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172 Cards in this Set
- Front
- Back
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Carbohydrates
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aldehydes & ketones containing # hydroxy groups on unbranched C chain & chem derivatives
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Most common sugars have mlclr formulas that fit
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hydrate of carbon pattern - Cn(H2O)m
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sucrose (table sugar)
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C12(H2O)11 or C12H22O11
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glucose & fructose
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prevalent in honey, C6(H2O)6 or C6H12O6
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Carbohydrates as abundant organic cmpds
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in polymerized form as cellulose, account for 50-80% of dry weight of plants, major source of food sucrose (table sugar) & lactose (milk sugar) shells of arthropods such as lobsters
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aldose
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carb w aldehyde carbonyl group
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ketose
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carb w ketone carbonyl group
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hexose
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6C carb
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pentose
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5 C carb
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pentulose
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5C ketose
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monosaccharides
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cannot be converted into simpler carbs by hydrolysis i.e. glucose, fructose
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disaccharide
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sucrose - can be converted by hydrolysis into two monosaccharides
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trisaccharides
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can be hydrolyzed to 3 monosaccharides
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oligosaccharides
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to a few monosaccharides
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polysaccharides
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to a large number of monosaccharides
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Are carbs soluble in water?
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Yes, many hydroxy groups
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Fischer projections
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almost all carbs are chiral and most have 1+ asymm C - many carbs have several contiguous asymm C in unbranched chain
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Fischer rules
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projection based on eclipsed mlclr conformation, bonds connecting asymm C arranged in vertical line, asymm C located @ intersections of vertical & horizontal bonds (not drawn explicitly) vertical bonds to asymm C recede, horizontal bonds emerge toward observer
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do fischer projections convey info about mlclr conformations?
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no - only absolute configuration of each asymm C
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can a fischer projection be turned 180?
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Yes, in the plane of the paper
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Can a Fischer projection be turned 90?
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No
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Can a Fischer projection be lifted from the plane of the paper and turned over?
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No
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Can the 3 groups at either end of a Fischer projection be interhanged in a cyclic permutation?
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Yes, all three groups can be moved at the same time in a closed loop so that each occupies an adjacent position
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An interchange of any 2 groups bound to an asymm C changes
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the configuration of that C
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it is easy to recognize enantiomers & meso cmpds from Fischer projections bc
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planes of symmetry in actual mlcs reduce to lines of symmetry in their projections
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If the group of lowest priority is in either of the two vertical positions
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apply R,S priority rules to remaining three groups
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If lowest priority group is in a horizontal position
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reverse the assignment
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Each diastereomer is
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a diff carb w diff properties known by a diff name
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fam of aldoses
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each monosaccharide has an
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enantiomer
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how to apply d,L system
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configuration of naturally occurring triose (+) glyceraldehyde is designated as D, enantiomer is L,
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aldoses or ketoses written in Fischer projection w C in straight vertical line & C numbered consecutively as would be in systematic nomenclature so that
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carbonyl C receives lowest possible #
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Asymm C of ___ # is designated as a reference C
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highest - if this C has the H, OH, CH2OH groups in same relative configuration as same three groups of D-glyceraldehyde, carb has D configuration
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Is there a general correspondence btwn configuration & sign of optical rotation?
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no
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difference btwn R,S & D,L system
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R,S used to specify configuration of each asymm C atom in a mlc D,L specifies particular enantiomer of a mlc that may contain many asymm C
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important aldoses to know
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D-Glucose, D-mannose, D-galactose
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epimers
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diastereomers that differ in configuration at only one of several asymm C
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important aldopentose
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D-Ribose
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D-Fructose
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important naturally occurring ketose
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y or delta-hydroxy aldehydes exist predominantly as
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cyclic hemiacetals
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aldoses and ketoses exist primarily as
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cyclic hemiacetals
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in many carb both 5 & 6 membered cyclic hemiacetals are possible depending on
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which hydroxy group undergoes cyclization
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furanose
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5 membered cyclic acetal form of a carb
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pyranose
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6-membered cyclic acetal form of a carb
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aldohexoses & aldopentoses exist predominantly as
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pyranoses in aq. soln, but furanose forms of some carbs are important
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to name a cyclic hemiacetal form of a carb
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start w prefix derived from name of carb followed by a suffix that dictates the type of hemiacetal ring
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the furanose or pyranose form of a carb has
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one or more asymm C than the open chain form - C1 in case of aldoses
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how many possible diastereomers of D-glucopyranose?
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two
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anomers
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two cyclic forms of a carb differ in configuration only at their hemiacetal C aka cyclic forms of carbs that are epimeric at the hemiacetal C
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anomeric carbon
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hemiacetal carbon (C-1 of an aldose)
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anomers are named with
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greek letters a & b
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in the a-anomer the hemiacetal OH group
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is on the same side of the Fischer projection as the O at the configurational C
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the b-anomer, the hemiacetal OH group
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is on the side of the Fischer projection opposite the O at the configurational C
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when a carb ring is drawn w the anomeric C on the right & ring O in the rear
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substituents on left in Fischer projection are up in Haworth projection or chair structures, groups on right are down in Haworth projection or chair structures
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Altho 5 membered rings of furanoses are nonplanar, they are close enough to planarity that
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Haworth projections are good approximations to their actual structures
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Haworth projection of B-D-ribofuranose
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when pure a-D-glucopyranose is dissolved in water its specific rotation is found to be
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+112 degrees mL g-1 dm-1
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with time the specific rotation of the soln
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dec, reaches stable +52.7 degrees
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When pure B-D-glucopyranose is dissolved in water it has a specific rotation of
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+18.7 degrees
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mutarotation
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change of optical rotation w time
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Mutarotation occurs when
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pure anomers of other carbs are dissolved in aq soln
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Mutarotation of glucose is caused by
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conversion of a- and b-glucopyranose anomers into an equil mix of both, formed from either pure a-D-glucopyranose or B-D-glucopyranose
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Mutarotation is cat by
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acid & base, but also occurs slowly in pure water
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mutarotation is characteristic of
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cyclic hemiacetal forms of glucose
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aldehyde cannot undergo mutarotation bc
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an aldehyde C is not an asymm C
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Mutarotation was one of the phenomena that suggested that aldoses might exist as
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cyclic hemiacetals
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Mutarotation occurs by
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opening of the pyranose ring to the free aldehyde form, the reverse of hemiacetal formation
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180 rotation about the C-C bond to the carbonyl group permits
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reclosure of the hemiacetal ring by the rxn of the hydroxy group @ the opposite face of the carbonyl C
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mutarotation of glucose is due to
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interconversion of 2 pyranose forms
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other carbs undergo
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more complex mutarotations
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the structures of the cyclic hemiacetal forms of D-fructose can be derived from
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its carbonyl (ketone) form
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the crystalline form of D-fructose
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B-D-fructopyranose
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When crystals of B-D-fructopyranose are dissolved in water
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it equilibrates to both pyranose & furanose forms
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Does glucose in soln contain furanose forms?
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Yes but in very small amts ~ 0.2% each
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A single hexose can exist in at least 5 forms
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acyclic aldehyde or ketone form, a & b-pyranose forms, a & B- furanose forms
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most aldohexoses & aldopentoses exist primarily as
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pyranoses, altho few have substantial amts of furanose forms
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most monosaccharides contain relatively
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small amts of their noncyclic carbonyl forms
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mixtures of a & B-anomers
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are usually found, but exact amts of each vary from case to case
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fraction of any form in soln @ equil
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determined by its stability relative to that of all other forms
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Lobry de Bruyn-Alberda van Ekenstein rxn - in base, aldoses & ketoses rapidly equilibrate to
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mixtures of other aldoses & ketoses
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altho glucose in soln exists mostly in its cyclic hemiacetal forms, it is also in equilibrium w
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a small amt of its acyclic aldehyde form
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This aldehyde like other carbonyl cmpds w a-hydrogens
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ionizes to give small amts of its enolate ion in base
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Protonation of this enolate ion @ one face of the db gives back ___
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glucose, protonation @ the other face gives mannose
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enediol
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enoalte ion protonated on oxygen to give a new enol
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enediol contains a
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hydroxy group @ each end of the db
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enediol derived from glucose is simultaneously
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the enol of not only the aldoses glucose & mannose but also the ketose fructose
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conversion of D-glucose-6-phosphate into D-fructose-6-phosphate occurs
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in the breakdown of D-glucose (glycolysis), series of rxns by which D-glucose is utilized as a food source
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Bc biochem rxns occur near pH 7
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too little hydroxide ion is present to cat rxn
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Instead rxn cat by an enzyme
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D-glucose-6-phosphate isomerase & involves enediol intermediate
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conversion of D-glucose derived from corn into D-fructose is
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enzyme-cat process, involves similar rxns - central to commercial prod of high-fructose corn syrup, widely used sweetener
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most monosaccharides react w alcohols under acidic conditions to yield
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cyclic acetals
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glycosides
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special types of acetals in which one of the O of the acetal group is the ring O of the pyranose or furanose
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contrast rxn of a cyclic hemiacetal (i.e. glucopyranose) w corresponding rxn of an ordinary aldehyde under same conditions
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glycoside one -OR group is incorporated, formation of aldehyde acetal 2 -OR groups are incorporated
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glycosides are named
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as derivatives of the parent carb
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pyranoside
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indicates that the glycoside ring is 6-membered
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furanoside
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5-membered ring
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Glycoside formation
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like acetal formation, is cat by acid, involves a-alkoxy carbocation intermediate
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like other acetals, glycosides are
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stable to base, but in dilute aq acid, hydrolyzed back to parent carb
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many cmpds exist naturally as
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glycosides
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glycoside formation plays an important role in
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removal of some chem from the body - carb is joined to an OH group of substance to be removed, added carb group makes substance more soluble in h2o, hence more easily excreted
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like simple methyl glycosides, glycoside of a natural prod can be
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hydrolyzed to its component alcohol or phenol & carb
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disaccharides
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2 monosaccharides connected by a glycosidic linkage
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(+)-Lactose
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example of a disaccharide, present in about 4.5% cows milk, 6-7% human milk.. D-glucopyranose mlc linked by O @ C-4 to C-1 of D-galactopyranose, so effectively glycoside in which galactose is the carb & glucose is the alcohol
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glycosidic linkage is an acetal & acetals
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hydrolyze under acidic conditions
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disaccharide structural basis
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carb that can be hydrolyzed to 2 monosaccharides
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Hydrolysis occurs at the
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glycosidic bond btwn the 2 monosaccharide residues
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stereochem of glycosidic bond in (+) lactose
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B
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B stereochem
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stereochem of O linking 2 monosaccharide residues in the glycosidic bond corresponds to B-anomer of D-galactopyranose
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higher animals possess an enzyme, B-galactosidase
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cat hydrolysis of B-glycosidic linkage near neutral pH
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hydrolysis allows lactose to act as
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source of glucose, a-glycosides of galactose inert to action of this enzyme
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bc C-1 of the galactose residue in (+)-lactose is involved in a glycosidic linkage
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it cannot be oxidized
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C-1 of the glucose residue is part of
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a hemiacetal group which is in equil w free aldehyde & can undergo characteristic aldehyde rxns
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reducing sugars
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carbs such as (+)-lactose that can be oxidized bc they reduce oxidizing agents
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glucose residue said to be
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at the reducing end of the disaccharide
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galactose residue is at the
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nonreducing end
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bc of its hemiacetal group (+) lactose
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also undergoes many other rxns of aldose hemiacetals such as mutarotation
