16.2 Monosacharides
497
Vibrational pattern of a
single lightwave in a
beam ofordinarylight
viewed from the side
\dhen you look at the vibration
head on, it looks like this.
Vibrations in a beam ofordinarv
light are in all directions.
A polarizing filter lets through light in
which vibrations are in only one direction.
Polarizing filter
sl2
t/,1\=
Figure16.5
Oneof a pairof opticalisomers
rotatesplane-polarized
lightto the
right.lts mirrorimagerotatesthe
lightto the left.Theinstrument
usedto measureopticalrotationis
calfeda polorimeter.
Polarizing filter
Sample tube
o-(+) glyceraldehyde
r-(-) glyceraldehyde
directions. Plane-polarized light is light in which uibrations are in only one
direction. stereoisomers ire often called optical isomers.optical isomers
are stereoisomersthat rotate plane-polarized light in oppositedirections.For
example, n-glyceraldehyde rotates plane-polarized light to the right, and
r-glyceraldehyde rotates plane-polarized light to the left. Sometimes you
may seethe name o-(+)-glyceraldehyde.The o refersto the handednessof
the particular glyceraldehyde;the plus sign refers to the direction of rotation of plane-polarLed light (Fig.16.3).Thereis no connection betweenthe
o and the (+); other D sugars may rotate plane-polarized light to the left.
Then the sugarwould be designatedo-(-).
16,2Monosacchorides
AIMS: To drow open-choinFischerproiectionsfor the common
simple sugars.Toidentify o sugor os D or t by looking ot
its Fischerproiectionformulo.
Focus
Most monosaccharides contain
four, five, or six carbons.
Thesimplest carbohydratemolecules,notbonded to any other carbohydrate,
are called simple sugars or monosaccharides. Many of the monosaccharides in nature contain four, five, or six carbons. Sugarscontaining an aldehyde functional group and consisting of four, five, and six carbons are
aldotetroses,aldopentoses,and aldohexoses,respectively.In this section we
will examine some of them.
498
CHAPTER
l6 Carbohydrates
The four-carbon
sugarsl
n-Threose,with a chain of four carbons, is a naturally occurring tetrose; the
aldehyde functional group makes threose an aldotetrose. TWo of the carbons of threose, carbon 2 and carbon 3, have four different groups
attached, so threose has two asymmetric carbons:
(a)
nofJ-H
nji-oH
nJ",o"
CHO
Ho-l-u
I
HToH
cH2oH
o-Threose
1
').
(b)
Fischer projection fbrmulas
tcno
tcno
zl
CHO
H-l-oH
l
HoTH
H-C-OH
HojJ-H
cH2oH
t-CH,OH
l
l-Threose
The Fischer projections of the stereoisomer have an -OH group on
each of their two asl.rnmetric centers,one pointing to the left and one to the
right. If they were not labeled for handedness,how could you tell which is
the o and which is the r isomer?First, draw a Fischer projection of the sugar
being considered.Then look at the hydroxyl group attached to the last
aqrnmetric carbon in the chain. If the hydroxyl group points to the lght,
the sugar belongs to the n family; if it points to the left, the sugar belongs to
the r family. In the caseof threose, the last asymmetric carbon in the chain
is at carbon 3, so structure (a) belongs to the o family of sugars.Structure (b)
belongs to the r family.
The structures of o- and r-threose are mirror images. However, since
there are two asymmetric carbons in a four-carbon sugar molecule,
another pair of stereoisomerscan exist. These stereoisomersare o- and
r-erythrose:
'cHo
,l
H-:C-OH
rl
H_:c_oH
nt",on
o-Erythrose
tcHo
,l
HO-:C-H
' 41
Ho_:g_H
nC*r,o"
l-Erythrose
It is easy to calculate the number of possible stereoisomers of a sugar
that contains multiple asymmetric carbons. This number is 2', where n is
the number of asyrnmetric carbons. Thus glyceraldehyde,with one asymmetric carbon, has 2', or 2, stereoisomers;these are the I and r isomers.
Four-carbon sugars with two asymmetric carbons can exist as22, or 4,
stereoisomers.Wecan calculate the number of mirror-image pairs by dividing the number of stereoisomersby 2. The four aldotetrosel fbrm two pairs
of mirror images.
