Download - CARBS b Fischer and Haworth
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Mffi CHAPTERl5 CARBOHYDMTES
ffi sryl g-*' " ryg! M gt o ugltrtgyeqg--
rn chapter 14, we learned that.chiral t"T::T,t*t:::::"y- images that canno{
LEARNIFJG GSAL
Use Fischer Proiections to draw
the P or L isomers of glucose'
galactose, and fructose'
yff; srlr sruDY Acrlvlw" #y Forms o{ CarbohYdrates
o*3_ TUTORIAL
f mC l ldentifying Chiral Carbons inE=J Monosaccharides
*- TUTORIALi me 3 Drawing Fischer Projections?q""r of Monosaccharides
d3,- rutoRtnLi wiL 6''' J ldentifying o and r Sugars
Fischer Projections
Let,s take a look again at the Fischer projection for the simplest aldose' glyceraldehyc
Byconvention,thecarbonchainiswrittenverticailywiththeaidehydegroup(mostoldized carbon) at the top. The lettel L is assigned to the stereoisomer if the -OtI group
on the left of rhe chirai carbon. In n-glyceraldehyde, the -OH is on the right' The carbr
atom in the -CH2OH gloup at the bottom of the Fischer projection is not chiral' becau
it does not have four different groups bonded to it: :
,"oin.i*t"u' M;"; monosaccharides exist as mirror images'
Most of the carbohydrates we will study have carbon chains with five or six carbt
atoms. Because there are several chiral carbons, the chiral carbon farthest from tl
carbonyl group is used to determine the o or L isomer' The following are the Fischer pr
jections for tnl o and L isomers of ribose, a five-carbon monosaccharide, and glucose'
six-carbon monosaccharide. In each of the mirror images, it is important to understal
that the -OH groups on all the ehiral carbon atoms ale reversed from one side to t'
other. For example, in l-ribose, the -OH groups are all written on the left side of the ht
izontal lines. In the mirror image, n-ribose, the -OH groupS are all written on the rig
side of the horizontal lines.
{t-CHO.l
IHO:I-H I'.-----1-cHzoH
l-GlyceraldehYde
t"LCHO
lH-+-oH I
-- cH2oHo-GlyceraldehYd
4CHO
OH
H
OH
cH2oHo-GIucose
How does the Fischer projection of l-galactose differ from that of o-glucose?
ANSWERIn the Fischer projections, the direction of the -OH group on the chiral carbor
atoms differs only at carbon 4, extending to the left in o-galactose and to the right tl
D-slucose.
tr.CHO
i'(-t-HO
il"CHO
Fischer
15.2 FISCHER PROJECTIONS OF MONOSACCHARIDES
ig ldentifying o and L lsomers of Sugars
Identify the following Fischer projection as D- or t'-ribose:
H,O\"/
Ho -l-nI
Ho -l-HHo *1-H
ct{2oH
SOLUTION
ln ribose, carbon 4 is the chiral atom farthest from the
t yaro*yt'gtoup on carbon 4 is on the left' this is t--ribose:
H.o\./
SAMPLE PROBLE
carbonYl grouP. Because the
HO
HO
HO
farthestgroup
tt",o
H-_{__ onIHo-f*H
Ho--l-Hn-t-o"
cH2oHD-Galactoss
CHrOHt- |n, al,{+ lo tl@ lt^*'ri
\ {{Jt lurt \ 3 F
c-oH
HO
OH
H
OH
OH
H
H
HO
H
H
H
OH
OH
cHzoHo-Fructqse
I
--l-HI-1-"
-+-f-:\"l \l\
CH2OH Chiral carbon
from carbonYl
STUDY CHECK
Draw the Fischer projection for o-ribose'
Some lmPortant Monosaccharides
Thehexosesglucose,galactose,andfructoseareimportantmonosaccharides.Althoughwecan draw Fischer prolections for o and t- isomers' 1he o isomers *" ::T:1::::T''Tffi;;. ffi;il;;ii; ;;; body. rhe Fischer projections for the n isomers are wrlt-
ten as follows: AHHO\"/
cH2oHu-Glucose
The most common hexose, n-glucose'sugar, is found in fruits, vegetables, corn
/nf'!,x t. ! ;' 'ffJ*fv-{a'.i 'Y
C6H12O6, also known as dextrose and blood
tv*p,i"a honey. It is a building block of the
',&CHAPTER 1 5
Fischer Projections of Monosaccharides
is a Fischer projection identified as a D or L isomer?
15.L2 Draw the Fischerprojection for o-glyceraldehyde and
r--glyceraldehyde.
ify each of the followi,ng as the D or L lsomer:
d. : CHO',''l:,
HO-_F_Hl,','
HO-].. 11 .'IHort]H
cH2oHRibose.
Identify the monosaccharide that fits each of the foilowingl^-^;.^+:^--.ug>LttPLlutt).
a. also called blood sugarb, not metabolized in galactosemiac. also called fruit sugar
15.22 Idenrify the monosaccharide that fits each of rhe followingdescriptions:a. high blood levels in diaberesb. obtained as a hydrolysis product oflactosec. the sweetest of the monosaccharides
c.
