Download Importance of Carbohydrates

11/2/16
Chapter 25
Importance of Carbohydrates
Distributedwidelyinnature
Keyintermediatesofmetabolism(sugars)
Structuralcomponentsofplants(cellulose)
Centraltomaterialsofindustrialproducts:
paper,lumber,fibers
• Keycomponentoffoodsources:sugars,flour,
vegetablefiber
• ContainOHgroupsonmostcarbonsinlinear
chainsorinrings
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Chemical Formula and Name
• CarbohydrateshaveroughlyasmanyO’sasC’s
(highlyoxidized)
• SinceH’sareaboutconnectedtoeachHand
Otheempiricalformulasareroughly
(C(H2O))n
• Appears to be “carbon hydrate” from formula
• Currentterminology:naturalmaterialsthat
containmanyhydroxylsandotheroxygencontaininggroups
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Chemical Formula and Name
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Sources
• Glucoseisproducedinplantsthrough
photosynthesisfromCO2 andH2O
• Glucoseisconvertedinplantstoothersmall
sugarsandpolymers(cellulose,starch)
• Dietarycarbohydratesprovidethemajor
sourceofenergyrequiredbyorganisms
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Why this Chapter?
• Toseewhatthestructuresand1˚biological
functionsofcarbohydratesare
• Tohaveanintroductiononhowcarbohydrates
arebiosynthesizedanddegradedinorganisms
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25.1 Classification of Carbohydrates
• Simplesugars(monosaccharides)can'tbe
convertedintosmallersugarsbyhydrolysis.
• Carbohydratesaremadeoftwoormoresimple
sugarsconnectedasacetals (aldehydeand
alcohol),oligosaccharidesandpolysaccharides
• Sucrose(tablesugar):disaccharidefromtwo
monosaccharides(glucoselinkedtofructose),
• Celluloseisapolysaccharideofseveralthousand
glucoseunitsconnectedbyacetal linkages
(aldehydeandalcohol)
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25.1 Classification of Carbohydrates
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Aldoses and Ketoses
• aldo- andketo- prefixesidentifythenatureof
thecarbonylgroup
• -ose suffixdesignatesacarbohydrate
• NumberofC’sinthemonosaccharide
indicatedbyroot(-tri-,tetra-,penta-,hexa-)
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Aldoses and Ketoses
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25.2 Depicting Carbohydrate
Stereochemistry: Fischer Projections
• Carbohydrateshavemultiplechiralitycenters
andcommonsetsofatoms
• AchiralitycenterCisprojectedintotheplane
ofthepaperandothergroupsarehorizontal
orverticallines
• Groupsforwardfrompaperarealwaysin
horizontalline.Theoxidizedendofthe
moleculeisalwayshigheronthepage(“up”)
• The“projection”canbeseenwithmolecular
models
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25.2 Depicting Carbohydrate
Stereochemistry: Fischer Projections
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Stereochemical Reference
• Thereferencecompoundsarethetwo
enantiomersofglyceraldehyde,C3H6O3
• Acompoundis“D”ifthehydroxylgroupat
thechiralitycenterfarthestfromtheoxidized
endofthesugarisontherightor“L”ifitison
theleft.
