Lecture notes on Carbohydrates

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Chemistry 108 Chapter 12 Lecture Notes Carbohydrates Chapter 12 Lecture Notes: Carbohydrates Educational Goals 1. Given a Fischer projection of a monosaccharide, classify it as either aldoses or ketoses. 2. Given a Fischer projection of a monosaccharide, classify it by the number of carbons it contains. 3. Given a Fischer projection of a monosaccharide, identify it as a D-sugar or L-sugar. 4. Given a Fischer projection of a monosaccharide, identify chiral carbons and determine the number of stereoisomers that are possible. 5. Identify four common types of monosaccharide derivatives. 6. Predict the products when a monosaccharide reacts with a reducing agent or with Benedict’s reagent. 7. Define the term anomer and explain the difference between α and β anomers. 8. Understand and describe mutarotation. 9. Given its Haworth projection, identify a monosaccharide either a pyranose or a furanose. 10. Identify the anomeric carbon in Haworth structures. 11. Compare and contrast monosaccharides, disaccharides, oligosaccharides, and polysaccharides. 12. Given the structure of an oligosaccharide or polysaccharide, identify the glycosidic bond(s) and characterize the glycosidic linkage by the bonding pattern for example: β(1⟶4). 13. Given the Haworth structures of two monosaccharides, be able to draw the disaccharide that is formed when they are connected by a glycosidic bond. 14. Understand the difference between homopolysaccharides and heteropolysaccharides. 15. Compare and contrast the two components of starch. 16. Compare and contrast amylopectin and glycogen. 17. Identify acetal and hemiacetal bonding patterns in carbohydrates. 1 Chemistry 108 Chapter 12 Lecture Notes Carbohydrates Introduction to Carbohydrates Carbohydrates are also known as_______________________. Carbohydrates are an abundant biomolecule. • More than 50% of the carbon in organic compounds is found in carbohydrates • Plants use photosynthesis to store energy in__________________, a simple sugar 6 CO + 6 H O + Energy à C H O + 6 O 2 2 6 12 6 2 Carbohydrates are a large class of naturally occurring polyhydroxy ___________________and ________________________. • ________________________(also known as simple sugars) are the simplest carbohydrates containing 3-7 carbon atoms. A sugar containing: – an aldehyde is known as an ______________________ – a ketone is known as a _________________________ Classification of Carbohydrates Carbohydrates are grouped into 3 classes: • ___________________________ are the simplest sugars and serve as the building blocks of larger molecules – Example: Glucose • __________________________ contain 2-10 monosaccharides bonded together (building block = residue) – Example: Sucrose • _____________________________contain more than 10 residues – Example: Complex Carbohydrates 2 Chemistry 108 Chapter 12 Lecture Notes Carbohydrates Monosaccharides Monosaccharides are _____________________________ ketones or aldehydes with 3 or more carbons. Naming Monosaccharides Carbohydrate nomenclature is unique to “sugar chemistry” — we do not name monosaccharides using the IUPAC rules. • Monosaccharide names end in “ose." • Monosaccharides can be classified by: • Carbonyl group: ______________________ or _______________________ • ______________________ of carbons: triose, tetrose, etc. • Both: aldotriose, ketotriose, and so on… 3 Chemistry 108 Chapter 12 Lecture Notes Carbohydrates Examples: You try it: Name each of the following monosaccharides as an aldose or ketose & according to its number of C atoms. _________________________ ______________________________ _____________________ 4 Chemistry 108 Chapter 12 Lecture Notes Carbohydrates Stereoisomers in Carbohydrates Carbohydrates are ________________________molecules since they have carbon atoms carrying four different groups. The simplest three-carbon sugar is glyceraldehyde. This sugar exists as a pair of ___________________. • Enantiomers have the same _______________ physical properties except they behave differently in the way they rotate polarized light and the way they are affected by catalysts. n Remember: Compounds with n chiral carbon atoms has a maximum of 2 possible stereoisomers and half that many pairs of enantiomers (mirror images). 