CHEM1239 Carbohydrate Lecture Notes PDF

CARBOHYDRATES Carbohydrates are natural products that perform many vital functions in both plants and animals. Through photosynthesis, plants convert carbon dioxide to carbohydrates such as starch, cellulose and various sugars. Cellulose is the major structural component of plants. Starch is the chi...

CARBOHYDRATES Carbohydrates are natural products that perform many vital functions in both plants and animals. Through photosynthesis, plants convert carbon dioxide to carbohydrates such as starch, cellulose and various sugars. Cellulose is the major structural component of plants. Starch is the chief form of stored carbohydrates for later use as food or energy. Other carbohydrates are important components of coenzymes, antibiotics, cartilage and bacterial cell walls. Carbohydrates are called saccharides or, if they are relatively small, sugars. Several classifications of carbohydrates have proven useful: Complexity Simple monosaccharides Complex disaccharides, oligosaccharides & polysaccharides Size Triose C–3 sugars Tetrose C–4 sugars Pentose C–5 sugars Hexose C–6 sugars Heptose C–7 sugars etc. C=O Function Aldose sugars having an aldehyde functional group or an acetal equivalent Ketose sugars having a ketone functional group or an acetal equivalent Reactivity Reducing sugars oxidized by Tollens' reagent: Ag[NH3]2+ (or Benedict's or Fehling's reagents) Non-reducing sugars not oxidized by Tollens' or other mild oxidising reagents The name carbohydrate has been used because the formula of many compounds of this type can be expressed as hydrates of carbon: Cn(H2O)m. eg. glucose C6H12O6 A better definition would be polyhydroxyaldehydes or polyhydroxyketones (or substances that give such compounds on hydrolysis). Carbohydrates are classified as monosaccharides, oligosaccharides and polysaccharides. "Sacc- haride" comes from the Latin for sugar, because of the sweet taste of simple carbohydrates. Polysaccharides can be hydrolysed into oligosaccharides and then to monosaccharides. The human body has a number of enzymes to facilitate this process. Examples of polysaccharides include starch and cellulose, which both consist of glucose units linked together. Oligosaccharides (Gk. oligos = few) contain 2 to 8 or 10 monosaccharides. Disaccharides contain two units, trisaccharides contain three, etc. Maltose is a disaccharide consisting of two glucose units. Sucrose (table sugar) consists of one unit of glucose, and one of fructose. MONOSACCHARIDES These are classified according to the number of carbons present (triose, tetrose, pentose, hexose, etc.) and according to whether the carbonyl group is present as an aldehyde (aldose) or a ketone (ketose). There are only two trioses – each has two hydroxyl groups and one carbonyl group: CHIRALITY IN MONOSACCHARIDES Glyceraldehyde has one chiral carbon atom, and hence can exist in one of two enantiomeric forms: For carbohydrates, glyceraldehyde is used as a reference for describing their structure. (+)–Glyceraldehyde is dextrorotatory, and is designated D. All monosaccharides with the conformation around the chiral carbon atom farthest from the aldehyde or ketone group the same as in D–(+)–glyceraldehyde are D– monosaccharides. Hence, the symbols D and L are not used to designate the sign of optical rotation, but to show the structural similarity between a compound and D–glyceraldehyde. Hence, D and L refer to the stereochemistry at the highest numbered chiral carbon. Glyceraldehyde is the simplest D-aldose. Each time a carbon is added, two new sugars are created – each new carbon creates a new stereocentre (chiral carbon), hence two new isomers are created. EPIMERS Pairs of monosaccharides which differ only in the configuration at one carbon are called epimers. For example, D–ribulose and D–xylulose are epimers, as they differ only in their orientation at C–3. L- SUGARS L-Sugars are the mirror images of the corresponding D sugars - the conformation at all chiral carbons is reversed. CYCLIC HEMIACETAL STRUCTURES OF MONOSACCHARIDES While it is possible for monosaccharides to exist as straight–chain compounds, they are usually found in the cyclic hemiacetal form, where the carbonyl carbon is converted to one with an ether group and a hydroxyl. A hemiacetal is formed by the reaction of an alcohol and