosaccharides that rotate plane-polarized light to the rightare known as (+) isomers while those that rotate it to the left are (–) isomers.
Note: The D and L designations do not indicate the direction of rotation, e.g., the D isomer of glucose could be either the (+) isomer or it can be the (–) isomer.
15
Learning Check
Identify each as the D or L isomer.
A. B. C.
__-Ribose __- Threose __- Fructose
CH2OH
HO H
HO H
HHO
O
C H
CH2OH
HO H
OHH
O
C H
CH2OH
H OH
H OH
HO H
O
CH2OH
16
Solution
Identify each as the D or L isomer.
A. B. C.
L-Ribose L-Threose D-Fructose
CH2OH
HO H
HO H
HHO
O
C H
CH2OH
HO H
OHH
O
C H
CH2OH
H OH
H OH
HO H
O
CH2OH
17
Learning Check
Write the projection formula for a D-aldopentose.
18
This is one example of a D-aldopentose. This molecule is a D-isomer because of the orientation of the hydroxyl group (see arrow). The molecule is an aldose because it is an aldehyde and a pentose because it contains five carbon atoms.
C
CH OH
C
C
HO H
CH2OH
H OH
OH
D-configuration
1919
Types of MonosaccharidesRing Structure
The cyclic form of monosaccharides that have five atomsin the ring are known as furanoses while those with six atomsare known as pyranoses based on the correspondingheterocyclic ring structures furan and pyran.
2020
Types of Monosaccharides
Anomeric Configuration
Monosaccharides that have an –OH below the ring at the anomeric carbon are known as (alpha) isomers while those with the –OH above the ring are (beta) isomers.
21
Disaccharides• A disaccharide yields two
monosaccharides – either alike or different – when hydrolyzed:
disaccharide + water 2 monosaccharidesH+ or
enzymes
22
Monosaccharides & Disaccharides
• Disaccharides are often used by plants or animals to transport monosaccharides from one cell to another.
• The monosaccharides and disaccharides generally have the ending –ose – for example, glucose, sucrose, and lactose.
• These are water-soluble carbohydrates, which have a characteristically sweet taste and are called sugars.
23
Oligosaccharides
• An oligosaccharide has two to six monosaccharide units linked together.
24
Polysaccharides• A polysaccharide is a macromolecular
substance that can be hydrolyzed to yield many monosaccharide units:
polysaccharide + water monosaccharidesH+ or
enzymes
• Polysaccharides are important structural supports, particularly in plants, and also serve as a storage depot for monosaccharides, which cells use for energy.
25
Importance of Carbohydrates
2626
Why are carbohydrates so important?
Carbohydrates are important because they are widely available and because they have exceptional utility.
The Utility of Carbohydrates
Energy-Yielding Nutrients(starch f rom plants )
Building Materials(cellulose is used in woodconstruction, paper, cottonbased clothing and cell wallsin higher plants)
Water-Soluble Molecules( mono- and disaccharides are used as sweetners)
27
MonosaccharidesMonosaccharides
2828
The most important monosaccharides are the pentoses and hexoses as shown in the diagram below .
Important Monosaccharides
Pentoses Hexoses
ribose deoxyribose glucose galactose fructose
( important source of celluar energy and in nutrition)
( important components of the nucleic acids DNA and RNA )
29
Monosaccharides
• The hexose monosaccharides are the most important carbohydrate sources of cellular energy.
• Three hexoses – glucose, galactose, and fructose – are of major significance in nutrition.– All three have the same formula, C6H12O6, and thus
deliver the same amount of cellular energy.– They differ in structure, but are biologically
interconvertible.
30
• “Tree” formulas• Shorthand formulas used in carbohydrate
configurations where the following symbols are used:
H C O=
= H C OH
= HO C H
= CH2OH
Example
is
H C O
H C OH
CH2OH
31
• Glucose is the most important of the monosaccharides.
• It is an aldohexose and is found in the free state in plant and animal tissue.
H
CHO
OH
HHO
OHH
OHH
CH2OH
or
3232
The concentration of glucose in blood is normally about
80-100 mg/100 mL
Glucose is known by either of the following names dextrose
(from dextrorotatory), grape sugar (found in grapes) , or blood
sugar (because it is transported in the blood).
Glucose
33
• Galactose is also an aldohexose and occurs, along with glucose, in lactose and in many oligo- and polysaccharides such as pectin and gums.
H
CHO
OH
HHO
HHO
OHH
CH2OH
or
34
• Fructose, also know as levulose, is a ketohexose that occurs in fruit juices, honey, and along with glucose, as a constituent of sucrose.
