carbohydrates

carbohydrates - SUMMARY Carbohydrates General Information...

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Unformatted text preview: SUMMARY . Carbohydrates General Information ' Carbohydrates 1 Molecular Representations of Carbohydrates Carbohydrates 2 Reactions of Carbohydrates ° Hemiacetal Formation Carbohydrates 3 0 Mutarotation Carbohydrates 4 0 Synthesis of Esters via Acid Anhydride Carbohydrates 5 0 Synthesis of Glycosides Carbohydrates 6 ' Synthesis of Ethers via Alkyl Halides (Permethylation) Carbohydrates 7 0 Reduction of Carbohydrates Carbohydrates 8 ' Oxidation of Carbohydrates Carbohydrates 9 I Kiliani-Fischer Synthesis Carbohydrates 10 0 Wohl Degradation Carbohydrates 11 GENERAL INFORMATION Carbohydrates 1 Carbohydrates [Cn(H20)nl Introduction: Carbohydrates, otherwise known as sugars, have the chemical formula Cn(H20)n. They are found widely throughout nature and are very important in biology. They form the structure of plants and give fruits their sweet taste. Additionally, they (glucose in particular) are used as the primary fuel in most of the tissues of the human body. Physical Properties: Increasing the carbohydrate concentration of an aqueous solution increases the viscosity of the liquid. Chemical Properties: I Carbohydrates are divided into two groups: Monosaccharides consist of one sugar molecule, while polysaccharides consist of multiple sugar molecules covalently linked to one another via glycosidie bonds. I Monosaccharides contain either an aldehyde or a ketone and have hydroxyl groups bonded to all the other carbon atoms. Most of these carbons are stereocenters. _ 0 The number of stereoisomers that can exist for a sugar molecule with 72 carbon stereocenters is equal to 2". Epimers are monosaccharides with the same number of carbons, but which differ in configuration at only one carbon stereocenter. ' In solution, a monosaccharide exists predominantly in ring form and transiently in linear form. In order to convert from linear to ring form, the sugar undergoes an intramolecular hemiacetal reaction (see Carbohydrates 3). - As a result of the intramolecular hemiacetal reaction, the carbonyl carbon is converted from 5132 to sp3 hybridization, thereby forming an additional carbon stereocenter known as the anomeric carbou. The OH group bonded to the anomeric carbon can be oriented so that it points above or below the plane of the ring. Sugars that differ in the orientation of this OH group are called anomers. 0 D-Sugars (defined below) predominate in nature. Nomenclature: 0 In general, carbohydrates have an -ose ending. 0 Monosaccharides can be classified according to (1) whether they contain an aldehyde or a ketone in the open form and (2) how many carbons are present. a. Sugars with an aldehyde are called aldoses and have the prefix aldo-, whereas those with a ketone are called ketoses and have the prefix keto-. b. The number of carbons in the sugar is indicated by tri— (three carbons), tetr- (four carbons), pent- (five carbons), bex- (six carbons), heptv (seven carbons), and so on; for example, six-carbon aldose— aldohexose, six-carbon ketose—ketohexose. ' A D sugar has an OH group on the right of the carbon stereocenter furthest from the carbonyl in a Fischer projection. (See Carbohydrates 2, Figure 1, for a Fischer projection of D-glucose. In this example, the car- bon stereocenter furthest from the carbonyl group is (2-5. Because the OH group attached to (3-5 is located to its right, this sugar molecule is D.) 0 An L sugar has an OH group on the left of the carbon stereocenter furthest from the carbonyl group in a Fischer projecrion. - In an or anomer, the OH group on the anomeric carbon is below the plane of the ring. I In