Carbohydrates Carbohydrate: A compound with multiple hydroxy and/or carbonyl groups that has the general formula C x (H 2 O) y ; a hydrate of carbon. The suffix –ose often indicates a saccharide (sugar). Carbohydrates make-up more more than 50% of the Earth’s biomass. Carbohydrates are classified in several different ways: one way is by how many simple sugars it is comprised of. Monosaccharides Disaccharides Polysaccharides

Monosaccharide: The simplest carbohydrate with a general formula C n H 2n O n. Mono = one, saccharide = sugar Monosaccharides are also known as simple sugars. The most common monosaccharides have from 3 to 7 carbons with biological sugars being mostly 5 and 6 carbons. The number of stereocenters in a monosaccharide is equal to the number of carbons minus two. Aldose: a monosaccharide containing an aldehyde group. Ketose: a monosaccharide containing a ketone group. Monosaccharides

Monosaccharides are classified also by their number of carbon atoms.

Monosaccharides Glyceraldehyde (an aldotriose) contains one stereocenter and exists as a pair of enantiomers. The carbons of a monosaccharide are numbered with the carbonyl carbon first (or in a ketosugar, second or third).

Fischer Projection Formulas Fischer projection: a two-dimensional representation that shows the configuration of a tetrahedral stereocenter (asymmetric carbons) without using wedge and slashes. The carbon atoms in a sugar are arranged vertically with carbon 1 (the carbonyl carbon) at the top. Horizontal lines represent bonds projecting forward (wedges). Vertical lines represent bonds projecting to the rear (slashes). The first and last carbons in the chain are written with a “C”; others are indicated by the crossing of bonds.

D- and L-Monosaccharides In 1891, without having the actual structures, Emil Fischer guessed(!!) the stereochemistry of the two optically active enantiometers of glyceraldehyde. His guess became the basis of the convention by which we now distinguish the stereochemistry of monsaccharides.

According to the convention proposed by Fischer D -monosaccharide: A monosaccharide that, when written as a Fischer projection, has the -OH on its penultimate (second from last) carbon on the right. L -monosaccharide: A monosaccharide that, when written as a Fischer projection, has the -OH on its penultimate carbon on the left. For monosaccharides, this convention is used independent of its optical activity. That is, it possible that a D-monosaccharide is actually levorotatory (as is D-fructose).

Cyclic Structure of a Monosaccharide Monosaccharides have hydroxyl and carbonyl groups in the same molecule. In solution, these groups, within the same molecule, react in order to create five- and six- membered cyclic hemiacetals. Anomeric carbon: The hemiacetal carbon of a cyclic form of a monosaccharide. Anomers: Monosaccharides that differ in configuration only at their anomeric carbons. Anomers have different optical activities. The hemiacetal conformation of a monosaccharide is often illustrated with a Haworth representation.

Haworth Projections 5- and 6-membered hemiacetals are represented as planar pentagons or hexagons viewed through the edge. To begin creating Haworth projection, rotate a Fischer projection 90  clockwise. Then make to acetal linkage between the oxygen of the hydroxyl group on carbon 5 and the anomeric carbon (carbon 1), moving carbon 5 to the back of the plane. Hydroxyl groups on the right in a Fischer projection are on the bottom of Haworth projection.

Haworth Projections Note the acetal linkage created between the oxygen of the hydroxyl group of carbon 5 and the anomeric carbon (carbon 1).

Most commonly written with the anomeric carbon on the right and the hemiacetal oxygen to the back right. With the hydroxyl group reacting with the carbonyl group, two different anomers result.  - means that -OH on the anomeric carbon is cis to the terminal (carbon 6) -CH 2 OH (both pointing up)  - means it is trans. (-OH down, -CH 2 OH up) A 6-membered hemiacetal is shown by using the infix ‑ pyran-. A 5-membered hemiacetal is shown by using the infix ‑ furan-.