an aldehyde or ketone, giving a carbon with one ether group and one hydroxyl group. For aldohexoses, the hydroxyl group of the C–5 carbon can react with the aldehyde carbon. This is an equilibrium reaction, heavily favouring the ring form. CONVENTIONS FOR W RITING MONOSACCHARIDE STRUCTURES Fischer structures can be readily adapted to show cyclic structures. Haworth formulas use planar hexagons to represent the cyclic structures. The ring oxygen is shown on the top right position. The methylol (CH2OH) group is up for D sugars, and down for L sugars. With Haworth structures, for D sugars the –CH2OH (methylol) group, and all groups on C-2, C-3 and C-4 on the left of the vertical line of a Fischer structure are “up”, and all groups on the right are “down”. The ring is numbered clockwise, starting from the carbon adjacent to the oxygen. Hydrogen atoms are not usually shown. For L sugars, the methylol group is “down”. To convert a Haworth structure to a chair structure, all the “up” groups lie above the equatorial plane, and all the “down” groups lie below. The –CH2OH group is generally equatorial. MONOSACCHARIDE ANOMERS The stereochemistry at carbons 2, 3, 4 and 5 is fixed in a given aldohexose. However, when the C–5 hydroxyl group of a monosaccharide reacts with the carbonyl carbon of such a straight chain sugar, there are two possible conformations of the newly formed hydroxyl group: α – with the hydroxyl group in the opposite orientation to the –CH2OH group ("down"); β – with the hydroxyl group in the same orientation to the –CH2OH group ("up"). The β form of D-glucose is the most stable of all the aldohexoses, because the CH2OH group and the four OH groups are all in the equatorial position. These two diasteriomers are called anomers. The carbonyl carbon which reacts to form the hemiacetal is the anomeric carbon. The two separate forms of the hemiacetal can be isolated from solution, and have different chemical and physical properties. For example, α–D–glucose has a specific rotation of +112˚, while the β form has [α] = +19˚. An aqueous solution of D–glucose, however, has a specific rotation of 52˚, which suggests that both forms, α and β , are present in equilibrium. Further, a freshly prepared solution of α–D–glucose has a specific rotation of 112˚, but this will decrease to an equilibrium value of 52˚. For β –D–glucose, the specific rotation will increase from an initial value of 19˚ to the same equilibrium value of 52˚. Hence there must be interconversion between the α and β isomers in solution – a process known as mutarotation. It is possible to calculate the relative amounts of each isomer in solution. If we assume that there is 100% of one form initially, then at equilibrium there will be X% of the α– form and [100 – X]% of the β – form. The equilibrium specific rotation will be the sum of the products of the fractions of the two forms by their specific rotations. Assuming X% of the α form, there will be 100 – X% of the β form, hence: 100 × [α]equil = X × [α]α + [100-X] × [α]β So for glucose: 100 × 52 = X × 112 + [100-X] × 19 So X, the fraction of α–D–glucose in solution is 36% and hence the β - form is 64%. PYRANOSES AND FURANOSES Aldohexoses always form six–membered rings on cyclisation, and these are called pyranoses, because of their relationship to pyran. Hence, the correct name for α–D–glucose is α–D– glucopyranose. Ketohexoses can form both five–membered rings – furanoses – and six–membered rings. Five membered rings arise where the C–5 hydroxyl forms a bond with the carbonyl carbon: These two cyclic forms are also hemiketals – analogous to the hemiacetals formed by aldohexoses. Six membered rings arise when a bond is formed between the C–6 hydroxyl and the carbonyl carbon: In all cases, the cyclic forms can be α– or β –, due to the different conformations possible at C–2. PENTOSES Most pentoses are found as furanoses. β –D–Ribose (β –D–ribofuranose) is an important component of ribonucleic acid. DEOXY SUGARS Deoxy sugars are those in which one or more hydroxyl groups have been replaced by a hydrogen. β – 2–deoxy–D–ribose is an example, with replacement of the C–2 hydroxyl. AMINO SUGARS These contain an amino group in place of a hydroxyl group. N–acetyl sugars have an acetyl group attached to the amine group. N-Acetylglucosamine (N-acetyl- D-glucosamine, or