a B anomer, the OH group on the anomeric carbon is above the plane of the ring. (See Carbohydrates 2, Figures 2 and 3, for examples of B-D-glucose. In both figures, C-l is the anomeric carbon.) Spectroscopy: Carbohydrates have complicated H NMR and IR patterns reflecting the presence of various functional groups. Important Carbohydrates in Biology 1. D-Ribose, which comprises part of the backbone of DNA and of the energy molecule ATP, is an aldopentose. ‘ 2. Biologically significant aldohexoses include D-glucose and D-galactose. Starch, glycogen (the storage form of glucose in humans), and cellulose (found in plant cell walls) are polymers of glucose. 3. D-Fructose, a component of table sugar, is a ketohexose isomer of glucose. MOLECULAR REPRESENTATIONS OF CARBOHYDRATES Carbohydrates 2 ' Monosaccharides can be depiCted in linear or ring form. Fischer projections represent the linear form (the first figure), whereas Haworth projections (the second figure) and chair conformations (the third figure) both represent the ring form of sugar molecules. Fischer projection Haworth projection Chair conformation 0 © © \\ CH2°H <3) CH20H 0—H G, to 0 OH HO H ® CH ® OH HO 6) H {3) OH H0 @ H @ <2: 0H (D 0H —D—Glucose H @ 0" BvD-Glucose B H OH (3} CHZOH C6) D-Glucose - Fischer projections are used to highlight epimers and whether a sugar molecule is an aldose or a ketose. They are written so that the carbonyl group is on top or close to the top. Carbons are numbered from top to bottom. The horizontal lines stand‘ for bonds above the plane of the paper {i.e., out toward you), and the vertical lines stand for bonds below the plane of the paper (i.e., away from you). Carbon stereocenters are represented as the intersections between the horizontal and vertical lines. 0 Haworth projections illustrate pyranose (i.e., six-membercd sugar rings that look like the cyclic ether pyran) and furanose (i.e., five-membered sugar rings that look like the cyclic ether furan) rings as if they were lying flat with the anomeric carbon on the right and the viewer looking down on them at an angle. They are artificial representations of sugar rings and highlight which OH groups are on the same or opposite sides of the ring. In order to convert from Fischer to Haworth projections (and vice versa), remember that OH groups on the right of the carbon in Fischer projections are below the plane of the ring in Haworth projec- tions. Conversely, OH groups on the left of the carbon in Fischer projections are above the plane of the ring in Haworth projections. The terminal —CH20H group is above the plane of the ring in D sugars and below the plane of the ring in I. sugars. O The chair conformation most closely represents an actual pyranose ring. The conformational rules of cyclohexane (see Alkanes 4 for more details) also apply to pyranose rings. Note that glucose is the only aldohexose with all groups equatorial in the chair conformation. REACTIONS OF CARBOHYDRATES Carbohydrates 3 HEMIACETAL FORMATION CHO H OH CH20H 0 Ho H “30* : OH T“; 0” H OH Ho H H OH 0H OH: OH B-D-Glucose (ring form) D-Glucose (linear form) Keys: 1. This is an intramolecular hemiacetal reaction (see Aldehydes/Ketones 8 for more details on hemiacetal formation). 2. This reaction is acid-catalyzed and reversible. 3. This reaction converts the sugar molecule from linear to ring form. 4. This reaction usually occurs to form a furanose {five-membered) or pyranose (six-membered) ring, since ring strain is minimal for these structures. Mechanism: 1. Protonation of the carbonyl oxygen generates a resonance-stabilized oxonium ion. 2. Nucleophilic attack on the carbonyl carbon by an OH group bonded to a stereocenter three or four carbons down the chain yields a five or six-membered hemiacetal ring, respectively. In the example, the nucleophilic attack is from the OH group bonded to carbon 4 to form a pytanose, a six-membered hemiacetal ring. 3. Abstraction of the proton occurs to yield the cyclic monosaccharide. :6 l O“ \\ H H \ C (EC — H H OH H OH Ho H 6) Ho H w; .7...