Chemistry Reference
In-Depth Information
glyceraldehyde(C
3
H
6
O
3
)isthesimplestfromthecommonaldoses,whereasdihydroxyacetoneisthe
simplestfromtheketoses.
Asymmetric carbon atoms (carbon atoms that are attached to four different types of atoms
or four different groups of atoms) in sugar molecules lead to the presence of stereoisomer forms.
Monosaccharidesanddisaccharideshavetheabilitytorotatepolarizedlightatdifferentdirections.
Stereoisomermoleculesareknownasdorlisomers(enantiomers),whereaswithregardstothedirec-
tionoftherotation,theyaresymbolizedas(+)(d)ifitisclockwiseor(-)(l)ifitiscounterclockwise.
The characterization of monosaccharide isomers is based on the glyceraldehyde molecule,
whichhasanasymmetriccarbonatom(C
2
)withtwoisomers(dorl)thatrepresenttwoenantiomers
(RorSaccordingtotheprioritiesdecreaseinaclockwiseorcounterclockwisefashion),asshown
inFigure2.1.
Therefore,usingglyceraldehydeasthepointofreference,monosaccharidesbelongtothed-or
l-series if the highest numbered asymmetric carbon atom has the same coniguration with the
middlecarbonatom(chiralcenter)ofd-orl-glyceraldehyde,respectively.Mostsugarsoccurring
innatureareinthed-form,andtheybelongtotheRform.l-sugarsoccurrarelyinnature;themost
importantofthemisl-arabinose.Somel-sugarsareofgreatinterestastheycanbeusedasalterna-
tivesweeteners.
Thedirectionofpolarizedlightrotationischaracteristicforeachsugarandisindependentfrom
thedorlenantiomerism(e.g.,d-fructoseiscounterclockwise).Thespeciicopticalrotation
[ α
20
refersto20°Candtomonochromaticlightdofsodiumandcanbedeterminedwithapolarimeter
thatcorrespondstotheangleofrotationcausedbypassingpolarizedlightthroughasugarsolution
of1g/mLwithapathlengthof1dm.Thespeciicopticalrotationisequalto
α
20
=
[
α
]
D
l
·
where
l
isthepathlength(indecimeters),and
c
istheconcentrationofthesugarsolution(ingrams
per milliliter) used for identiication and quantiication (Robyt 1998; Coultate 2002; Izydorczyk
2005).
Sugarsimportantforfoodsareconsidered:xylosefrompentoses(nonfermentablesugars)and
d-glucose, d-galactose, d-mannose (aldoses), and d-fructose (ketose) from hexoses (fermentable
sugars).Themostimportantofallsugarsisglucose,whichisfoundatthehighestconcentrationin
foodsasfreeorbound(disaccharides,polysaccharides,andglycosides).
Sincecertainreactionscouldnotbejustiiedusingthelinearformofsugarmolecules,itwas
concludedthatthestructureofsimplesugarscanalsobecyclicthroughtheformationofaninter-
mediatehemiacetalform(Kuszmann2006;Horton2008).Thecircularformsofsugarsaregivenby
theHaworthprojectionorbyChair/Boatconiguration,asshownford-glucose(Figure2.2).
Themostimportantd-aldosesarepresentedinFigure2.3.
CH
2
OH
H
O
H
O
C
C
1
1
C
O
H
C
OH
HO
C
H
2
2
CH
2
OH
CH
2
OH
CH
2
OH
3
3
D
-Glyceraldehyde
(R)-Glyceraldehyde
(+)-Glyceraldehyde
L
-Glyceraldehyde
(S)-Glyceraldehyde
(-)-Glyceraldehyde
Dihydroxyacetone
Figure 2.1
Fischer projections of glyceraldehyde and dihydroxyacetone.
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