Carbohydrates Summary 2023-2024 PDF

What are they? Carbohydrates are polyhydroxy aldehydes or ketones. “Carbohydrate”= carbon and water Saccharide is another name for a carbohydrate. Classifications Monosaccharides contain a single unit. ◦ Glucose, fructose Disaccharides consist of two monosaccharide units. ◦ Sucrose Oligosacch...

What are they? Carbohydrates are polyhydroxy aldehydes or ketones. “Carbohydrate”= carbon and water Saccharide is another name for a carbohydrate. Classifications Monosaccharides contain a single unit. ◦ Glucose, fructose Disaccharides consist of two monosaccharide units. ◦ Sucrose Oligosaccharides contain from 3 to 10 units. ◦ Raffinose Polysaccharides contain very long chains of hundreds or thousands of monosaccharide units. ◦ Cellulose, glycogen, starch Monosaccharides Monosaccharides are the simplest carbohydrates. They contain two or more hydroxyl groups. Their basic chemical formula is (CH2O)n. They either contain aldehyde or ketone groups. ◦ They are called aldoses or ketoses.  Numbering carbons Carbons in a monosaccharide are numbered such that the aldehyde group is carbon number one or the ketone group is carbon number two. The carbonyl group must be “up”  Drawing carbohydrates Fischer projections are a good way to represent mirror images in two dimensions. Fischer projections  Monosaccharides classification They can be classified by the number of carbon atoms they contain. The smallest monosaccharides contain three carbons (n = 3). ◦ They are referred to as trioses. There are two trioses: dihydroxyacetone and glyceraldehyde. ◦ Dihydroxyacetone is a ketose, whereas glyceraldehyde is an aldose. Four, five, six, and seven carbon atoms are called tetroses, pentoses, hexoses, and heptoses, respectively. Glyceraldehyde Glyceraldehyde has a single asymmetric carbon. ◦ An asymmetric carbon is one attached to four different chemical groups and is also known as a chiral carbon A chiral molecule, like in glyceraldehyde, is one that cannot be superimposed on its mirror image. Isomerism Isomers Constitutional Stereoisomers isomers Enantiomers L-isomers D-isomers Stereoisomers The two forms of glyceraldehydes are known as stereoisomers (or geometric isomers). ◦ Stereoisomers are molecules that have the same molecular formula and atomic connectivity but have a different arrangement of the atoms in space. Number of possible stereoisomers How many stereoisomers can a monosaccharide molecule have? The number of possible stereoisomers is 2n where n is the number of chiral carbons in a sugar molecule. Isomers of glucose Enantiomers When two stereoisomers are mirror images of each other, they are known as enantiomers. Designation They are designated with the prefix d- and l- indicating the position of the –OH group in relation to the carbon being right or left (“L” means left and “D” means right). So, the two isomers are D-glyceraldehyde and L-glyceraldehyde. The sugars in our bodies are D-sugars. Naming D- or L When there is more than one chiral center in a carbohydrate, look at the chiral carbon farthest from the carbonyl group. If the hydroxy group points to right, it is the D-isomer, and when the hydroxy group points to the left, it is the L-isomer. Dextrose The form most commonly found in living organisms, D-glucose, medically known as dextrose, has only one mirror image (look for it). Cyclic sugars In the cell 99% of pentoses and hexoses are in the ring form because they more stable. The ring forms when the aldehyde or ketone reacts with one of the hydroxyl groups. The aldoses, ribose and glucose, form pyranose ring structures. Cyclic fructose Similarly, fructose mainly forms a furanose when oxygen of the keto carbon (C2) reacts with the C-5 hydroxyl group. Ring structures Two ring structures can form: pyranose and furanose rings depending on the carbons involved in the bond formation.  Anomers  For an aldohexose such as glucose, two ring pyransoe structures can be formed: - glucopyranose and -glucopyranose. α means that the hydroxyl group attached to C-1 is below the ring; β means that it is above the ring. C1 is known as the anomeric carbon. The α and β forms are called anomers. These two forms are in equilibrium where they can inter-convert by mutarotation.  Other aldohexoses Ribose: an aldopentose