Download 16.5 Glycosides

16.5Clycosides 505
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PRACTICE
EXERCISE
I5.7
Draw the Haworth projection of
B-o-grucopyranose.Identiff the hemiocetSlcarbon.
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EXERCISE
.. PRACTICE
I5.S
Draw the.Haworth projection of a-o-ribofuranose.
Identiff the hemi,, a.c€td carbon.
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PRACTICE
EXERCISE
I6.9
,
Prury the Haworth projection of a-o-fructofuranose. Identify the
hemiketal carbon.
Interconversion
of stralght-chain
aa4 ring for.trs of sugars
The straight-chain sugar.forms are in equilibrium
with the ring forms. The
ring forms are usually quite predominant. For
example, ifstlieZcfremically
pure a-o-glucopyranoseis dissolved in an
acidic iol'tion, the ring will
open and close repe.atedry.In reclosing, some
B-o-glucopyranose is
formed. The final equitibrium *i"t.tt" .orrists
of about 63voB-o-grucopyranose, about 37To a-o-gtrucopyranose,and
only u ti"y uriolrr, of the
straight-chain aldehyde. From ihe percentages products
of
formed, we can
say that B-o-glycgn{anose is_only srightry morapreferred
than a-o-glucopyranose and that both o-glucopyranos-e
*o-"r,
u." -.r.h -ore pre_
ferred than the straight-chai" -aa"frya" form of
t_glucose.
CHO
I
H-C-OH
cH2OH
I
HO-C-H
I
H-C_OH
OH (cr)
H-C_OH
I
cH2OH
o.-o-Glucopytanose
(about 37%)
oroH (9)
o-Glucose
(lessthan l%)
OH
B-o-Glucopyranose
(about 63%)
16.5Glycosides
AIM: To describethe formotion of grycosidic hoinds
ond the
- '
products of their hydrotysis.
Focus
Alcohols react with closedchain forms of sugars to form
glycosidic bonds.
The closed-chain hemiacetal or hemiketal forms
of sugars may react with
tgrgr1nSCerdso{rcfals
(see
s".. rs.ol.Theacitatsorietalsofsugl-""Itt
arsarecalledglycosideC.- Glycosidic bonds
Thecoualentetherlink betweenthesugarhydroxyr
and thearcohoris asycosidicbond. A simplearcoholsuchai ''"ihurroi
u. o-o_
""J;;;;;lr"n
ll
l,l
r'f
506
CHAPTER16 Carbohydrates
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_o
J\
ucHroH
Figure15.7
Someof the common
glycosidic
bondsfoundin
polysaccharides.
Theacetal
portionsof
andhemiacetal
areshbwn
the molecules
in color.Thewavyline
the hydroryl
connecting
groupto carbonI indicates
that the carbon-orygen
linkagemaybe eitheralpha
or beta.
_o
5\
4
I
O, ,.-90-+6)
J
Clycosidicbond
glucopyranoseproduce methyl a-o- glucopyranoside:
cHzoH
i.",o".ts4E>Z*.:::,ji*.
Ho\|--{ocH3
OH
ct-o-Glucopyranose
Methanol
Methyl a-o-glucopyranoside
The alcohol used to make a glycosidic bond is often more complex than
methanol-in fact, sugarsthemselvesare alcohols.As shovvnin Figure 16'7'
the individual saccharide units are attached through glycosi<lic bonds. Glycosidic bonds between sugars are designated according to the position
numbers of the carbons of the sug:us that are linkdd and also according to
the stereochemistry of the linkage. For example, suppose the beta hydroxyl
group at carbon I in a hexose is linked by a glycosidic bohd to carbon 4 of
another hexose. This linkage is called a BQ-4) glycosidic bond. Other
common linkagesare c(l-4), a(l-6), and B(l-6). Once the anomeric
-OH group of a sugar is tied up as an acetal, it is no longer freeto go from
the ring form to the straight-chain form.
r6.t0
PRACTTCE
EXERCISE
=
.. The hydroxyl group of carbon I in a-o-glucopyr:rnoseis lin\ed bya glyt cosidic bond to carbon 4 of another o-glucopyranose molecule. Draw
?,= the structure of the glycoside that is formed. Identifii the acetal car-bon.
16.6 Polysaccharides
Hydrolysis
of glycosidic
507
bonds
Glycosidic bonds may be cleaved by hydrolysis reactions. We can take the
hydrolysis of an a(1-4) glycosidic bond between two hexosesas an example. For simplicity, only the carbon skeleton and the glycosidic bond are
shown:
Fo.
L-o.
L-o.
L-o.
L)"(_)+Ho-H=(_),*n_L)
The chemical hydrolysis of most complexsugars can be done byheating an
aqueous solution of the carbohydrate.A trace of acid is added as a catalyst.
En4rmes act as the catalyst in biological systems.Hydrolysis reactions will
be important as we proceed into biochemistry since they are the means by
which sugars, fats, and proteins are broken dornmto simple materials by
digestion.
PRACTICE
EXERCISE
I6.II
,, The glycosidic bondin the following compound is hydrolyzed.What are
-: the structures of
the products?
cH2oH
F-O
]
t/'
\
\oH /
"oHl
rol
o
cH2oH
16.6Polysaccharides
AIM: To list the structures,sources,ond usesof the following
polysoccharides:
storch,omylose,amylopectin,glycogen,
ond cellulose.
Focus
Polysaccharidesare composed
of many monosaccharide units
connected by glycosidic
linkages.
Individual sugar units may be connected to one another to forrir linear,
branched, or circular polymers, as shor,rmin Figure 16.8. Polysaccharides
haue many monosaccharidesbonded togetherto form a long polymer chain.
The bonds connecting the sugar units are glycosidic. The 1-4 and L,*6
linkages are the ones most commonly found in natural polysaccharides
consistingofhexoses.