浙大食化考研.docx
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浙大食化考研.docx
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浙大食化考研
Chapter2Carbohydrates
Carbohydratescomprisemorethan90%ofthedrymatterofplants.Theyareabundant,widelyavailable,andinexpensive.Theyarecommoncomponentsoffoods,bothasnaturalcomponentsandasaddedingredients.Theiruseislargeintermsofboththequantitiesconsumedandthevarietyofproductsinwhichtheyarefound.Theyhavemanydifferentmolecularstructures,sizes,andshapesandexhibitavarietyofchemicalandphysicalproperties.Theyareamenabletobothchemicalandbiochemicalmodification,andbothmodificationsareemployedcommerciallyinimprovingtheirpropertiesandextendingtheiruse.Theyarealsosafe(nontoxic).
Starch,lactose,andsucrosearcdigestiblebyhumans,andthey,alongwithD-glucoseandD-fructose,arehumanenergysources,providing70-80%ofthecaloriesinthehumandietworldwide.IntheUnitedSlates,theysupplylessthanthatpercentage,infactonlyabout50%,ofthehumancaloricintake.HealthorganizationsrecommendthatthepercentageofthetotalcaloriestheaverageAmericanconsumesintheformoffat(about33%)bereducedtonomorethan30%andthatthedifferencebemadeupwithcarbohydrate,especiallystarch.
Thetermcarbohydratesuggestsageneralelementalcomposition,namelyCx(H2O)y,whichsignifiesmoleculescontainingcarbonatomsalongwithhydrogenandoxygenatomsinthesameratioastheyoccurinwater.However,thegreatmajorityofnaturalcarbohydratecompoundsproducedbylivingorganismsdonothavethissimpleempiricalformula.Rather,mostofthenaturalcarbohydrateisintheformofoligomers(oligosaccharides)orpolymers(polysaccharides)ofsimpleandmodifiedsugars.Lowermolecularweightcarbohydratesareoftenobtainedbydepolymerizadonofthenaturalpolymers.Thischapterbeginswithapresentationofthesimplesugarsandbuildsfromtheretolargerandmorecomplexstructures.
3.1Monosaccharides
Carbohydratescontainchiralcarbonatoms.Achiralcarbonatomisonethatcanexistintwodifferentspatialarrangements(configurations).Chiralcarbonatomsarethosethathavefourdifferentgroupsattachedtothem.Thetwodifferentarrangementsofthefourgroupsinspace(configurations)arenonsuperimposablemirrorimagesofeachother.Inotherwords,oneisthereflectionoftheotherthatwewouldseeinamirror,witheverythingthatisontherightinoneconfigurationontheleftintheotherandviceversa(Fig,1).
D-Glucose,themostabundantcarbohydrateandthemostabundantorganiccompound(ifallitscombinedformsareconsideredbelongstotheclassofcarbohydratescalledmonosaccharides.Monosaccharidesarecarbohydratemoleculesthatcannotbebrokendowntosimplercarbohydratemoleculesbyhydrolysis,sotheyaresometimesretortedtoassimplesugars.Theycanbejoinedtogethertoformlargerstructures,namely,ollgosacccharidesandpolysaccharides,thatcanbeconvertedintomonosaccharidesbyhydrolysis.
D-Glucoseisbothapolyalcoholandanaldehyde.Itisclassifiedasanaldose,adesignationforsugarscontaininganaldehydegroup(TableI).Theending-osesignifiesasugar;ald-signifiesanaldehydegroup.WhenD-glucoseiswritteninanopenorvertical,straight-chainfashion,knownasanacyclicstructuretoorganicchemists,withthealdehydegroup(positionl)atthetopandtheprimaryhydroxylgroup(position6)atthebottom,itisseenthatallsecondaryhydroxylgroupsareoncarbonatomshavingfourdifferentsubstituentsattachedtothem.
Figure1Chiralcarbonatom.A,B,D,andErepresentdifferentatoms,functionalgroups,orothergroupsofatomsattachedtocarbonatonC.Wedgeindicatechemicalbondsprojectingoutwardfromtheplaneofthepage;dashindicatechemicalbondsprojectingintoorbelowtheplaneofthepage.
Table1ClassificationofMonosaccharides
Thesecarbonatomsarethereforechiral.Glucosehasfourchiralcarbonatoms:
C-2,C-3,C-4,andC-5.NaturallyoccurringglucoseisdesignatedistheDform,specificallyD-glucose.Ithasamolecularmirrorimage,termedtheL-form.specificallyL-glucose.Sinceeachchiralcarbonatomhasamirrorimage,thereare2narrangementsfortheseatoms.Therefore,inasix-carbonaldose,thereare24or16differentarrangementsofthecarbonatomscontainingsecondaryhydroxylgroups,allowingformationof16differentsix-carbonsugarswithanaldehydeend.EightofthesebelongtotheD-series(seeFig.3);eightaretheirmirrorimagesandbelongtotheLseries.Allsugarsthathavethehydroxylgrouponthehighestnumberedchiralcarbonatom(C-5inthiscase)positionedontheright-handsidearearbitrarilycalledD-sugars,andallwithaleft-handpositionedhydroxylgroupontilehighestnumberedchiralcarbonatomaredesignedL-sugars.TwostructuresofD-glucoseinitsopen-chain,acyclicform(calledtheFischerprojection)withthecarbonatomsnumberedintheconventionalmanneraregiveninFigure2.Inthisconvention,eachhorizontalbondprojectsoutwardfromtheplaneofthepageandeachverticalbondprojectsintoorbelowtheplaneofthepage.Itiscustomarytoomitthehorizontallinesforcovalentchemicalbondstothehydrogenatomsandhydroxylgroupsasinthestructureontheright.Becausethelowermostcarbonatomisnonchiral,itismeaninglesstodesignatetherelativepositionsoftheatomsandgroupsattachedtoit.Thus,itisusuallywrittenas-CH2OH.