H
HO
H
HCHO
Ho-1-sIHrT-oHcH2oH
Threose
b. cH2oH
C:Oso-l-n
IH-1-oHcH2oH
Xylulose
c. CHO d. CHO
u-]-on u-l-oHIIH-l-oH H-f-oHHo-f-H H-+-oH
tlHo-t-H H-f-oHcHzoH cH2oH
Mannose Allose
15.14 ldentify each of the following as the o or L isomer:
a. cH2oH b. cH2oHtIC-o C:oI ^'- Ho-l--nH-J- oH
II H-FOHH --J-oH II Ho-T-t
cH2oH cH2oHRi bulose
CARBOHYDRAlES
\ " :: ff::$:ffi:;J11R];l}.i,1 *?# fli:';:; Hl*ffi.ff:oms arc wn[e1
\
I .HroHili c:o1 ,o_-L'l""l-I "o-1-"I cH2oH
I
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I srunv cHEcl<
I *u, type of carbohydrate is ribulose?
k
CHO
_fo,,l--i"
-_|-o"-_]--oH
cH2oHulucose
@.t?).u* the Fischer projections for the mirror images for a-d inVoroblem 15.13.
15.16 ;; the Fischer projections for the mirror images for a-d inDroDlem lf.l4.
@;..- the Fischer projections for o-glucose and l-glucose.
15.18 Draw the Fischer projections for o-fructose and L-fructose.
Q5.19)}iow does the Fischer projection for o-galactose differ from\--l
D-slucose /
15.20 HJw does the Fischer proiection for o-fructose differ fromo-glucose?
o
8-r \
_o/
H
STEP 2 Fold the carbon chain into a hexagon by moving carbon 5 above carbon 3. Inall o-monosaccharides including o-glucose, the -CH2OH
group (carbon 6) isplaced above carbon 5, and the --OH group on carbon 5 is written trext to the car-bonyl carbon. To complete the Haworth structure, draw a bond from the oxygen ofthe --OH group to the carbonyl carbou
15.3 HAWORTH STRUCTURES OF MONOSACCHARIDES
ffi l!rySlglgggrqg|l4ggg11..br i d e s
ln Chapter 14, we saw that an aldehyde group reacts with one alcohol molecule to form a
lremiacetal. This same reaction occurs when a carbonyl group and an -OH group are in
the same molecule, which forms a cyclic hemiacetal. While the carbonyl group in the
open chain could react with several of the -OH groups, the most stable form of pentoses
and hexoses are their hemiacetals with five- or six-atom rings. For the aldohexose in the
following diagram, the oxygen atom in the hydroxyl group on carbon 5 forms a bond withthe carbonyl carbon 1 to produce a helerocyclic six-atom ring containing an oxygen atom
and an --*OH group on carbon 1:
O.rHuoJ,=Ck:
LEARNIf{G 6OALDraw and identify the Haworthstructures of monosaccharides.
@ Y*l$y.ri.sus,,,
Open chain
H' /-o, ,H----*\ rr-OH
Heterocyclic hemiacetal
Drawing Haworth Structures for Cyclic Forms
kt's look at how we draw the Haworth structure, a representation of the cyclic herni-acetals of the rnonosaccharides for some p isomers, starting with the open-chain sffuctureof o-glucose. Traditionally, Fischer projections represent the chiral carbon intersectionsof vertical and horizontal lines. In this text, we will show the carbon chain when it helpsunderstanding.
STEP 1 Thrn the open-chain structure of o-glucose clockwise. This places the
-OH groups pn the right of the vertical open chain below the carbon atoms andthe -*{H group on the left of the open chain above its carbon atom:
'lH-zC - OH
HO 3C-HH 4?*oHH-5C-OH
uCH2OHD-U
H H OHH651+lrlrir: HocHz-f-f-c-c-crrllOHOHH OH
lucose (open chain)
CH,OH6lI
H;g-gHt,/l | ,.otr/ u \"/+fq oH H ZY\t| \l l,/ lr -:+
HO )C--^C' rr
'| 'lHOH
Calbon-5 oxygen bonds to carbonyl
6cH2oH-tLr )c_-o'i/t \ ,oH
| / H \../ Hydroxyl group on
oYt OU H ,/Y\ newchiiaica'bon
,-l\l l,/ HHO -C--C''l'iHOH
Cyclic hemiacetal srlucture
CHAPTERl5 CARBOHYDMTES
STEP 3 In the Haworth structure for the hemiacetal, the -OH group forms orcarbon 1. In the Haworth stmcture, the comers of the ring represent carboratoms. There are two ways to draw the -OH on carbon 1, either up or d6yen
which gives two isomers called anomers. The -OH group is drawn down irthe a (alpha) anomer and up in the B (beta) anomer:
l')' cHroH
HOHa-p-Glucose
HOHB-o-Glucose
f!'i1e:
Ll,\a,fr
L lh e s[ntjh l(,s, rrt ,(' a-
A'4,a,,pg *ghari,lt t + q <
&rna,4tcl'it C : l-lre
