Post on 20-Dec-2015
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Carbohydrates
• Carbohydrate: a ________________ or ______________ , or a substance that gives these compounds on hydrolysis
• Monosaccharide: a carbohydrate that cannot be _____________ to a simpler carbohydrate– they have the general formula CnH2nOn, where n varies
from 3 to 8– aldose: a monosaccharide containing an aldehyde
group– ketose: a monosaccharide containing a ketone group
Monosaccharides• There are only two trioses
– often aldo- and keto- are omitted and these compounds are referred to simply as trioses; although this designation does not tell the nature of the carbonyl group, it at least tells the number of carbons
Dihydroxyacetone (a ketotriose)
Glyceraldehyde (an aldotriose)
CHO
CHOH
CH2OH
CH2OH
C=O
CH2OH
Monosaccharides• Glyceraldehyde contains a stereocenter and
exists as a pair of enantiomers
(S)-Glyceraldehyde(R)-Glyceraldehyde
CHO
C
CHO
CH OH
CH2OH CH2OH
HHO
Fischer Projections• Fischer projection: a two dimensional
representation for showing the configuration of tetrahedral __________________ – ____________ lines represent bonds projecting
forward – vertical lines represent bonds projecting to the
______
(R)-Glyceraldehyde
CHO
CH OH
CH2OH
(R)-Glyceraldehyde
convert to a Fischer projection
H OH
CHO
CH2OH
D,L Monosaccharides
• In 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde
(L)-Glyceraldehyde(D)-Glyceraldehyde
CHO
C
CHO
CH OH
CH2OH CH2OH
HHO
[]25 = +13.5°D
[]25 = -13.5°D
D,L Monosaccharides
• According to the conventions proposed by Fischer– D-monosaccharide: a monosaccharide that,
when written as a Fischer projection, has the -OH on its ___________ carbon on the right
– L-monosaccharide: a monosaccharide that, when written as a Fischer projection, has the -OH on its penultimate carbon on the ______
• Following are the two most common D-aldotetroses and D-aldopentoses
D,L Monosaccharides
D-ErythroseD-Threose D-Ribose 2-Deoxy-D- ribose
CH2OH
CHO
OH
CH2OH
CHO
OH
CH2OH
CHO
OHOH
OH
HH
H
HHO
H
CH2OH
CHO
HOH
OH
HH
H
OHH
H
D,L Monosaccharides• and the three common D-aldohexoses.
Note that one of these is an amino sugar
D-GlucosamineD-Glucose D-Galactose
CH2OH
CHO
OH
OHH
OH
H
HHO
H
CH2OH
OH
OH
HOH
H
CH2OH
CHO
OH
HH
OH
H
HOHO
H
CHO
NH2H
H
Cyclic Structure
• Monosaccharides have hydroxyl and carbonyl groups in the same molecule and exist almost entirely as five- and six-membered cyclic ______________ – __________ carbon: the new stereocenter
resulting from cyclic hemiacetal formation– _______ : carbohydrates that differ in
configuation at their anomeric carbons
Haworth Projections
• Haworth projections– five- and six-membered hemiacetals are represented as
planar ________ or ________ , as the case may be, viewed through the edge
– most commonly written with the ________ carbon on the right and the hemiacetal oxygen to the back right
– the designation means that -OH on the anomeric carbon is cis to the terminal -CH2OH; ___ means that it is trans
Mutarotation
• Mutarotation: the change in specific rotation that occurs when an or form of a carbohydrate is converted to an equilibrium mixture of the two
•Mutarotation: the change in specific rotation that occurs when an a or b form of a carbohydrate is converted to an equilibrium mixture of the two
D-Glucose
CH2OH
CHO
OHOHHOH
HH
HOH
-D-Glucopyranose (-D-Glucose)
C
H OH
HHO
HOH
H
CH2OHOH
OH()
H OH
HHO
HH
OH
H
CH2OHO
O
H
H
H OH
HHO
HOH()
OH
H
CH2OHO
-D-Glucopyranose (-D-Glucose)
+
anomeric carbon
5
5 5
5
44
4
(-D-Galactose)(-D-Galactose) -D-Galactopyranose
-D-Galactopyranose
D-Galactose
()
()
CH
OH
HO
HO
CH2OH
OOH
OHOH
HO
HO
OCH2OH
OCH2OH
HO
HO
OHOH
[]25 = +52.8°D
[]25 = +150.7°D
Reduction to Alditols• The carbonyl group of a monosaccharide
can be reduced to an hydroxyl group by a variety of reducing agents, including NaBH4 and H2/M
Ni+
D-Glucitol (D-Sorbitol)
D-GlucoseCH2OH
CH2OH
OH
OHH
OH
CH2OH
CHO
OH
OHH
OH
H
HHO
H
H
HHO
H
H2
Oxidation to Aldonic Acids
Precipitates as a silver mirror
+
O O
RCH Ag(NH3)2+ RCO
- NH4
+Ag
Tollens' solution
NH3, H2O+
