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Highlights of Carbohydrates I
1. Simple carbohydrates are monosaccharides, also called sugars. These include glucose, galactose, and mannose.
2. The suffix '-ose' is used to designate saccharides. The number-related prefixes 'tri', 'tetr', 'pent', 'hex', 'hept', and 'oct' are used to designate saccharides with 3,4,5,6,7, and 8 carbons, respectively.
3. Monosaccharides with an aldehyde group are called aldoses. Those is a ketone group are called ketoses.
4. Glyceraldehyde and dihydroxyacetone are the simplest saccharides we call carbohydrates.
5. Carbon can have as many as four different molecular groups attached to it. If this happens, the carbon is chiral (or asymmetric) and the groups can be arranged in two different ways. These arrangements are called stereoisomers.
6. The letters 'D' and 'L' are an older nomenclature system used to designate whether a particular stereoisomer rotated polarized light rightwards or leftwared, respectively. We use the convention today that the 'D' isomer corresponds to the stereoisomer in which the lowest asymmetric carbon from the top (closest to the bottom) is written on the right side of the molecule.
7. Most biological sugars are in the D configuration.
8. Stereoisomers, such as D-glyceraldehyde and L-glyceraldehyde, are mirror images of each other.
9. When sugars differ in stereoisomeric configuration, they are called diastereomers. When stereoisomers are mirror images of each other, they are called enantiomers. When they differ in configuration of only one carbon, they are called epimers. When they differ only in the configuration of the anomeric carbon, they are called anomers.
10. Cyclization of monosaccharides leads typically to five member or six member rings. These are called furanoses (five member rings) and pyranoses (six member rings). Cyclization arises from fromation of hemiacetals in aldoses and hemiketals in ketoses.
11. Cyclization creates a new asymmetric carbon. This carbon is called the anomeric carbon and it can exist in the alpha (down position) or beta (up position) configurations. If the hydroxyl group on the anomeric carbon is unaltered, the ring and linear forms of the sugar can reversibly form. Thus, a sugar in the beta configuration can, in solution, 'flip' to the alpha form by going to the linear form and then reverting back to the ring structure in the alpha configuration. If the hydroxyl of the anomeric carbon is altered (by methylation, for example), the linear structure cannot form and 'flipping' cannot occur.
12. Altering the hydroxyl group on the anomeric carbon results in creation of a glycoside. Glycosides are commonly created during formation of disaccharides and longer carbohydrates.
13. A given sugar such as beta-D-glucose can have different conformations that have different shapes. These are referred to as 'boat' and 'chair' forms. Chair forms are generally favored over boat forms due to less steric hindrance.
14.Linking together of more than one sugar residues creates higher order saccharides. These include disaccharides (two sugars), trisaccharides (three sugars), oligosaccharides (several sugars), and polysaccharides (many sugars).
15. Most of the linkages in higher order saccharides involve glycosidic bonds.
16. Disaccharides include sucrose (glucose + fructose), lactose (glucose + galactose), and maltose (two glucoses). Linkages of these are shown in the figures in the notes.
17. Sucrose is a non-reducing sugar (has no free anomeric hydroxyl), whereas lactose is a reducing sugar (has a single free anomeric hydroxyl). The sucrose figure in your book shows a confusing structure. HERE is a better one. You are responsible for the structure of sucrose and the linkages for the disaccharides I described in class.