2 • The aldotetrose (below), has 2 chiral carbon atoms and a total of 2 = 4 possible stereoisomers (2 pairs of enantiomers). 5 Chemistry 108 Chapter 12 Lecture Notes Carbohydrates The D and L Families of Sugars: Drawing Sugar Molecules Fischer Projections represent three-dimensional structures of stereoisomers on a flat page. • A chiral carbon atom is represented in the Fisher projection as the ___________________of two crossed lines. • Bonds that point __________________ the page are shown as horizontal lines. • Bonds that curve _________________ _______ ______________the page are shown as vertical lines. • In a Fischer projection, the aldehyde or ketone carbonyl group of a monosaccharide is always placed toward the top of the page. 6 Chemistry 108 Chapter 12 Lecture Notes Carbohydrates Example: Glucose Monosaccharides are divided into _________________ families: • D form and L form sugars. • D: the –OH group on the chiral C furthest from the C=O comes out of the plane of paper and points to the_______________________. • L: the –OH group on the chiral C furthest from the C=O comes out of the plane of paper and points to the ______________________. 7 Chemistry 108 Chapter 12 Lecture Notes Carbohydrates Monosaccharides We will briefly survey some important pentoses and hexoses, and their derivatives. D-glucose, also called ___________________or_________________ ___________________________, is the most important monosaccharide in human metabolism. D-fructose, or fruit sugar, is most common natural ketose • Honey is 40% fructose Monosaccharide Derivatives In ________________ __________ a hydrogen atom replaces one or more of the -OH groups in a monosaccharide. • D-ribose and its derivative D-2-deoxyribose (deoxy = minus one oxygen atom) are found in various coenzymes and in DNA. 8 Chemistry 108 Chapter 12 Lecture Notes Carbohydrates Monosaccharide Derivatives (continued) In __________________ _______________an -OH group of a monosaccharide has been replaced by an amino (-NH ) group. 2 • D-glucosamine is an example. • D-glucosamine is an amino derivative in which an amino group replaces one hydroxyl group In __________ ________________the carbonyl group of a monosaccharide has been reduced to an alcohol group. o Sorbitol is an example. Sorbitol and Xylitol are used as sweeteners Ribitol is found in the coenzyme FAD 9 Chemistry 108 Chapter 12 Lecture Notes Carbohydrates Monosaccharide Derivatives (continued) • In ______________ ______ sugars, an aldehyde group of a monosaccharide has been oxidized to form a carboxyl group D-Gluconic Acid is an example. Reactions of Monosaccharides Reactions of monosaccharides are reactions of carbonyl and hydroxyl groups (chapter 11). • Aldehyde and ketone groups can be reduced • Aldehyde and alcohol groups can be oxidized Reduction of Monosaccharides • The reduction of the C=O group in an aldehyde or ketone produces _____________ _______________. Oxidation of Monosaccharides The oxidation of the aldehyde C=O group produces __________________________ _______________ _________________ . 10 Chemistry 108 Chapter 12 Lecture Notes Carbohydrates Oxidation of Monosaccharides (continued) Benedict’s reagent is a copper compound that will oxidize only _________________ ________________ (aldoses) and not alcohols. A sugar that reacts with Benedict’s solution is called a reducing sugar since it reduces 2+ + the ion Cu à Cu NOTE: Some ketoses give positive results for Benedict’s test because they ______________________ to aldehydes in the strongly basic Benedict solution. 11 Chemistry 108 Chapter 12 Lecture Notes Carbohydrates Monosaccharides: Their Cyclic Form Review: A hydroxyl group in a monosaccharide can react with the carbonyl to form a _________ _____________ • Hemiacetals are made by the reaction of an _________________with an ___________________. A hemiacetal contains a C atom bonded to an ______________ and an ________________group. A monosaccharide contains both an alcohol and an aldehyde group. • It can react with _____________ to form a cyclic hemiacetal. 