— <—> H OH H OH H OH H OH CH; OH CH2 OH n-Glucose Resonancc‘slabilizcd oxonium ion imermediate \ q — H CHZOH A CHZDH C9 / H , 0 OH 1 0 OH ® 2 OH f - 10H H0 H HO H OH OH B—D-Glucose REACTIONS OF CARBOHYDRATES Carbohydrates 4 MUTAROTATION OF GLUCOSE CHon CHon O O OH H30+ (or OH‘) OH T———— —N‘ OH HO OH HO H OH OH Ot-D-GlLlCOSC B—D—Glucose Keys: 1. Mutarotation is an acid- 0r base—catalyzed reversible reaction that changes the orientation of the OH group on the anomeric carbon (i.e., conversion of [3 to or anomers, or vice versa). 2. This phenomenon involves the breakdown and subsequent re-formation of the cyclic hemiacetal and proceeds through a linear intermediate. 3. Mutarotation occurs only in aqueous solutions. 4. The B anomer is more stable than the oc anomer; for glucose the ratio of B to (I anomers in solution is approximately 64 : 36. Notes: . 1. The specific rotation of the U. anomer is +1122”, while that of the B anomer is +187". 2. When either anomer in pure form is placed in aqueous solution, the specific rotation changes and equilibrates at +52.6° (this specific rotation corresponds to :1 [VIII anomer ratio of 64:36). m Acid-catalyzed 1. 2. Jun.» Protonation of the oxygen in the fix: H H pyranose ring. crion / CHEUH cnzon DH 2—5 Dlssoc1at10n of the bond °= OH 3) 8* (“EH :2. OH in" on? [0" DH : DH : 0” c\ *—> u" 'Jc\ "O H H0 H H0 H MD H rolalion of between the protonated oxygen and the anomeric carbon, form- 0” OH on ing a resonance—stabilized “WWW oxonium ion. Rotation of the carbonyl group. Re-formation of the cyclic hemiacetal ring. Loss of the proton from the oxygen in the pyranose ring. B-[J-Glucme Resonanceestahilized oxomurn Ion CH20H o H 5. —“n on f no on HO OH a-D-Glucosc Base-catalyzed 1. 2. Abstraction of the proton from the anomeric OH group by a hydroxide ion (OH‘). Formation of the carbonyl bond and dissociation of the bond between the pyrannse oxygen and the anomeric carbon. Rotation of the carbonyl OH OH group. ‘ no on HO §L Nucleophilic attack on the “OH on [ ' I carbonyl carbon by the Q'DGIWDQ “6‘” negatively charged pyranose oxygen. Abstraction of a proton from water by the negatively charged oxygen bonded to the anomeric carbon and regeneration of the base. CHZOH CHIOH :4va Km :11: a 0 {—— DH & H0 N no H HO 0 H OH on on B-D-Giucme <2"on rotation of carbon hnnd CHZOH REACTIONS OF CARBOHYDRATES Carbohydrates 5 SYNTHESIS OF ESTERS VIA ACID ANHYDRIDE 0 H CH io~c—CH CH20H I? I? 2 :40 0 °” CH3COCCH1 on —[email protected]—* CHJCOO Na HO H OH B-D—Glucme Penta-O—aeelyl-B-D-glucose Keys: 1. In this reaction, every OH group in the monosaecharide eventually undergoes esterification. 2. Esterifieation proceeds via a nucleophilic acyl substitution mechanism (see Acid Anhydrides 2b for ester synthesis, and Acyl Derivatives 2 for details on nucleophilic aeyl substitution). 3. This reaction is carried out with the aid of a mild base (e.g., Na acetate or pyridine). Notes: 1. Unlike sugars, a-anomeric ester derivatives are more stable than B-anomeric ones. 2. Unlike sugars, monosaccharide ester derivatives are soluble in organic solvents. Mechanism: 1. Nucleophilic attack on one carbonyl group of acetic anhydride by an OH group of B-D-glucose results in the formation of a tetrahedral intermediate. 2. The oxyanion re-forms the carbonyl group with the loss of an acetate ion. 3. Acetate abstracts a proton from the oxygen of the alkoxy group. 