CH2OH
O
HHO
OHH
OHH
CH2OH
It is the sweetest common sugar being about two times sweeterthan glucose.
3535
Structures of the Pentoses (Ribose and Deoxyribose)
(both monosaccharides are aldopentoses)
36
Structures of Glucose and Other Aldoses
37
Epimers• Any two monosaccharides that differ
only in the configuration around a single carbon atom are called epimers.
• In the case of glyceraldehyde this atom is carbon 2.
3838
The D-family of aldoses. The red –OH group indicate the new chiral carbon added in case from top to bottom of the diagram.
39
Learning Check Draw the enantiomer of D-allose using the previous
slide.
40
The enantiomer of D-allose is L-allose. These molecules are mirror images.
CHO
OHH
OHH
OHH
OHH
CH2OH
D-allose
CHO
HO H
HO H
HO H
HO H
CH2OH
L-allose
41
Cyclic Structure of Glucose; Mutarotation
4242
Bonds that are missing atoms are understood to have H atoms.
Haworth Formulas
Haworth formulas are structural formulas that represent cyclicsugars. In the case of glucose the formula is drawn as a flat hexagon withH and –OH written above and below the plane of the ring.
Haworth formulas are sometimes shown in a abbreviated form as shown here.
OH’s removed
43
OH O
H
O O
H
44
OH O
H
O O
H
HHH H H H H H H HH HHH
HH
H
45
Most naturally occurring monosaccharides occur in the chair conformation shown.
-D-glucopyranose
4646
What is an anomer?
Anomeric carbon
This hydroxyl group on theanomeric carbon is above thering which means this is the anomer.
This is an anomeric carbon.
This hydroxyl group on theanomeric carbon is below thering which means this is the anomer.
The structure below is an alpha anomer.
An anomer is the or form of a monosaccharide as shown here.
4747
What is mutarotation? Mutarotation is the change in specific rotation of an anomer as it is converted into an equilibrium mixture of the and forms.
Fischer projection formulas showing mutarotation of D-glucose.
Emil Fischer
1852-1919
4848
Haworth formulas showing mutarotation of D-glucose.
Sir (Walter) Norman Haworth1883-1950
49
Comparison of Formulas
50
OH O
H
O O
H
H
51
Conversion of Fischer to Haworth Formulas
http://faculty.chemeketa.edu/lemme/CH%20123/Handouts/HaworthFormulas.pdf
52
Hemiacetals and AcetalsHemiacetals and Acetals
5353
Hemiacetals are structures that contain an alkoxy group and
a hydroxyl group on the same carbon atom.
Hemiacetals
This is the alkoxy part of the hemiacetal.
This is the hydroxyl group part of the hemiacetal.
5454
The cyclic structures of monosaccharides are intramolecular hemiacetals.Five and six -membered ring hemiacetals are stable but
theserings can open in aqueous solution to the straight-chain aldehyde.
5555
Acetals
Acetals are structures that contain two alkoxy groups on
the same carbon atom.
5656
Glycosides
Cyclic acetals are known as glycosides and glycosides are
derivatives of hemiacetals.
5757
Glycosides
Glycosides like the methyl isomers shown below are
less reactive than the corresponding monosaccharide.
The methyl isomers shown below will not undergo mutarotation.
58
Structures of Galactose, Fructose,
Ribose, andDeoxyribose
5959
Structure of Galactose
Galactose has the same structure as glucose except the configuration at carbon four is reversed as shown here.
6060
Structure of Galactose
Galactose is an aldohexose like glucose and like glucose it also exists in the alpha and beta cyclic pyranose forms.
6161
Structure of Fructose
Fructose is a ketohexose and like glucose it also exists in the open-chain and cyclic forms as shown here.
( open-chain form) ( cyclic form)
6262
Structure of Ribose and Deoxyribose
D-Ribose and its derivativeD-2-dexoyribose are pentosesfound in nuclei acids RNA and DNA.
Notice that the 2-deoxy in D-2-deoxyribose means an oxygen is omitted from theD-ribose molecule at carbon two.
63
DisaccharidesDisaccharides
6464
DisaccharidesDisaccharides are carbohydrates consisting of two monosaccharides.
The two monosaccharides are connected by a glycosidic linkage as shown here for the disaccharide lactose .
lactose
6565
DisaccharidesSucrose and lactose are important disaccharides foundin the free state in nature.
Sucrose is known as table sugar while lactose is known asmilk sugar. Both undergo hydrolysis in the presence of an acid or the enzymes sucrase or lactase respectively.
6666
Disaccharides
Maltose is not found in the free state but is the productwhen a polysaccharide is degraded during the sprouting of
grain. Maltose is known as grain sugar.