GlcNAc, or NAG) and N-acetylmuramic acid, or MurNAc (the ether of lactic acid and N-acetylglucosamine) are found in bacterial cell walls. REACTIONS OF MONOSACCHARIDES Oxidation: Mild Although aldoses exist primarily in the cyclic hemiacetal form, there is a small equilibrium concentration of the straight–chain aldehyde. These aldehyde groups can be readily oxidised to give the carboxylic acid. Mild oxidising agents are sufficient for the conversion of aldoses to aldonic acids. Common oxidising agents of this type include: Bromine Water Tollens Reagent – Br2 / H2O  2 Br Ag+ (as Ag[NH3]2+)  Ag ↓ red colourless colourless silver solution solution solution mirror Benedict’s or Fehling’s Reagent Cu2+  CuI2O blue brick-red solution precipitate In these reactions, the sugar is oxidised, and the reagent is reduced. Carbohydrates that reduce Cu(II) to Cu(I) or Ag(I) to Ag metal are classified as reducing sugars. Those that do not reduce these reagents are called non–reducing sugars. Reducing sugars contain either an aldehyde group or an α–hydroxyketone. Under the basic conditions of the reaction with the oxidising agents, α–hydroxyketones are in equilibrium with the aldehyde form: Oxidation: Strong Stronger oxidising agents, such as aqueous nitric acid, convert both the aldehyde group and the terminal (primary) alcohol group to carboxylate groups. Diacids formed from hexoses are called aldaric acids. Reduction The carbonyl group of a monosaccharide can be reduced to an alcohol using various reducing agents, including NaBH4, and hydrogen in the presence of a catalyst. FORMATION OF OSAZONES As aldehydes, aldoses react with phenylhydrazine (Ph–NH–NH2) to form phenylhydrazones: With excess phenylhydrazine, the reaction proceeds to produce an osazone. Osazones are crystalline derivatives of monosaccharides, with sharp melting points, and are often used for identification of unknown sugars. Osazone formation destroys the stereochemistry at C–2 of the monosaccharide. Thus, compounds which differ only at C–2 will give the same osazone. As well, C-2 ketoses give osazones. Thus, D–glucose, D–mannose and D–fructose all give the same osazone, as the conformations at C– 3, C–4 and C–5 are all the same. FORMATION OF GLYCOSIDES (ACETALS) In the hemiacetal form, monosaccharides react with an equal amount of an alcohol to form acetals (carbons with two ether groups). Only the anomeric –OH group is replaced by the –OR group. Such acetals are called glycosides. Glycosides are stable in both water and aqueous base, but are hydrolysed in aqueous acid to give the monosaccharide and the alcohol: Glycosides are stable in aqueous solution or base. Since they are not hydrolysed to the open–chain aldehyde by the base in Tollens' or Fehling's reagent, glycosides are non–reducing sugars. Glycosides are widespread in nature, and many biologically important molecules contain glycoside linkages. Glycosides can also undergo enzymatic hydrolysis using emulsin and maltase. Emulsin hydrolyses β linkages only. Maltase hydrolyses only α linkages. DISACCHARIDES Carbohydrates which contain two monosaccharide units are called disaccharides. Disaccharides are formed from monosaccharides: the two units are joined by a glycoside bond between the anomeric carbon of one unit and an –OH group of the other. Maltose Maltose is found in germinating grain, corn syrup and can also be obtained by the partial hydrolysis of starch. Maltose consists of two units of D–glucose joined by a glycoside bond between carbon 1 (the anomeric carbon) of one glucose, and carbon 4 of another. The oxygen atom of C–1 is α, so the bond joining the two units as called an α–1,4–glycoside bond. Maltose is a reducing sugar. The anomeric carbon on one unit remains as the hemiacetal form, in equilibrium with the aldehyde form. This can be oxidised to the carboxylic acid, with the corresponding reduction of the reagent. Maltose can be hydrolysed by the enzyme maltase, which hydrolyses α–glycosidic linkages. Cellobiose Cellobiose is a disaccharide obtained by the par- tial hydrolysis of cellulose. It is identical with maltose, except that the two glucose units are joined by a β –1,4– linkage. Cellobiose can be hydrolysed by the enzyme emulsin, which hydro- lyses β –1,4–glycoside bonds. Lactose Lactose (milk sugar) occurs in the milk of mammals. Hydrolysis of