Mutarotation of e- and B-o-glucoseIn aqueous solution, the Haworth sffucture of a-o-glucose opens and closes to formB-o-glucose. In this process called mutarotation, each anomer converts to ttre openchain and back again. As the ring opens and closes, the hydroxyl (-OH) group on car-bon I forms either the a or the B anomer. At equilibrium, a glucose solution contains a
mixture of 36Vo of the a anomer and 64Vo of the B anomer. Although the open chain isan essential part of mutarotation, only a trace amount of the open chain is present at anysiven time:Cg/"*y[ , Tln'iJ ('uf
ilffi,i, h.,,.fu
YD/ ilele(wiqv wh;c h
L il fhe l'at)t'4{it (,: +l- l,il [.* 1I4: r+i ft^. o^,L in Lfu ,,nh , tk^l|t
,l;recllv fu"lt {"
-
t
W'ovyL(;tth, vrrii"p-xi,l,zeJ
L ,'v" tht" , rnl ,
Open chain with
groupAnomeric carbon
erHO
eIHO
H
OHHHOHH
H 611 a Anomer
a-o-Glucose(367o in equilibrium mixture)
Flaworth Structures of Galactose
Galactose is an aldohexose that differs from glucose only in the arrangement of the
-OH group on carbon 4. Thus, its Haworth structure is similar to glucose, except that ingalactose the -OH on carbon 4 is up. Galactose aiso exists as a and B anomers and under-goes mutarotation via the open-chain form in aqueous solution:
HOHo-Glucose
open chain (trace)
lD' cH2oH
H
HI
criI
OH
ooo-
H
OH.lC4
I
H
A 6g F Anomet
(64Vo in equilibrium mixture)
1o
-CH'OHOt
15.3 HAWORTH STRUCTURES OF MONOSACCHARIDES
flaworth Structures of Fructose
Sqcontrast to glucose and galactose, fructose is a ketohexose. The Haworth structure for
{-f uctose is a stable five-atom ring. The hemiacetal forms when the hydroxyl group 0n car-'
t{fn 5 reacts with the ketone group on carbon 2. In fructose, the anomeric carbon, which is
{-rfubon 2, is bonded to -CH2OH and a hydroxyl group (-OH). Mutarotation of the
tLhomeric carbon 2 gives a and B anomers:
,qHzoHf
lLr
ti- 4nu'"'14*t, i t
',#
Ho-3?-HH-4C-OH
H*59-oH
uCH2OHD-Fructose
H2OH
H rcH2oH
OHHB-o-Fructose
E Anomers
r. Why is tlre Haworth structure of n-galactose a hemiacetal?r. What is the difference petween a and B anomers of o-galactose?
\NSWERr. o-galactose consists of both a carbonyl group as the aldehyde and several hydroxyl
groups. A stable six-atom cyclic structure forms when the hydroxyl group oncarbon 5 reacts with the carbonyl group at carbon l. The resulting structure is acyclic hemiacetal.
r. When the hemiacetal forms, an -OH appears on carbon 1. Tlvo isomers calledanomers are possible because the -{H can form above or below the ring. In thea anomer, the -OH is drawn down, and for the B anomer, the ---OH is drawn up.
sAMPLE PROBLEM ffitrl Drawing Haworth Structures for Sugars
"D-Munoor", a carbohydrate found in immunoglobulins, has the following open-chaintructure. Draw the Haworth structure for F-p-mannose anomer:
\clI
HO-C-HI
HO-C-HI
H_ C_ OHI
H__C--_OHI
cH2oHo-Mannose
o
4)
ItHofH3
"+ CN
CHAPTERl5 CARBOHYDMTES
SOLUTION
STEP 1 Tllrn the open-chain structure to the right.
.,o1//C
H
Fold the carbon chain into a hexagon (move carbons 4, 5, and 6 clock_wise), Draw the -CH2OH group above carbon 5 and the -OH groupnext to the carbonyl. Complete the Haworth structure by bonding the o of thl--{H group with the C of the carbonyl group:
STEP 3 In the Haworth structure, drawmake the B-n-mannose anomer:
the hydroxyl group on carbon 1 up to
OH
H H OHOH.-t.t_t2lno"cn,-'E -t-'i-.--llll
OHOHH H
HoV
HOH OH
HO
HHB-o-Mannose
STUDY CHECK
Draw the Haworth structure for a-n-glucose.
Guide to Drawing Haworth Structures
(HOH
Haworth Structures of Monosaccharides
{'5.23}ame the kind and number of atoms in theg!4g44!g of thev Haworth sFucture of glucose.
15.24 Name the kind and number of atoms in the ring portion of theHaworth structure of fructose.
the Haworth stmctures for the a and B anomers of
15.28 Identify each of the following Haworth structures as the a or
B anomer:
a. CH2OH
15.26 Draw the Haworth structures for the a and B anomers ofD-fmctose.r\
(15.27y'dentifu each of rhe following Haworth srnrctures as the a or\----. p̂ anomel:
HOCH2-.-o\ cH2oHV\l\ H HO ,/lOH
f- t'.'HOHH