tài liệu carbohydrates (english)

Saccharides Saccharide is another name for a carbohydrate. Simple saccharides are the monosaccharides, commonly called sugars. Glucose is an example of a monosaccharide. Others are shown in Figure 11.2 and Figure 11.3. We use the terms monosaccharide, oligosaccharide, or polysaccharide to refer to compounds composed of a single sugar, several sugars linked together, or many sugars linked together, respectively. The term carbohydrate derives from the fact that many of them have a formula that can be simplified to (CH2O)n. Some of these compounds are chemically modified, however, and do not fit the formula due to the modification. Saccharides play a variety of roles in living organisms, including energy storage (monosaccharides and oligosaccharides), structural roles (polysaccharides), and cell identity (oligosaccharides). Monosaccharide Nomenclature Monosaccharides are the simplest sugars, having the formula (CH2O)n. The smallest molecules usually considered to be monosaccharides are those with n = 3. Monosaccharides can be categorized according to their value of n, as shown below: n Category 3 Triose 4 Tetrose 5 Pentose 6 Hexose 7 Heptose 8 Octose Monosaccharides can exist as aldehydes or ketones and are called aldoses or ketoses, respectively. For example, THIS shows the structures of glyceraldehyde, an aldotriose, and dihydroxyacetone, a ketotriose. Glyceraldehyde and dihydroxyacetone have the same atomic composition, but differ only in the position of the hydrogens and double bonds. Carbons in a monosaccharide are numbered such that the aldehyde group is carbon number one or the ketone group is carbon number two. The three dimensional arrangement of atoms around a carbon atom are such that if four different groups are attached to it, they can be arranged in two different ways. Such a carbon is described as chiral or asymmetric. The two molecules with different threedimensional arrangement are mirror images of each other, and the two different forms are called stereoisomers. For example, Dglyceraldehyde and Lglyceraldehyde (HERE) are mirror images of each other (stereoisomers) and cannot be superimposed on each other. Such molecules with these properties are called enantiomers. The designation D or L is an older nomenclature still used widely in biochemistry. It originally described whether the compound rotated a plane of polarized light to the right (D for dextro) or left (L for left). This is not absolute, however, because it depends on the reference compound chosen. The RS nomenclature, which is an absolute naming scheme for organic chemistry will not be used here. The predominant monosaccharides found in nature have the D configuration. Sugars with more than one asymmetric carbon have many possible three dimensional configurations. In general a molecule with m chiral centers will have 2m stereoisomers. The multiple stereoisomeric forms means that not all stereoisomers will be mirror images of each other. Stereoisomers that are not mirror images of each other are called diastereomers. Ketosealdose pairs of sugars frequently are named by inserting the letters ul in the name of the corresponding aldose to derive the name of the ketose. An example is erythrose erythrulose.