Cyclic Structures Aldopentoses also form cyclic hemiacetals. The most prevalent forms of D -ribose and other pentoses in the biological world are furanoses. The prefix deoxy- means “without oxygen”.

D -Fructose, a 2-ketohexose, also forms  and  cyclic hemiacetals. Five-membered rings are close to planar so that Haworth projections are adequate to represent furanoses.

The six-membered rings of pyranoses are more accurately represented as chair conformations.

Mutarotation Mutarotation: The change in specific rotation that occurs when the  or  forms of a carbohydrate are converted to an equilibrium mixture of the two.

Mutarotation of D-Glucopyranose

Glycosides Glycoside: A carbohydrate in which the -OH on its anomeric carbon is replaced by -OR. Glycosidic bond: The bond from the anomeric carbon of the glycoside to an -OR group.

Sucrose Table sugar (from sugar cane or sugar beets)  -D-glucopyranose +  -D-fructofuranose Glycosidic bond between carbon 1 (in  configuration) of glucose and carbon 2 of fructose. (  -1,2-glycosidic bond)

Lactose Milk sugar (human milk, 5–8% 4–5% cow's milk)  -D-galactopyranose + D-glucopyranose Glycosidic bond between carbon 1 (in  configuration) of galactose and carbon 4 of glucose. (  -1,4-glycosidic bond)

Maltose From malt, the juice of barley and other cereals.  -D-glucopyranose + D-glucopyranose Glycosidic bond between carbon 1 (in  configuration) of glucose and carbon 4 of another glucose. (  -1,4-glycosidic bond)

Reduction to Alditols The carbonyl group of a monosaccharide can be reduced to an hydroxyl group by a variety of reducing agents, including NaBH 4.

Name alditols by replacing the -ose of the name of the monosaccharide by -itol. Sorbitol is found in the plant world in many berries and in cherries, plums, pears, apples, and seaweed; it is about 60% as sweet as sugar. Other common alditols include

Oxidation to Aldonic Acids The –CHO group can be oxidized to –COOH. Reducing sugar: Any carbohydrate that reacts with an oxidizing agent to form an aldonic acid. Reducing sugars (many common monosaccharides) behave much differ biochemically than non-reducing sugars.

Tests for Reducing Sugars Several oxidizing agents can be used to determine if a sugar is a reducing sugar. Each oxidizing agent is a key reagent is several named tests. Benedict’s test uses Cu 2+ (blue) in alkaline solution that reduces to Cu 2 O (brick red). Fehling’s test uses Cu(C 4 H 4 O 6 ) 4- (blue) that reduces to Cu 2 O (brick red) Tollen’s test uses Ag+ that reduces to Ag metal. Remember Tollen’s test is used to identify aldehydes. Both Benedict’s test and Fehling’s test are used to detect glucose in the urine of a diabetic.

Polysaccharides Starch is used for energy storage in plants. It can be separated into two fractions; amylose and amylopectin Amylose is composed of unbranched chains of up to 4000 D ‑ glucose units joined by  ‑ 1,4-glycosidic bonds. Amylopectin is a highly branched polymer of D ‑ glucose; chains consist of units of D -glucose joined by  ‑ 1,4-glycosidic bonds and branches created by  ‑ 1,6-glycosidic bonds.

Amylopectin with both  -1,4 and  -1,6 glycosidic bonds between  -D-glucose units.

Glycogen is the reserve carbohydrate for animals. A nonlinear polymer of D -glucose units joined by  -1,4- and  -1,6-glycosidic bonds. The structure of glycogen is similar to amylopectin; however, glycogen is much more heavily branched (like dendridic polymer). The total amount of glycogen in the body of a well- nourished adult is about 350 g (about 3/4 of a pound) divided almost equally between liver and muscle.

Cellulose Cellulose is a linear polymer of D ‑ glucose units joined by  -1,4-glycosidic bonds. It has an average molecular weight of 400,000, corresponding to approximately 2800 D -glucose units per molecule. Both rayon and acetate rayon are made from chemically modified cellulose.