12 Chemistry 108 Chapter 12 Lecture Notes Carbohydrates Open Chain to Cyclic Form Mechanism (MECHANISM NOT ON EXAM) Cyclic forms of monosaccharides are usually drawn with the Haworth Projection in which the ring is viewed from the side at an angle. • The edge of the ring closest to the viewer is drawn with a bold line for perspective. • Substituents on the ring in a Haworth projection are either “up” or “down” The pair of cyclic hemiacetals with the OH on the hemiacetal carbon in different positions are called __________________. • For D-sugars, the α-anomer has the OH pointing ________________ • For D-sugars, the β-anomer has the OH pointing ________________ 13 Chemistry 108 Chapter 12 Lecture Notes Carbohydrates • Example: The open-chain form of D-galactose with its cyclic anomers. In solution, the open chain and cyclic forms of a monosaccharide are in______________________: • If we start with a pure open chain or cyclic form in solution, the optical rotation of the solution will change until equilibrium is achieved and the concentrations of the different forms remain constant. • The change in optical rotation observed as the system approaches equilibrium is called ________________________. The cyclic forms of monosaccharides can be named as derivatives of the heterocyclic ethers furan (5 members) and pyran (6 members). 14 Chemistry 108 Chapter 12 Lecture Notes Carbohydrates Example: The aldopentose D-ribose forms a cyclic _____________ (the deoxy form is also shown below) 15 Chemistry 108 Chapter 12 Lecture Notes Carbohydrates Oligosaccharides Oligosaccharides are short polymers containing 2-10 monosaccharide _________________________. The residues are bonded to each other by ___________________ _____________. A glycosidic bond is the ether linkage formed when an acetal is made by reacting a __________________ of a monosaccharide with a __________________on another sugar. 16 Chemistry 108 Chapter 12 Lecture Notes Carbohydrates The glycosidic bond in maltose is referred to as an __________________ bond since the monosaccharide on the left reacts it’s α-anomer hemiacetal at C-1 with a hydroxyl at C-4 on the second monosaccharide. Formation of α and β anomers The glycosidic bond can be either ________ or_________. – Example: Cellobiose is a disaccharide formed when the polysaccharide cellulose is broken down. – A disaccharide is composed of two monosaccharide residues. – Cellobiose is made by connecting two glucose molecule by a β(1à4) glycosidic bond. – Cellobiose cannot be used as a source of glucose by humans since we lack the enzyme to hydrolyze the glycosidic bond. 17 Chemistry 108 Chapter 12 Lecture Notes Carbohydrates Example: Lactose is a disaccharide found in milk • Lactose consists of a galactose connected to a glucose residue by a β(1à4) glycosidic bond • Sounds like cellobiose But the OH on C-4 is up in galactose and down in glucose Lactose intolerance is the inability to hydrolyze lactose due to an enzyme deficiency. Sucrose, or table sugar, is a disaccharide with two twists: ___________________ _______________ and___________ ____________________________. • The glycosidic bond in sucrose is formed between the hemiacetal C of α-glucopyranose and the hemiacetal C of β-fructofuranose • This is an α,β-(1↔2) glycosidic bond 18 Chemistry 108 Chapter 12 Lecture Notes Carbohydrates Note that, as is the case with monosaccharides, the oligosaccharides can be in equilibrium with their anomers Only the end, hemiacetal residue can open and close 19 Chemistry 108 Chapter 12 Lecture Notes Carbohydrates Most common oligosaccharides are disaccharides The following are found in peas and beans Raffinose (a trisaccharide composed of three monosaccharide residues) Stachyose (a tetrasaccharide composed of four monosaccharide residues) Verbascose (a pentasaccharide composed of four monosaccharide residues) These oligosaccharides are indigestible since they contain galactopyranose residues involved in α-(1à6) glycosidic bonds that humans lack the enzyme to hydrolyze. _____________________ are sugar-containing lipids that: • Are present in nerve cell membranes. • Serve as identifying markers on cell surfaces. The hemiacetal of a sugar residue is connected to alcohol group of a lipid by a ________________ bond. 20