PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 47Slide 48Slide 49Slide 50HMonosaccharidesGlycosidic bondsPolysaccharidesO6-1MCB 450Lecture 6Carbohydrates• Most abundant biomolecules on• Oxidation of carbohydrates iscentral energy-yielding pathwayin most non-photosynthetic cells• Roles of carbohydrate polymers….• structural (in plants, insects, fungi, bacteria, archaea)• storage (starch, glycogen)• protective (lubrication of joints)• cell-cell recognition and adhesion6-2Polysaccharides• chains > ~ 20 to 1000s of monosaccharides in length• linear: e.g. cellulose (glucose)n• branched: e.g. glycogen & starch (glucose)n• depending on the sugar residues in a polysaccharide & thelinkages between them, polysaccharides can havevery different biological rolesClasses of saccharides6-3Monosaccharides= single polyhydroxy alcohol or ketone unit, cyclized, (e.g. 6-carbon glucose, most abundant in nature)Oligosaccharides= short chain of 2- ~20 monosaccharides joined by “glycosidic bonds”(e.g. disaccharide sucrose = glucose-fructose)• oligosaccharides > 3 residues are often joined to protein or lipidin “glycoconjugates”• Have unbranched carbon chains in which all C atoms are linked by single bonds• In the linear (“open chain”) form, one of the Cs is double-bonded to O (= carbonyl),each of the other Cs has an -OH: H2O-soluble• If the C=O is at:the end of a carbon chain (i.e. in an aldehyde), monosaccharide = aldose at any other position (i.e. in a ketone group), monosaccharide = ketoseMonosaccharides-16-4Simplest monosaccharideshave 3 carbons...Common monosaccharides have 5 and 6 carbons... Monosaccharides-26-5Monosaccharides have asymmetric centersStereoisomers of glyceraldehyde: • All monosaccharides (except dihydroxyacetone) haveone or more asymmetric carbons• In glyceraldehyde, middle C = chiral center,so molecule has 2 different optical isomers or enantiomers (= stereoisomers that are non-superimposable mirror images of one another)• By convention, one enantiomer = the D isomer, the other, L6-6Ways of representing stereoisomersHORIZONTAL:PROJECTS OUTFROM PLANEVERTICAL:PROJECTSBEHIND PLANE6-7• Stereoisomers of monosaccharides > 3 C divided into 2 groups: differ in the configuration about the chiral center most distant from the carbonyl C:-OH GROUP ON LEFT,CONFIGURATION = L-OH GROUP ON RIGHT,CONFIGURATION = DMost of the hexoses in living organisms are D-isomers123456MIRRORD and L configurations of monosaccharides6-8HIGHEST NUMBEREDASYMMETRIC CARBON,MOST DISTANT FROMCARBONYL CARBON• In general, a molecule with n chiral centers can have 2n stereoisomersIN ALDOHEXOSES, C-2,C-3, C-4, AND C-5= CHIRAL CENTERS,SO 24 = 16 POSSIBLEALDOHEXOSES:8 D AND 8 L123456MIRRORD and L configurations of monosaccharides6-9HOW MANY INALDOPENTOSES?D or L?_ glucose(aldose)_ fructose(ketose)6-10D or L?_ erythrulose(ketose)_ arabinose(aldose)12345416-11Series of D-aldoses6-12Epimers= 2 sugars that differ only in the configuration around one carbon6-13• Have 1 chiral center less than aldoses• C-4 and C-5 ketoses designated by adding “ul” into the name of their corresponding aldose, e.g. D-ribulose = ketopentose corresponding to D-riboseSeries of D-ketoses6-146-15In aqueous solution, monosaccharides with ≥ 5 C occur mainly ascyclic (ring) structures in which the carbonyl group has formed acovalent bond with the O of an -OH group along the chainIntramolecular cyclization of pentoses & hexoses-1Rotation aboutC-C & C-O bondspermits cyclizationCCOHCHOC OHC OHCH2OHO HC OHHOHCOCCCOHOHCH2OHOHHCOCOHHOH2C CC COHOHReaction between C-1 aldehyde group and an OH at C-5forms intramolecular HEMIACETAL....Intramolecular cyclization of pentoses & hexoses-26-16ALDEHYDEat C-1ALCOHOLat C-5COHC:OCHH+CYCLICHEMIACETALC*OHOHCCH+and the aldehyde Cbecomes anew chiral centercalled theanomeric carbonReaction between C-2 keto group and an OH at C-5forms intramolecular HEMIKETAL.....Intramolecular cyclization of pentoses & hexoses-36-17KETONEat C-2ALCOHOLat C-5COR2C:OR1HH+C*OR2COR1H+HHEMIKETALand the keto C becomesa new chiral centercalled theanomeric carbonHAWORTH PERSPECTIVE(HEAVY LINE PROJECTS OUTWARD6-18..6-196-20 Fructose can also form 6-membered ringsPyranose rings formed if C-2 keto reacts w. C-6 OHOHOH6-21C OHHOHCOCCCOHOHCH2OHCCHOCCCCH2OHOHOHOHHO-D-GlcupdowndownC OHOCCCCOHOHCH2OHOH l6-22 How to draw the pyranose form of an D-aldohexoseCCHOCCCCH2OHOH- on right = downOH- on left = upCH2OH on top = D-sugarCH2OH on bottom = L-sugaranomeric C:= OH = OH123456Final position in Haworth:OCH2OHOCH2OHL-Glcconfig unspecifiedInterconversion between anomers• and anomers interconvert in solution via the linear form: = mutarotation• D-glucose solution forms an equilibrium mixture of ~ 62 , 38% 6-23THIS FORM HAS CHEMICAL PROPERTIESOF AN ALDEHYDEConformations of pyranoses• Pyranose ring is not planar, as suggested by Haworth perspective,(because of tetrahedral geometry of saturated C atoms) but can adopt two classes of conformations…..6-24EQUATORIAL: APPROX or AXIAL: or Substituents on thering carbons havetwo orientations:Chair form of -D-Glcpredominates becauseall axial positions areoccupied by H:Disfavored because ofsteric hindranceStereoisomers: Same structural formula, but different spatial configurationsEnantiomers: Stereoisomers (about chiral center) that are non-superimposablemirror images of each other e.g. fructose: 3 chiral centers, D and L enantiomersDefinitions-16-25D-monosaccharides predominate in nature(just as L-amino acids predominate)Diastereoisomers:Stereoisomers that have opposite configurations at one or morechiral centers, but which are not mirror images of each otherEpimers: Diastereoisomers that differ only in the stereochemistry at only oneof their chiral carbons (e.g. at C-2 for Glc and Man) D-Glucose/D-Talose = diastereoisomers D-Glucose/D-Mannose = epimersAnomers: Isomeric forms of monosaccharides that differ only in theirconfiguration about the hemiacetal or hemiketal (= ring-formingcarbon). The hemiacetal or carbonyl carbon = anomeric carbonexample: and
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