Maltose undergoes hydrolysis in the presence of acid ormaltase to produce two molecules of glucose.
67
Structures and Properties of Disaccharides
Structures and Properties of Disaccharides
6868
Formation of Maltose
6969
Structure of Lactose Shown here are Haworth structures using the stacked position convention and the bent structure convention.
stacked position
bent structure
7070
This is a Haworth projection formula of the disaccharide sucrose
71
This is an alternate Haworth projection formulas of the disaccharide sucrose
1
2
3 4
5
6
72
Sweeteners and Diet
Sweeteners and Diet
Invert sugar is sucrose that has been hydrolyzed to glucose and fructose.
7373
Importance of Sucrose as a Sweetener
Sucrose represents 40-60% of all sweeteners and is 20-30%
of the average caloric intake in the United States because of
its low price and sweet taste.
Sucrose is hydrolyzed to prevent crystallization in certain
food preparations and in these cases it is known as invert
sugar.
7474
Artificial Sweeteners
Artificial sweeteners have been developed with the intent to
balance the concerns for safety, relative sweetness, and
aftertaste.
Many artificial sweeteners have a higher relative sweetness
than the common sweeteners like sucrose, glucose or fructose
as shown in the table on the next slide.
75
Relative Sweetness of Sugars and Sugar Substitutesbased on fructose = 100
SugarsRelative
sweetnessSugar substitutes
Relative sweetness
Fructose 100Sucralose
(Splenda)3.5 × 104
Invert Sugar 75Saccharin(Sweet ‘N Low)
1.7 × 104
Sucrose 58Acesulfame potassium
(Sweet One)1.2 × 104
Glucose 43Asparatame
(Equal)1.0 × 104
Maltose 19Rebiana(Truvia, PureVia)
1.2 × 104
Galactose 19
Neotame
4.1 × 105
Lactose 9.2
Stevia
3.0 × 104
Xylitol
58
76
NameCalories /
GramSweetness
IndexGlycemic
Index
Calories / Spoon-Equiv
Fructose 4 1.7 23 9
Sucrose 4 1 65 16
Glucose 4 0.75 100 21
Dextrose 4 0.75 100 21
Trehalose 4 0.45 70 36
Galactose 4 0.3 23 53
Maltose 4 0.3 105 53
Lactose 4 0.15 45 107
Sugars
77
NameCalories /
GramSweetness
IndexGlycemic
Index
Calories / Spoon-Equiv
Erythritol 0.2 0.65 1 1
Xylitol 2.4 1 12 10
Maltitol 2.4 0.9 35 11
Mannitol 1.6 0.5 2 13
Isomalt 2.1 0.5 2 17
Sorbitol 2.6 0.55 4 19
Lactitol 2 0.4 3 20
HSH 3 0.4 36 30
Sugar Alcohols
78
NameCalories /
GramSweetness
IndexGlycemic
Index
Calories / Spoon-Equiv
Honey 4 1.1 50 14
Maple Syrup 4 1 54 15
Coconut Palm Sugar
4 1 35 15
Sorghum Syrup
4 1 50 15
Natural Caloric Sweeteners
79
NameCalories /
GramSweetness
IndexGlycemic
IndexCalories /
Spoon-Equiv
Thaumatin 4 2,000 0 0
Monellin 4 1,500 0 0
Brazzein 4 1,000 0 0
Pentadin 4 500 0 0
LuoHanGuo 0 300 0 0
Stevia 0 300 0 0
Natural Zero Calorie Sweeteners
80
NameCalories /
GramSweetness
IndexGlycemic
IndexCalories /
Spoon-Equiv
Tagatose 4 0.92 0 7
Agave Syrup 4 1.5 15 10
HFCS-90 4 1.6 31 10
HFCS-55 4 1.2 58 13
HFCS-42 4 1.1 68 14
Golden Syrup
4 1.1 60 15
Barley Malt Syrup
4 0.5 42 32
Brown Rice Syrup
4 0.5 25 32
Modified Sugars
81
NameCalories /
GramSweetness
IndexGlycemic
IndexCalories /
Spoon-Equiv
Advantame 0 20,000 0 0
Neotame 0 8,000 0 0
Sucralose 0 600 0 0
Saccharin 0 300 0 0
AcesulfameK 0 200 0 0
Aspartame 4 180 0 0
Cyclamate 0 40 0 0
Artificial Sweeteners
82
Discovered in 1976
83
Discovered in 1879
84
Discovered in 1967
85
Discovered in 1965
86
Agave nectar (sometimes called agave syrup) is most often produced from the Blue Agaves that thrive in the volcanic soils of Southern Mexico. Agaves are large, spikey plants that resemble cactus or yuccas in both form and habitat, but they are actually succulents similar to the familiar Aloe Vera.