lactose yields one unit of D–galactose and one of D–glucose. The C–1 of the galactose is joined to C–4 of the glucose by a β –1,4– link. The glucose unit still has a free hemiacetal system, so lactose mutarotates, and is a reducing sugar. Sucrose Sucrose (table sugar) is obtained from sugar cane, and consists of a glucose and a fructose unit, joined by an α–1,2–glycoside bond. C–1 of the glucose unit is joined to C–2 of the fructose. Sucrose has no free anomeric carbons (no hemi- acetal or hemiketal groups) and is hence a non– reducing sugar. When sucrose is hydrolysed, a 50:50 mixture of glucose and fructose is formed, called invert sugar. The name arises because sucrose has a specific rotation of +66.5˚, whilst the hydrolysed product has [α] = –19.9˚. POLYSACCHARIDES Starch Starch is the reserve carbohydrate for plants, and can be separated into two main polysaccharides – amylose and amylopectin. Both are polymers of α–glucose. Amylose consists of continuous unbranched chains of up to 4000 D–glucose monomers joined by α–1,4–glycoside bonds. Amylopectin has a highly-branched structure with 24 to 30 monomer units, joined by α–1,4– and α– 1,6– glycoside bonds. Glycogen Glycogen is the reserve carbohydrate for animals. Glycogen consists of highly branched chains of D–glucose joined by α–1,4– and α–1,6– glucoside bonds, as in amylopectin, but has a lower molecular weight, and is more highly branched. In the bodies of animals, starches are enzymatically hydrolysed to glucose, using the enzyme amylase. Glucose molecules pass through the intestinal walls into the bloodstream and are carried to tissues that need energy, such as muscle cells. Excess glucose is transported to the liver, where it is polymerised, and stored as glycogen. During exercise, or whenever glucose is consumed rapidly, stored glycogen is hydrolysed to maintain an adequate blood sugar level. Once glycogen reserves are exhausted, the body begins to oxidise fat as an energy source. Cellulose Cellulose makes up almost half the cell wall material of wood. Cotton is almost pure cellulose. Cellulose is a linear polymer of D–glucose monomers joined by β –1,4–glycoside bonds. It has an average molecular weight of 400 000, corresponding to approximately 2 800 glucose units. Humans, and most other animals, cannot use cellulose as a food because their digestive system does not contain β –glycosidases – enzymes that catalyse hydrolysis of β –glucoside bonds. Their systems contain only α–glycosidases, and hence can only use starch and glycogen as energy sources. Hydrogen bonding between adjacent cellulose chains is responsible for the high strength of cellulose-based materials, such as wood and cotton. Modified Sugars Modified sugars, typically deoxy sugars and/or amino sugars, are found in a range of natural and synthetic compounds. Kanosamine The Gram-positive bacterium Bacillus cereus UW85 produces two antibiotics that contribute to its ability to suppress certain fungal plant diseases. One of these is kanosamine, 3-amino-3-deoxy-D-glucose. Bacillus cereus UW85 Neuraminic acid Neuraminic acid (5-amino-3,5-dideoxy-D-glycero-D-galacto-non-2-ulosonic acid) is a nine-carbon mono- saccharide, an amino derivative of a ketononose (nine-carbon keto sugar). Influenza viruses have an enzyme – neuraminadase – which cleaves the glycosidic linkages of neuraminic acids as part of the process allowing the viral particle to enter the host cell. There are many variants of the enzyme. The N1 form is found in swine flu, hence the name H1N1. Glucuronic acid In drug metabolism, modified sugars such as glucuronic acid are added to some molecules to increase their solubility, and hence the rate of their excretion: Aldgamycin E Aldgamycin E is a neutral, macrolide (large-ringed) antibiotic isolated from Streptomyces lavendulae. It has two modified sugars: 6-deoxy-2,3-di-O-methyl-β-D-allose and aldgarose – derived from L-arabinose, with a spiro carbonate ring attached. Daunomycin Daunomycin (Daunorubicin) is chemotherapeutic of the anthracycline family that is given as a treatment for some types of cancer. It is most commonly used to treat specific types of leukaemia (acute myeloid leukaemia and acute lymphocytic leukaemia). It was initially isolated from Streptomyces peucetius. Isoquinocycline A An antibiotic from Streptomyces aureofaciens.