• D-glyceraldehydeis(R)-2,3-dihydroxypropanal
• L-glyceraldehydeis(S)-2,3-dihydroxypropanal
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Stereochemical Reference
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Working With Fischer Projections
• Ifgroupsarenotincorrespondingpositions,
theycanbeexchangedthreeatatimein
rotation– workwithmolecularmodelstosee
howthisisdone
• Theentirestructuremayonlyberotatedby
180°
• WhileR,Sdesignationscanbededucedfrom
Fischerprojections(withpractice),itisbestto
makemolecularmodelsfromtheprojected
structureandworkwiththemodel
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Working With Fischer Projections
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25.3 D, L Sugars
• Glyceraldehydeexistsastwoenantiomers,first
identifiedbytheiroppositerotationofplane
polarizedlight
• Naturallyoccurringglyceraldehyderotatesplanepolarizedlightinaclockwisedirection,denoted
(+)andisdesignated“(+)-glyceraldehyde”
• Theenantiomergivestheoppositerotationand
hasa(-)or“l”(levorotatory)prefix
• Thedirectionofrotationoflightdoesnot
correlatetoanystructuralfeature
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NaturallyOccurringDSugars
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25.5 Cyclic Structures of Monosaccharides:
Anomers
• Alcoholsaddreversiblytoaldehydesand
ketones,forminghemiacetals
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Internal Hemiacetals of Sugars
• Intramolecular nucleophilic additioncreates
cyclichemiacetals insugars
• Five- andsix-memberedcyclichemiacetals are
particularlystable
• Five-memberedringsarefuranoses.Sixmemberedarepyranoses
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Internal Hemiacetals of Sugars
• Formationofthethecyclichemiacetal creates
anadditionalchiralitycentergivingtwo
diasteromeric forms,designateda andb
• Thesediastereomers arecalledanomers
• Thedesignationa indicatesthattheOHatthe
anomeric centerisonthesamesideofthe
Fischerprojectionstructureashydroxylthat
designateswhetherthestructureusDorL
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Converting to Proper Structures
• TheFischerprojectionstructuresmustbe
redrawntoconsiderrealbondlengths,and
youalsoseethe“Pyran”form
• Pyranoseringshaveachair-likegeometrywith
axialandequatorialsubstituents
• Ringsareusuallydrawnplacingthe
hemiacetaloxygenatomattherightrear
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Converting to Proper Structures
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Converting to Proper Structures
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25.6 Reactions of Monosaccharides
• ¾OHgroupscanbeconvertedintoestersand
ethers,whichareofteneasiertoworkwith
thanthefreesugarsandaresolubleinorganic
solvents.
• Esterification by treating with an acid chloride
or acid anhydride in the presence of a base
• All ¾OH groups react
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25.6 Reactions of Monosaccharides
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Ethers
• Treatmentwithanalkylhalideinthepresence
ofbase—theWilliamsonethersynthesis
• Usesilveroxideasacatalystwithbasesensitivecompounds
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Glycoside Formation
• Treatmentofamonosaccharidehemiacetal
withanalcoholandanacidcatalystyieldsan
acetal inwhichtheanomeric¾OHhasbeen
replacedbyan¾ORgroup
• β-D-glucopyranose with methanol and acid
gives a mixture of a and β methyl Dglucopyranosides
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Reduction of Monosaccharides
• Treatmentofanaldoseorketose withNaBH4
reducesittoapolyalcohol(alditol)
• Reactionviatheopen-chainforminthe
aldehyde/ketonehemiacetal equilibrium
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Oxidation of Monosaccharides
• Aldosesareeasilyoxidizedtocarboxylicacids
by:Tollens'reagent(Ag+,NH3),Fehling's
reagent(Cu2+,sodiumtartrate),Benedict`s
reagent(Cu2+,sodiumcitrate)
• Oxidations generate metal mirrors; serve as
tests for “reducing” sugars (produce metallic
mirrors)
• Ketosesarereducingsugarsiftheycan
isomerizetoaldoses
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Oxidation of Monosaccharides
• Aldosesareeasilyoxidizedtocarboxylicacids
by:Tollens'reagent(Ag+,NH3),Fehling's
reagent(Cu2+,sodiumtartrate),Benedict`s
reagent(Cu2+sodiumcitrate)
• Oxidations generate metal mirrors; serve as
tests for “reducing” sugars (produce metallic
mirrors)
• Ketosesarereducingsugarsiftheycan
isomerizetoaldoses
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Oxidation of Monosaccharides
with Bromine
• Br2 inwaterisaneffectiveoxidizingreagent
forconvertingaldosestocarboxylicacid,
calledaldonic acids(themetalreagentsarefor
analysisonly)
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Formation of Dicarboxylic Acids
• WarmdiluteHNO3 oxidizesaldosesto
dicarboxylic acids,calledaldaric acids
• The¾CHOgroupandtheterminal¾CH2OH
groupareoxidizedtoCOOH
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Summary
• Weonlycoveredmonosaccharidesandtheir
reactions.
• Highlyrecommendreadingthischapterespecially
ifyouplantotakebiochemistryinthefuture.
• Wecovered25.1,25.2,25.3,25.5,25.6(through
oxidationpg.853).
• DonotneedtoknowKoenigs-Knorrreaction(pg.
850-851)orBiologicalEsterFormation(Pg.851852).
• Forfinalexam,practiceproblemsfromtextbook:
1-7,11-20.
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