EXERCISE
I6.4
.. PRACTICE
Draw Fischer projections for the two aldotetroses that belong to the o
'
family.
16.2 Monosacharides
499
Ihe frve-carbon sugars
several aldopentoses are found in nature. n-Arabinose and o-xylose are
five-carbon sug.us produced by plants. o-Arabinose is sorrr-etimescalled
pectin sugar Pectin, the polysaccharide from which it is obtained, forms
gels that are usefrrl in making jelly. Because it is isolated from wood, oxylose is sometimes called wood sugar.
tcHo
tcHo
. )|
H-:C-OH
tl
HO--:C-H
njJ-oH
HojJ-H
al -,"
H-c+on''
H-1i-oH:
-l_l
'cHroH
"cHrort
o-Arabinose
o-Xylose
other important aldopentoses are o:ribose and l-2-deoxyribose, a related
compound that lacks an -OH group at carbon 2:
tcHo
tcno
nlJ-on
njl-on
uZt-u
ujC-on
n-43-oH
H-11-oH
uCnroH
uJ",o"
o-Ribos-e
o-2-Deoxyribose
Thesetwo sugarsare an integral part of the hereditarymaterials ribonucleic
acid (RNA) and deoxyribonucleic acid (DNA). The structures of RNA and
DNA are discussedin Chapter 20.
Ihe slx-carbon sugafsl
Only three aldohexosesappearin nature: o-glucose,o-galactose,and Dmannose:
The relative sweetnessof sugars
and sugar substitutes varies over a
wide range. Lactoseis about onesixth as sweet and glucose about
three-quarters as sweet as sucrose
(table sugar). Fructose is not quite
twice as sweet as sucrose.The
artifi cial sweetenersaspartame
(NutraSweet)and saccharin are
about 150 and 500 times as sweet
as sucrose.
tctto
njJ-on
nojJ-n
nll-oH
Hjl-oH
uJ",o"
o-Glucose
tcHo
ujJ-on
nojl-H
no-lJ-H
Hjl-on
uJ",o"
o-Galactose
tc'o
? u' i b
noll-n
uoji-n
n-1C-on
Hjl-on
uJ",o"
o-Mannose
The o form of glucose has a central role in the nutrition of virtually all
species, including plants and humans. The biochemistry of glucose is so
500
l6 Carbohydrates
CHAPTER
important that Chapter 24 is devoted to it. l-Glucose is abundant in all life
forms. Depending on the source, it has been called grape sugati corn sugai
and blood sugar.Urine usually contains a trace of o-glucose, but the concentration is greatly increased in the urine of patients with untreated diabetes mellitus. r-Galactose is a constituent of lactose,also called milk sugar
(seeSec.16.7).o-Mannose is a major constituent of polymeric molecules
called mannans,which are found in severalplants.
I
Monosaccharidesthat contain a ketonefunctional group -i- are called
ketoses. Ketosescontaining three, four, five, and six carbons ate ketotrioses,
ketotetroses,ketopentoses,and ketohexoses,respectively. No discussion of
hexoseswould be complete without including o-fructose, a ketohexose
becauseof the presenceof a ketone carbonyl group in the molecule at carbon2. o-Fructose and o-glucose differ in structure only at carbons I and2.
The identical stereochemistry at carbons 3, 4, and 5 exists because the
breakdornmof o-glucose in living systemsinvolves conversion of o-glucose
to n-fructose.
1
CH,OH
-^ cl : o
^t
Ho-:C-H
t1
H_]C-OH
_l
u-lc-on
^l
cH2oH
D-FruCtOSe
o-Fructose occurs in a large number of fruits and in honey. It is also the only
sugar found in human semen. o-Fructose is one of those sugarswhich
belongs to the o family but rotates plane-polarized light in a left-handed
direction.
I6.5
PRACTICE
EXERCISE
,:
Identifu each structure as D or L.
(b) cHo
(a)
CHo
I
H-l-oH
Ho-fH
CH2OH
(c)
cH2oH
HToH
CH2OH
t)
(d)
cHo
Fo
HO
H
l"I
To"
H
To"
cH2oH
HO
HO
cH2oH
0
PRACTICE
EXERCISE
I6.6
projection
formula for o-glucose. Number the carFischer
Draw the
carbonwith an asterisk.
identiff
each
asymmetric
bons, and