To make the agave nectar, sap is extracted from the pina, filtered, and heated at a low temperature, which breaks down the carbohydrates into sugars. Lighter and darker varieties of agave nectar are made from the same plants. Because of the low temperatures used in processing many varieties (under 118°F) raw foods enthusiasts generally regard agave nectar as a raw food.
The taste of agave nectar is comparable, though not identical, to honey. Many people who do not like the taste of honey find agave a more palatable choice. It also has none of the bitter aftertaste associated with artificial sweeteners.
http://www.allaboutagave.com/
87
Stevia (sweetleaf, sweet leaf or sugarleaf)
This sweetener is made from a crude preparation (powder or liquid) of dried stevia leaves. It may contain a mixture of many substances, only some of which are sweet.
88
Truvia™ natural sweetener is made from rebiana, the best tasting part of the stevia leaf, erythritol and natural flavors.
Rebiana is the common or usual name for a food-grade high-purity extract of the stevia leaf that is at least 97 percent rebaudioside-A, the best tasting sweet substance found in the stevia leaf.
Chemically, erythritol is simply a four-carbon sugar alcohol. Erythritol is made by fermenting glucose then separating andpurifying the resulting product.
89
Most fruits, berries and plants contain xylitol (also called wood sugar), the richest natural sources being plums, strawberries, raspberries, cauliflower and endives.
90
Xylitol is extremely toxic to dogs
The toxic dose of xylitol is 0.1 gm/kg body weight, while liver failure results from doses greater than 0.5 g/kg body weight. Translating these numbers into something usable in the every-day world is a little harder to do, since the amount of xylitol varies from one product to another. Two sticks of gum is enough to cause a serious drop in blood sugar for a small (under 20 lb) dog, while it might take 8 to 10 sticks to affect a large (over 60 lb) dog, but these amounts are only an estimate. As for baked goods containing xylitol, again, the amount in each cookie or muffin will vary. In one case, a Standard Poodle died after eating 5 or 6 cookies sweetened with xylitol.
9191
The history of sodium cyclamate illustrates the difficultyin balancing consumer safety with the needs of the consumer market.
This sweetener was banned in 1970 because of research that indicated risks of cancer from consuming the sweetener.
Discovered in 1937
92
Redox Reactions of Monosaccharides
Redox Reactions of Monosaccharides
9393
Oxidation of AldohexosesThe aldehyde group in monosaccharides can be oxidized
to
monocarboxylic acids with a mild oxidizing agent.For example glucose is oxidized to gluconic acid in thepresence of bromine water.
9494
Oxidation of AldohexosesDicarboxylic acids are formed when aldohexoses are
treated
with stronger oxidizing agents.For example glucose is oxidized to glucaric acid in the presence of nitric acid.
9595
Reduction of Aldohexoses
Hexahydric alcohols ( six –OH groups) are formed when
aldohexoses are treated with reducing agents.
For example glucose is reduced to glucitol (sorbitol) in the presence of H2/Pt.
Sorbitol is in many moisturizers
96
Learning Check
Write the products of the mild oxidation and reduction of D-mannose.
H
O
CH2OH
H OH
H OH
HO H
HHO
C
D-Mannose
97
Solution
Write the products of the mild oxidation and reduction of
D-mannose.
D-Mannitol D-Mannose D-Mannonic acid
9898
Redox Tests for Carbohydrates
A reducing sugar is a compound that will reduce Ag + → Ag or Cu2+ → Cu+.
A reducing sugar will have one of the following groups;
(a) An aldehyde group ( e.g. glyceraldehyde)(b) A hydroxyketone ( e.g. fructose) (c) A cyclic hemiacetal group ( e.g. glucose or maltose)
9999
Redox Tests for Carbohydrates
The Benedict, Barfoed, and Fehling tests are based on the formation of a brick red copper(I) oxide precipitate as a positive result while the Tollens test is based on theformation of a silver mirror.
100
The Barfoed test (a solution of cupric acetate and acetic acid) is more sensitive in that it can distinguish a reducing monosaccharide from a reducing disaccharide.
Monosaccharides form a precipitate within 3 minutes and Disaccharides take a bit longer.
– +
101
Redox Test for Carbohydrates
Benedict test for reducing sugars. The tube on the right contains Benedict reagent. The tube on the left shows the brick-red precipitate of Cu2O when glucose is added.
