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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

I

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