4. Repetition of steps 1 through 3 with the remaining four OH groups results in the formation of penta-O- acetyl-B-Drglucose. 9"? ‘u’ 9-- Cfls—c—O—C—CHa Eff (I?) ll CHzOH CHZOH CH3 —c—O—C—CH3 CH20H t: —CH;. .. a / U / O '08 O o\ o Q?) 4.; H OH «x—— OH L OH H Ho H Ho H Ho H 0 c .. El OH OH OH rig—c —CH3 fliDiGlucose l? CHzoccn3 0 CHon 0 n ll OCCH; o DCCH3 @ (H) OH HSCCO o H Ho H u occm OH Pentaioiucelyl-B-D-gl ucose REACTIONS OF CARBOHYDRATES Carbohydrates 6 SYNTHESIS OF GLYCOSIDES CH20H o OCHZCHa OH + H20 «0% Ho H cnon chfi k 0 OH O 3 0" a Ethylifiiniglucoside OH (glycoside) HO H 05901,)08 CHZOH 0” 550* 0 H B-D-Glucose 0H + H20 HO OCHZCHS OH EthylAaiDiglucoside Keys: (glycoside) 1. This reaction is equivalent to the formation of an acetal (glycoside) from a hemiacetal (monosaccharide) (see Aldehydes/Ketones 8). Acetal formation occurs only at the anomeric carbon. A glycoside is a cyclic acetal formed from a reaction between a sugar and an alcohol. 2. This reaction is reversible and acid-catalyzed. 3. This reaction proceeds through an oxonium ion intermediate that is stabilized by resonance (see Mechanism). CHZOH CHon 4. Both 0:- and B-glycosides can be formed from a particular monosac- o H ‘3 charide substrate since the alcohol can attack the oxonium ion inter— OH Ho mediate from either above or below the plane of the ring (see Ho 0 cnzon Mechanism). OH OH 5. Unlike cyclic hemiacetal monosaccharides, glyeosides are stable and k—v—J ‘——v—J do not exhibit mutarotation. «70431110056 B-D-FruclOSe 6. The formation of glycosidic bonds (see figure) between two or more Glycosidiclmkage monosaccharides yields polysaccharides [e.g., glucose + fructose —> -___,,_._.a sucrose (table sugar)]. Therefore, this reaction is extremely important Sucmqe in biology. Notes: 1. Glycosides are named by replacing the terminal -e of the parent monosaccharide from which they are derived with the suffix -ide (e.g., polysaccharides synthesized from glucose are called glucosides). 2. Even though they are both composed of two bonded glucose units, maltose (a degradation product of starch and glycogen) and cellobiose (a degradation product of cellulose) have different glycosidic bonds and consequently distinct biological functions. A ten-Io" ]. A-B-Glycusidic hnnd no ‘ ’0 ll v ‘U/ I {CHIN No i. 1 CHIDH N H Maltose has a 1,4-0t—glycosidic bond (an (I linkage 5“ "° . H between anomerlc C-1 of one glucose molecule and 7 a H cunnhime , I. drueGchosidlc I on C-4 of another), whereas cellobiose has a 1,4-B- mu m. glycosidic bond ([3 linkage). Maliosc Mechanism: 1,14. 1. The hemiacetal OH group undergoes protonation. CM" 2"" cw. \ 2. Water is lost from the anomeric carbon and yields a resonance- 0 " 7 i: ’5) (3") stabilized oxonium ion intermediate. m, m > (N ‘ no a” H 3. Nucleophilic attack of the anomeric carbon above or below the cm 3:“ plane of the ring by an alcohol yields a protonated acetal. ' cum 4. Water abstracts the proton, thus forming a- and B-glycosides “*5 +_,% OH ,‘ on (acetals). no Ho 1 V on C‘IH , Oxonium ion intermediate a) \H % "i, omen "PH?" cute“ f‘ N . t:an CH;GH ——o .33 o H on on an N no 7 on an a“ Alkyl-IA-irglu comdc Alkylruruglucoaide REACTIONS OF CARBOHYDRATES Carbohydrates 7 S YNIHESIS OF ETHERS VIA ALKYL HALIDES (PERMETHYLATION) l? CH20H Cfli—O—fi*O’CH3 eI-tzocn-Ia 0 CH 0 0 OCH 0 a (dimethyl sulfate) 3 OH ——> OCH3 NaOH "0 H H300 H OH OCH3 Methylifiiniglueoside B-DuGluCOSE pentamethyl ether Keys: 1. In this reaction, every OH group in the glycoside is eventually converted to an ether. 2. This reaction is the same as the Williamson ether synthesis 8N2 reaction {see Ethers 2) and is therefore subject to similar restrictions. Yields are greatest when 1° or methyl sulfates or iodides are utilized. 3. A variant of this reaction uses silver oxide (AgZO) as a mild base because the stronger bases normally used in the Williamson ether synthesis reaction would degrade the monosaccharide. CHZOH CH206H3 0 I 0H 0 OCHa CH3] OH ‘w—> ocna Agzo HO H H360 H OH ocu3 . BiuiGlucose BiniGlucose pentarnethyl ether. Notes: 1. Under mild acidic conditions, only the anomeric 0CH3 undergoes hydrolysis to an OH group. CH20CH3 CHEOCHa O 0 OH OCH3 H10+ OCH; —'_’ OCH: H360 H H300 H OCH, OCH; B-D—Glueose pentamethyl other B-n-Glucose tetramethyl ether 2. Unlike sugars, monosacccharide ether derivatives are soluble in organic solvents. Mechanism: 1. A hydroxide ion abstracts a proton from a hydroxyl group on the glycoside. 2. Nucleophilic (5N2) attack by the negatively charged oxygen atom on one carbon atom of dimethyl sulfate results in the formation of a B-D-glucose methyl ether. It is important to note that only one of the two methyl groups of dimethyl sulfate can be used to methylate a hydroxyl group. 3. Repetition of steps 1 and 2 for the remaining three OH groups results in the formation of B-D-glucose pentamethyl ether. crion CHon CHZOH 0 ocn3 , 0 OCH: 0 OCH: (1.. = . 0H —"' on —"—> OH Ho H Ho H HO H 07H :93; fi Den3 U 3:6” - Kcmfoifiioicua - o MelhyI-B-D-glucosme cmocmz 0 arms OCHS Haco H ocn3 B-D-(ilucosc penlamelhyl elher REACTIONS OF CARBOHYDRATES REDUCTION OF CARBOHYDRATES 0H0 H HO H H OH; OH n-Glucose Keys: OH H OH OH NBBH4 or NifH; OH: OH H OH Ho H H OH H OH CH2 OH D-Glucitol (alditol) 1. In this reaction, the aldose (or ketose) is reduced to an alditol. 2. Reduction occurs only when the monosaccharide is in its linear form. 3. Reduction can be carried out using NaBH4 or catalytic hydrogenation with Ni/Hz. Note: Reduction of D-glucose yields D—glucitol (otherwise known as D-sorbitol). Carbohydrates 8 Mechanism: Opening of the pyranose ring by base 1. Deprotonation of the anomeric carbon OH group. 2. The oxygen atom donates a pair of electrons, thereby forming an aldehyde group and opening the ring. 3. The alkoxide ion is protonated by water. Reduction of the aldehyde using NaBH4 (refer to Alcohols 5 for more details) 4. NaBH4, a hydride {H‘} equivalent, attacks the carbonyl carbon to form an alkoxide intermediate. 5. The negatively charged oxygen in the alkoxide intermediate abstracts a proton from water to yield the alditol. (—3:§H O \ CHZOH \h c"2°" . \c_“ O 0/ ° J : L/ IA L H OH on «\— 0“ “~— Ho H W H Ho H (SJ—H 0H 0" H OH H B—D-Glucosc H cu2 0H GK :9 :c‘):‘—..H H (a 1H from NaBH.4 cHon "—0—" 0y H OH H OH H 79? H H / H0 H 1 Ho H o i A H 0H 3 H 0” 4%? H OH H OH H OH H OH CH2 OH CH2 OH CH2 OH D-GluCilul REACTIONS OF CARBOHYDRATES OXIDATION OF CARBOHYDRATES Keys: HO HO CHO H H Br}. H20 OH H3O+ OH CH2 OH D-Mannose COOH H0 H H0 H H OH H OH CH2 0H DiMannonic acid (aldonic acid) Carbohydrates 9 1. This reaction oxidizes only the aldehyde group of an aldose in linear form to a carboxylic acid. Molecules that have one COOH group as a result of the oxidation of a monosaccharide are called aldonic acids. 2. Under slightly basic conditions, Benedict’s solution [Cu(II}SO4 in aqueous sodium citrate}, Fehling’s solution [Cu(II)SO4 in aqueous sodium tartrate], and Tollens’ solution {Ag+ in aqueous NH3] can also be utilized as oxidizing agents‘ Monosaccharides that, when in linear form, can reduce these reagents are called reducing sugars. ' a. Reaction with Benedict’s and Fehling’s solution: H0 H0 CH; OH DAManuosc OH OH + ZCu2+ + 50H‘ 4» c—§= (I; + 3H20 + Cuzo.