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(+) sucrose
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table sugar, another important disaccharide
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sucrose consists of
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D-glucopyranose residue & a D-fructofuranose residue connected by glycosidic bonds @ anomeric C of both monosaccharides
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glycosidic bond in (+) sucrose is diff from one in lactose
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only one of the residues of lactose (galactose residue) contains an acetal (glycosidic) C, both residues of +sucrose have an acegtal C
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glycosidic bond in + sucrose bridges
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C2 of fructofuranose residue & C1 of glycopyranose residue
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carbonyl C become
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the acetal or hemiacetal C in the cyclic forms
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neither the fructose nor the glucose part of sucrose
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has a free hemiacetal group, so +sucrose cannot be oxidized by bromine water nor undergo mutarotation
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nonreducing sugars
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carbs such as +sucrose that cannot be oxidized by bromine water
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sucrose is hydrolyzed by
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aq. acid or by enzymes (invertases) to an equilmolar mixture of D-glucose & D-fructose, sometimes called invert sugar
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as hydrolysis of sucrose proceeds
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positive rotation of soln changes to a neg rotation characteristic of the glucose-fructose mixture
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Rotation is negative because the strongly negative rotation
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of fructose has a greater magnitude than the positive rotation of glucose (dextrose)
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Fructose
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sweetest of the common sugars, accounts for intense sweetness of honey
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polysaccharides
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any # of monosaccharide residues can be linked together w glycosidic bonds to form chains
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cellulose
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principal structural component of plants, most abundant organic cmpd on earth
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cotton
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pure cellulose
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wood
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cellulose combined w polymer called lignin
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5 x 10^14 kg of cellulose is
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biosynthesized & degraded annually on the earth
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cellulose is a regular polymer
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of D-glucopyranose units connected by B-1,4-glycosidic linkages
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polysaccharides can be hydrolyzed?
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yes
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mammals lack the enzymes that cat hydrolysis of B-glycosidic linkages of cellulose
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why humans cannot digest grasses but cattle can (bacteria in their rumens provide appropriate enzymes that break down plant cellulose to glucose)
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uses of processed celulose
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spun into fibers (rayon) or made into wraps (cellophane)
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nitration of cellulose hydroxy groups
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gives nitrocellulose, a powerful explosive
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cellulose acetate
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hydroxy groups of cellulose are esterified w acetic acid, known by trade names Celanese, Arnel, so on, used in knitting yarn & decorative household articles
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cellullose as an alternative E source
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biomass is largely cellulose & cellulose is polymerized glucose
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glucose derived from hydrolysis of cellulose
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can be fermented to ethanol, which can be used as a fuel (as in gasohol) & plants obtain E to manufacture cellulose from sun
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important research: how to convert abundant sources of cellulose such as wild grasses
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into glucose w/o expending large amt of E - soln would reduce or elim need to use cultivated crops such as corn, a source of ethanol
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starch
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like cellulose, a polymer of glucose, mixture of 2 diff types of glucose polymer
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amylose
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glucose units are connected by a-1,4-glycosidic linkages
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conceptually chem diff btwn amylose & cellulose
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stereochem of glycosidic bond
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amylopectin
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branched polysaccharide, contains relatively short chains of glucose units in a-1,4-linkages & branches that involve a-1,6-glycosidic linkages
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starch
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important storage polysaccharide in corn, potatoes & other starchy veggies
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Humans have enzymes that cat hydrolysis of
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a-glycosidic bonds in starch, can use starch as source of glucose
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Chitin
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polysaccharide that occurs widely in nature - notably in shells of arthropods (lobsters, crabs)
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Chitin is a polymer of
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N-acetyl-D-glucosamine
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Residues of chitin are connected by
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B-1,4-glycosidic linkages within chitin polymer
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N-Acetyl-D-glucosamine is liberated when
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chitin is hydrolyzed in aq. acid
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Stronger acid brings abotu hydrolysis
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of the amide bond to give D-glucosamine hydrochloride & acetic acid
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amino sugars
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glucosamine & N-acetylglucosamine - a number occur in nature
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Amino sugars linked to proteins (glycoproteins)
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are found at the outer surfaces of cell membranes, some responsible for blood-group specificity
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polysaccharides
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mostly long chains w some branches - no highly cross-linked, 3D networks - some cyclic oligosaccharides known
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linkages btwn monosaccharide units
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in every case glycosidic linkages so monosaccharides can be liberated from all polysaccharides by acid hydrolysis
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A given polysaccharide contains
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only one stereochem type of glycoside linkage, so glycoside linkages in cellulose are all B, those in starch are all alpha
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