citrate or tartrate buffer
Precipitates as a brick-red solid
++
O ORCH Cu
2+RCO
-Cu2O
• Oxidation of the -CHO group of an aldose to a -CO2H group can also be carried out using Tollens’, Benedict’s, or Fehling’s solutions
Oxidation to Aldonic Acids
• 2-Ketoses are also oxidized by these reagents because, under the conditions of the oxidation, 2-ketoses equilibrate with isomeric _________
An aldoseAn enediolA 2-ketose
CH2OH
C=O
CH2OH
C-OH
CH2OH
CHOH
CHOH
CH2OH
CHO
(CHOH)n (CHOH)n (CHOH)n
Glucose Assay
• The analytical procedure most often performed in the clinical chemistry laboratory is the determination of glucose in blood, urine, or other biological fluid– this need stems from the high incidence of
diabetes in the population
Glucose Assay
• The glucose oxidase method is completely specific for D-glucose
+
+
glucose oxidase
D-Gluconic acid
Hydrogen peroxide
-D-Glucopyranose
OHOH
HOHO
CH2OHO
CH2OH
CO2H
OH
OHH
OH
H
HHO
H
H2O2
O2 + H2O
Glucose Assay
– the enzyme glucose oxidase is specific for -D-glucose
– molecular oxygen, O2, used in this reaction is reduced to hydrogen peroxide H2O2
– the concentration of H2O2 is determined experimentally, and is proportional to the concentration of glucose in the sample
– in one procedure, hydrogen peroxide is used to oxidize o-toluidine to a colored product, whose concentration is determined spectrophotometricallyperoxidase +colored product +o-toluidine H2O2 H2O
Glycosides
• _________ bond: the bond from the anomeric carbon of the glycoside to an -OR group
• __________ are named by listing the name of the alkyl or aryl group attached to oxygen followed by the name of the carbohydrate with the ending -e replaced by -ide– methyl -D-glucopyranoside– methyl -D-ribofuranoside
Formation of Glycosides• Glycoside: a carbohydrate in which the -
OH of the anomeric carbon is replaced by -OR
Methyl -D-glucopyranoside (methyl -D-glucoside)
O
CH2OH
H
OH
OCH3()H
HOH
OHH
H
OCH2OH
HOHO
OHOCH3()
Haworth projectionChair conformation
The –OR group of the glycoside may be another monosaccharide – the resulting glycoside is then a disaccharide
Examples of important disaccharides follow:
Maltose
• From malt, the juice of sprouted barley and other cereal grains
O
OHHO
HOCH2OH
OO
OHHO
CH2OH
OH
-1,4-glycoside bond
-maltose because this -OH is beta
Lactose• The principle sugar present in milk
– about 5% - 8% in human milk, 4% - 5% in cow’s milk
O
OHHO
OHCH2OH
OO
OHHO
CH2OH
OH
-1,4-glycoside bond
-lactose because this -OH is beta
D-galactose unit D-glucose
unit
Sucrose
• Table sugar, obtained from the juice of sugar cane and sugar beet
O
OHHO
HOCH2OH
O
-1,2-glycoside bond
D-glucopyranose unit
O
CH2OH
HCH2OH
OH H
H HO
1
1
2
D-fructopyranose unit
-2,1-glycoside bond
Cellulose
• Cellulose is a linear polymer of D-glucose units joined by -1,4-glycoside bonds– it has an average molecular weight of 400,000,
corresponding to approximately 2800 D-glucose units per molecule
Starch
• Starch is used for energy storage in plants– it can be separated into two fractions; amylose and
amylopectin. Each on complete hydrolysis gives only D-glucose
– amylose is composed of continuous, unbranched chains of up to 4000 D-glucose units joined by -1,4-glycoside bonds
– amylopectin is a highly branched polymer of D-glucose. Chains consist of 24-30 units of D-glucose joined by -1,4-glycoside bonds and branches created by -1,6-glycoside bonds
Glycogen
• Glycogen is the reserve carbohydrate for animals• Like amylopectin, glycogen is a nonlinear
polymer of D-glucose units joined by -1,4- and -1,6-glycoside bonds bonds
• The total amount of glycogen in the body of a well-nourished adult is about 350 g (about 3/4 of a pound) divided almost equally between liver and muscle
Blood Group Substances
• Membranes of animal plasma cells have large numbers of relatively small carbohydrates bound to them
• These membrane-bound carbohydrates are part of the mechanism by which cell types recognize each other; they act as antigenic determinants
• Among the first discovered of these antigenic determinants are the blood group substances