(rcd OH OH CH2 OH DiMannonale prec 1p itate} b. Reaction with Tollens’ solution: 0 CI \\ \\ e CAH c—o HO H H0 H HO H HO H + l“9(NH3); #—> + "H; + Ag;(silver mirror) H OH H OH H OH H OH CH2 OH CH2 OH D—Mannosc DiMannonalc The primary advantage of using these three reagents over Brz is that ketoses can be oxidized as well. This phenomenon occurs because ketoses can undergo keto—enol tautomerization under basic conditions to form an aldose (see Carbonyl a—Substitution 2 for more information on keto-enol tautomerization). Because they result in a lower yield of aldonic acid production than with Brz, these reagents are used primarily in diagnostic tests for CHO COOH the presence of sugar. 3. Because it is a stronger oxidizing agent than Brz, nitric acid Ho H Ho H (HNO3) ox1dizes monosaccharldes to yield two COOH Ho H HNOa Ho H groups. Molecules that have two COOH groups as a result of the oxidation of a monosaccharide are called aldaric H OH H OH acids (see figure to the right}. . H OH H OH 4. Because of the presence of COOH and OH groups, aldomc CH20H COOH (or aldaric) acid can undergo an inframolecular esterifica- tion to form lactones (especially if a five- or six-membered D—Mannose " D-Mannaric acid (aldaric acid) ring is formed). Note: Aldonic (or aldaric) acids are named by replacing the terminal -ose of the parent monosaccharide from which they are derived with the suffix -onic (-aric) acid. (For example, the aldonic acid derived from mannose is called mannonic acid, and the aldaric acid derived from mannose is called mannaric acid.) REACTIONS OF CARBOHYDRATES Carbohydrates 10 KILIANI—FISCHER SYNTHESIS {1] G) (-3 CHO CHO CHO H ® 0H 1)HCN H Q OH HO ® H __—, H ‘3 OH 2>H2-Pdea304 H (3 OH + H @ 0H 6) 3)H30* A ? “CHZOH H 9“ OH H U 0H ©CH2 0H ®CH2 0H D-Erylhrose D—Ribose D-Arabinose m 1. This reaction increases the carbon chain length of the starting sugar by one. (3—1 in the starting monosac- charide substrate becomes C-Z in the product. 2. This reaction proceeds via many intermediates (cyanohydrin, imine, irninium ion, and carbinolamine). 3. Because this reaction is not stereospecific, two epimers at the new chiral carbon ((3-2) are formed. The mechanism is beyond the scope of this discussion In general, it involoves (1) forming a cyanohydrin, (2) reduction to an imine, and (3) hydrolysis of the imine to a new aldehyde group. REACTIONS OF CARBOHYDRATES Carbohydrates 11 WOHL DEGRADATION ®CHO H a)CHC) HO “HzNOH HO ® H a) e ——u—) + CH 0 O H ll ll 9 © H ® 0H 2} CH3COCCH3.CH3COO Na , H H ‘5’ OH 3]NaOH ®CH20H D—Glucose D-Arabinose Keys: 1. This reaction decreases the carbon chain length of the starting sugar by one. The carbonyl carbon (C-l) in the starting monosaccharide substrate is eliminated as CN’. 2. This reaction proceeds via several intermediates (e.g., oxime and cyanohydrin). 3. It is most useful in situations where the starting materials are pentoses and hexoses. Note: The yield from this reaction is relatively low. m Ui-ILDJNH Formation of an oxime (see Aldehydes/Ketones 5, Notes, item 2, for detailed mechanism). . Acetylation of all of the OH groups. . Dehydration of the oxime to a cyano group with heat yields an acetate ester of a cyanohydrin. . Removal of the esters by treatment with a base. . Elimination of the CN group yields a new monosaccharide with one less carbon than the parent monosaccharide. o \\ H on /°" row” 0 o | a “I _ \\ 5H3 \\ H-N—H \\ “‘1 @qq/G °::/ °‘“ ° " ' i? H OH H 0H 0H occHJ n H0 H (3 HO H ® CHaco H9 H OH H OH i.) I? H occm CH3COCCH3 ("3 H OH H OH H OOCCHg II CH2 OH CH2 OH CH2 OCCHa D-Glucose Oxin‘te (3 l l o c 39-“ c ll ] o "—0 0—H H other: H U o 3 a ll Ho H Ho H CH3CO Ho ® (4) e cug + H OH ‘ H OH ' e H ohm; OH 0 ll H OH H OH H cJocrm, H CH2 OH 0”: 0H CH2 OCCH; D—Arabinose Cyanohydt‘in Acetate ester of a cyanohydrin ...
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carbohydrates - SUMMARY Carbohydrates General Information...

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