102
103103
Reduction of Hemiacetals
Sugars with the hemiacetal structure can be reduced under alkaline conditions because the ring opens as shown belowforming an aldehyde group.
Therefore glucose, lactose, and maltose have the hemiacetal structure and are reducing sugars but the disaccharide sucroseis not a reducing sugar because it does not have the hemiacetal structure.
104
Osazone FormationOsazone Formation
105
Phenylhydrazine (C6H5NHNH2) reacts with carbons #1 and #2 of reducing sugars to form derivatives called osazones. The formation of these distinctive crystalline derivatives is useful for comparing the structures of sugars. Glucose and fructose react as shown on next slide:
106
C
C
C
C
C
CH2OH
H
H OH
HO H
H
H OH
OH
O
C6H5NHNH2
C
C
C
C
C
CH2OH
H
H
HO
H
H
OH
H
OH
OH
NNHC6H5
C6H5NHNH2
C
C
C
C
C
CH2OH
H
HO
H
H
H
OH
OH
NNHC6H5
O
D-glucose
107
C
C
C
C
C
CH2OH
H
HO
H
H OH
OH
H
NNHC6H5
NNHC6H5
osazone
C6H5NHNH2
108
CH2OH
C
C
C
C
CH2OH
HO
H
H OH
OH
H
O
C6H5NHNH2
CH2OH
C
C
C
C
CH2OH
HO H
H OH
H OH
NNHC6H5
C6H5NHNH2
C
C
C
C
C
CH2OH
HO
H
H OH
OH
H
HO
NNHC6H5
D-fructose
109
C
C
C
C
C
CH2OH
H
HO
H
H OH
OH
H
NNHC6H5
NNHC6H5
osazone
C6H5NHNH2
110
Identical osazones are obtained from D-glucose and D-fructose. This demonstrates that carbons #3 through #6 of D-glucose and D-fructose molecules are identical. The same osazone is also obtained from D-mannose. This indicates that carbons #3 through #6 of the D-mannose molecule are the same as those of D-glucose and D-fructose molecules. In fact, D-mannose differs from D-glucose only in the configuration of the –H and –OH groups on carbon #2.
111
112
113
114
115
Polysaccharides Derived from
Glucose
Polysaccharides Derived from
Glucose
116116
Major Polysaccharides Derived from Glucose
Cellulose (used to construct cell walls in plants)
Glycogen (energy-storage in animals i.e. liver and muscle tissues)
Starch (energy-storage in plants)
Glucose Based Polysaccharides
There are three types of naturally occurring polysaccharides; cellulose, glycogen, and starch as shown below.
117117
Starch, glycogen, and cellulose all yield D-glucose
when hydrolyzed as shown here.
118118
StarchStarch is a polysaccharide composed of amylose and
amylopectin.
Amylose is a large molecule consisting of unbranched
chains composed of about 25-1300 -D-glucose units
joined by -1,4-glycosidic linkages.
Amylopectin is a large molecule with branched chains
composed of -1,4-glycosidic linkages in the main chain
and -1,6-glycosidic linkages at branch points as seen
in the next slide.
119119
Molecular Structure of Amylose
Amylose Glucose units
in Amylose
No Branching
120120
Molecular Structure of amylopectin.
Amylopectin
Glucose unitsin Amylopectin
Branching
121121
Hydrolysis of Starch
An important reaction during digestion is the hydrolysis
of starchy foods as shown below.
Starch is not soluble in cold water and will form a colloidal dispersion
in hot water.
Starch solutions form a blue-black
color in the presence of free iodine.
122122
Glycogen
Glycogen is a carbohydrate polymer that is stored in the liver
and muscle tissues in animals.
Glycogen has a structure similar to amylopectin except it
is more highly branched with the -1,6-glycosidic linkages
occurring more frequently along the polymer chain.
123123
Cellulose
Cellulose, like starch and glycogen, is a glucose based
polymer.
Cellulose is the most abundant organic substance found in nature and it is the chief structural component of plants and wood.
124124
Cellulose
However the glucose units in cellulose are join by -1,4-glycosidic linkages instead of -1,4-glycosidiclinkages.
This change in stereochemistry at the anomeric carbon
allows extensive hydrogen bonding in cellulose as shown
in the next slide.
125125
Two representations of cellulose. In the three-dimensional model note the hydrogen bonding that links the extended cellulose polymers to form cellulose fibers.
Haworth formula “Bent”
Three-dimensional model of cellulose
126
Cellulose: stacked structure
CH2OH
OH
OH
O
O
O
OH
OH
O
O
CH2OH
OH
OH
O
O
CH2OH
OH
OH
O
O
CH2OH
127