Glucose has an aldehyde group (-CHO) on carbon atom number one and is therefore called an “aldose,” also it has six carbon atoms (a hexose) so it can be called an “aldohexose.” The reactive group on fructose, however, is a ketone group (-C=0) on carbon number two. For fructose, there are only three asymmetric carbons, so only 8 or 2 3 stereoisomers can be produced. However, not all of these actually exist in nature. In theory, in glucose, the position of the OH group on each of the asymmetric carbon atoms, numbers two, three, four, and five could be flipped, producing a distinct stereoisomer each time, for a total of 16 or 2 4 stereoisomers. We then number the remaining carbons in order through the end of the chain. First we must find the reactive end of the molecule and assign its carbon the number one. In this example we will look at the numbering of D-Glucose. When numbering stereoisomers that have more than three carbon atoms we look at the position of the OH group on the penultimate or next to last carbon atom because this determines whether it is an L or D stereoisomer. Each carbon atom is then numbered in order through the end of the chain. The L is taken from the Latin word for left, Laever, and the D is taken from the Latin word for right, Dexter.Ĭarbon atoms are numbered beginning from the reactive end of the molecule, the CHO (aldehyde) or “C” double bonded “O” (carbonyl) end of the molecule. To distinguish between the isomers, they are labeled L and D isomers. An L-isomer has the OH on the left of the center carbon:Īnd the D-isomer has the OH on the right of the center carbon. These differences do not affect the physical properties but can affect the biochemical properties due to changing the shape of the molecule. The center C-atom of glycerose is called an asymmetric C-atom because it can be rearranged in space to produce two different structures. Therefore, sugars can exist as pairs of stereoisomers or enantiomers which are mirror images of one another. Some isomers differ only in the way in which the atoms are arranged in 3-D space, that is, the atoms are bonded to each other in the same way, but are arranged differently in 3-D space. These types of compounds are called isomers. As an example both glucose and fructose are hexoses (C 6H 12O 6) but they have different chemical and physical properties. Within each of these groups there are different compounds, each with the same molecular formula. The monosaccharides can be divided into groups based on the number of carbon atoms in the molecules, thus: trioses have 3-C atoms, tetroses have 4-C atoms, pentoses have 5-C atoms, and hexoses have 6-C atoms.
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