D-Glucoseandallothersugarscontainingsixcarbonatomsarecalledhexoses,themostcommongroupofaldoses.Thecategoricalnamesareoftencombined,withasixcarbon-atomaldehydesugarbeingtermedanaldohexose.
Figure2D-Glucose(open-chainoracryclicstructure).
Therearetwoaldosescontainingthreecarbonatoms.TheyareD-glycerose(D-glyceraldahyde)andL-glycerose(L-glyceraldehyde),eachpossessingonlyonechiralcarbonatom.AIdoseswithfourcarbonatoms,thetetroses,havetwochiralcarbonatoms;aldoseswithfivecarbonatoms,thepentoses,havethreechiralcarbonatomsandcomprisethesecondmostcommongroupofaldoses.Extendingtheseriesabovesixcarbonatomsgivesheptoses,octoses,andnonoses,whichisthepracticallimitofnaturallyoccurringsugars.DevelopmentoftheeightD-hexosesfromD-glyceroseisshowninFigure3.Inthisfigure,thecirclerepresentsthealdehydegroup;thehorizontallinesdesignatethelocationofeachhydroxylgrouponitschiralcarbonatom,andatthebottomsoftheverticallinesaretheterminal,nonchiralprimaryhydroxylgroups.ThisshorthandwayofindicatingmonosaccharidestructuresiscalledtheRosanoffmethod.SugarswhosenamesareinitalicsinFigure3arecommonlyfoundinplants,almostexclusivelyincombinedforms.Theythereforearepresentinourdietsincombinedforms.D-Glucoseistheonlyfreealdoseusuallypresentinnaturalfoods,andthenonlyinsmallamounts.
L-SugarsarelessnumerousandlessabundantinnaturethanaretheDformsbutneverthelesshaveimportantbiochemicalroles.TwoL-sugarsfoundinfoodsareL-arabinoseandL-galactose,bothofwhichoccurasunitsincarbohydratepolymers(polysaccharides).
Figure3RosanoffstructureoftheD-aldosescontainingfromthreetosixcarbonatoms.
Intheothertypeofmonosaccharide,thecarbonylfunctionisaketonegroup.Thesesugarsaretermedketoses.(Ket-signifiestheketonegroup.)Thesuffixdesignatingaketoseinsystematiccarbohydratenomenclatureis-ulose(Table1).D-Fructoseistheprimeexampleofthissugargroup.Itisoneofthetwomonosaccharideunitsofthedisaccharidesucroseandmakesupabout55%ofhigh-fructosecornsyrupandabout40%ofhoney.D-Fructosehasonlythreechiralcarbonatoms,C-3,C-4.andC-5.Thus,thereareonly23or8D-ketohexoses.D-Fructoseistheprincipalcommercialketoseandtileonlyonefoundfreeinnaturalfoods,but,likeD-glucose,onlyinsmallamounts.
3.2MonosaccharideReactions
Allcarbohydratemoleculeshavehydroxylgroupsavailableforreaction.Simplemonosaccharideandmostotherlow-molecular-weightcarbohydratemoleculesalsohavecarhonylgroupsavailableforreaction.Formationofpyranoseandfuranoserings(cyclichemiacetals)andofglycosides(acetals)ofmonosaccharideshasalreadybeenpresented.
3.2.1OxidationtoAldonicAcidsandAldonolactones
Aldosesarereadilyoxidizedtoaldonicacids.Thereactioniscommonlyusedforquantitativedeterminationofsugars.OneoftheearliestmethodsforquantitativemeasurementofsugarsemployedFehlingsolution.Fehlingsolutionisanalkalinesolutionofcopper(II)thatoxidizesanaldosetoanaldonateandintheprocessisreducedtocopper(I),whichprecipitatesasbrick-redCu2O.Variations,theNelson-SomogyiandBenedictreagents,atestillusedfordeterminingamountsofreducingsugarsinfoodsandotherbiologicalmaterials.
Because,intheprocessofoxidizingthealdehydegroupofanaldosetothesaltofacarboxylicacidgroup,theoxidizingagentisreduced,aldosesarecalledreducingsugars.Ketosesarealsotermedreducingsugarsbecause,underthealkalinecondi6onsoftheFehlingtest,ketosesareisomerizedtoaldoses.Benedictreagent,whichisnotalkaline,willreactwithaldosesbutnotwithketoses.
AsimpleandspecificmethodforquantitativeoxidationofD-glucosetoDgluconicacidusestheenzymeglucoseoxidase,withtheinitialproductbeingthe1,5-lactone(anintra-molecularester)oftheacid(Fig.4).Thereactioniscommonlyemployedtomeasuretheamountof~glucoseinfoodsandotherbiologicalmaterials,includingtileD-glucoselevelofblood.D-Gluconicacidisanaturalconstituentoffruitjuicesandhoney.
Figure4OxidationofD-glucosecatalyzedbyglucoseoxidase.
ThereactiongiveninFigure4isalsousedforthemanuf
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