Chapter 17: Carbohydrates 17.1 ─ Classes of Carbohydrates 17.2 and 17.3 ─ Stereochemistry and Fischer Projections 17.4 and 17.6 ─ Monosaccharides 17.5 ─ Cyclic Structures and Chemical Properties of Monosaccharides 17.7 ─ Disaccharides 17.8 ─ Polysaccharides

17.1 Carbohydrates  Most abundant organic compounds in nature  Composed of the elements C (“carbo”), H and O (“hydrate”)  Also called saccharides, which means “sugars”  A major source of energy from our diet  A source of C for synthesis of other biomolecules  Can be linked to cell membranes or proteins

Carbohydrate Structure  Polyhydroxy aldehydes or ketones  Examples:

Types of Carbohydrates  Monosaccharides are the simplest carbohydrates  Empirical formula = CH 2 O  Contain one aldehyde or ketone unit (saccharide)  Disaccharides consist of two monosaccharides  Oligosaccharides contain a few monosaccharides (3-10)  Polysaccharides contain many monosaccharides

17.2 Chiral Molecules  Chiral molecules Have “handedness” Nonsuperimposable on mirror images Ex: hand, shoe  Achiral molecules Do not have handedness Ex: glass, spoon  Chiral molecules contain carbons with 4 different groups Called chiral carbons or chiral center or stereocenter

Chiral Molecules  Compounds can have more than one chiral carbon  Example: Glucose contains 6 carbons. How many of these carbons are chiral?

Chiral Molecules

17.3 Fischer Projections  Used to represent carbohydrates  Places the most oxidized group (carbonyl) at the top  All chiral carbons are represented as intersection of lines C is not shown  Implied 3D arrangement of atoms Horizontal lines for bonds that come forward Vertical lines for bonds that go back

D and L Notations  The —OH on the chiral atom farthest from the carbonyl group is used to assign a D or L configuration  By convention, the letter L is assigned to the structure with the —OH on the left  The letter D is assigned to the structure with —OH on the right

D and L Monosaccharides

D D L

Learning Check  Indicate whether each is the D or L isomer: Ribose Threose Fructose

17.4 Classification of Monosaccharides  Monosaccharide = unbranched chain of 3 or more carbon atoms one C is carbonyl, other C’s attached to -OH  Aldose = monosaccharide with an aldehyde group (1 st carbon)  Ketose = monosaccharide with a ketone group (2 nd carbon) Aldose Aldose Ketose CH 2 OH

Monosaccharides  Monosaccharides are also classified according to the number of carbon atoms  A triose has three carbons; a tetrose has four carbons; a pentose has five carbons; and a hexose has six carbons. triose tetrose hexose aldotriose aldotetrose ketohexose CH 2 OH

Identify each as triose, tetrose, pentose or hexose, and as aldose or ketose: Learning Check A B

17.6 Important Monosaccharides  Ribose and deoxyribose  Glucose  Galactose  Fructose

D-Ribose and D-Deoxyribose  Pentoses  Used in the synthesis of DNA (deoxyribose) and RNA (ribose)

D-Glucose  Most common hexose  Found in fruits, corn syrup, and honey  An aldohexose with the formula C 6 H 12 O 6  Known as blood sugar in the body  Building block for many disaccharides and polysaccharides

D-Galactose  Aldohexose C 6 H 12 O 6  Differ from D-glucose at C4 C1 is at the top  Not found in the free form in nature  Obtained from lactose, a disaccharide (milk products)  Important in cellular membranes in CNS (brain sugar)

D-Fructose  Ketohexose C 6 H 12 O 6  Differ from glucose at C1 and C2 (location of carbonyl)  The sweetest carbohydrate (73% sweeter than sucrose)  Found in fruit juices and honey  Formed from hydrolysis of sucrose  Converts to glucose in the body

Learning Check Draw the structure of D-fructose: