Isomers Show IsomersThe 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. Within each of these groups there are different compounds, each with the same molecular formula. As an example both glucose and fructose are hexoses (C6H12O6) but they have different chemical and physical properties. These types of compounds are called isomers.  StereoisomersSome 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. Therefore, sugars can exist as pairs of stereoisomers or enantiomers which are mirror images of one another. The center C-atom of glycerose is called an asymmetric C-atom because it can be rearranged in space to produce two different structures. These differences do not affect the physical properties but can affect the biochemical properties due to changing the shape of the molecule. An L-isomer has the OH on the left of the center carbon: and the D-isomer has the OH on the right of the center carbon. To distinguish between the isomers, they are labeled L and D isomers. The L is taken from the Latin word for left, Laever, and the D is taken from the Latin word for right, Dexter. Back to top Numbering of Carbon Atoms[Video of Numbering of Carbon Atoms] Numbering Transcript: Carbon atoms are numbered beginning from the reactive end of the molecule, the CHO (aldehyde) or “C” double bonded “O” (carbonyl) end of the molecule. Each carbon atom is then numbered in order through the end of the chain. 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. In this example we will look at the numbering of D-Glucose. First we must find the reactive end of the molecule and assign its carbon the number one. We then number the remaining carbons in order through the end of the chain. Back to top Asymmetric Carbon AtomsIn 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 24 stereoisomers. However, not all of these actually exist in nature. For fructose, there are only three asymmetric carbons, so only 8 or 23 stereoisomers can be produced. 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. It is therefore called a “ketose” or a “ketohexose.” Only a few of the monosaccharides exist free in nature. Most of them are usually found as sugar units in polysaccharides or in more complex molecules. They can then be obtained by hydrolysis (breaking down) of the complex CHO’s. Monosaccharides are often called simple sugars, and are sub-divided according to the number of C-atoms. Back to top Trioses and Pentoses1. Trioses: (C3H6O3) 2. Pentoses (C5H10O5) D-ribose – a component of RNA, ribonucleic acid, vitamins (riboflavin), and coenzymes. It is also important in the high-energy compounds, ATP and ADP. In its reduced form, deoxyribose, it is a component of DNA. L-arabinose – occurs in conifer heartwood and is a component of hemicelluloses where it occurs with xylose. It is also a component of pectin and can be a major component of gums (gum Arabic). Bacterial action in making silage can yield free arabinose. Arabans are polymers of arabinose. D-xylose – there are small amounts of D-xylose free in fruits, but it occurs mainly in hemicellulose, as xylans and hetero-xylans. Hemicellulose is a polysaccharide of xylose and arabinose (a heteroxylan). The ratio of xylose to arabinose seems to affect digestibility as digestibility is reduced as the proportion of xylose increases. Hemicelluloses constitute a considerable portion of the cell walls of plants so herbivores eat large amounts of them. These sugars are all aldopentoses. Back to top Hexoses3. Hexoses (C6H12O6) D-glucose – an aldohexose with various common names, including grape sugar, dextrose, corn sugar (made from cornstarch). Occurs free in plants, fruits, honey, body fluids, including CSF, blood, lymph. It is the major end-product of CHO digestion by non-ruminants and is therefore a primary energy form for non-ruminants. It is a major component of many oligosaccharides (with galactose forms lactose) and polysaccharides (such as starch and cellulose). [Video] D-Glucose Ring Transcript: In solution D-glucose exists as an equilibrium mixture of the straight chain form with two pyranose ring forms. Effectively, carbon atom number one reacts with carbon atom number five forming a ring. In fact two forms of the structure exist, called anomers. If the hydrogen atom is above carbon atom one then it is called an alpha anomer but if the hydrogen atom is below the carbon atom it is called a beta anomer. This structural information is very important because it governs how molecules of glucose join together to form larger molecules. Starch is a polymer of the a-form and is water soluble and digestible by animal enzymes. Cellulose is a polymer of the b-form, it is not soluble and is not digestible by animal enzymes. Changing from a to b via an open chain structure is called mutarotation, and it requires the O-C bond to be broken to allow the C to swivel the H and OH upside down. Then the bond is remade. The other monosaccharides also make ring forms. Back to top D-Fructose – a ketohexose, found in honey, green leaves, seeds, and stems of many plants, as the main unit in fructans which are common in young grasses, in roots as the storage polysaccharide inulin, and as a component of the disaccharide sucrose (with glucose). It also forms pyranose rings but when it reacts to make oligo- or polysaccharides it does so as a FURANOSE ring structure (like FURAN). In this case the anomeric carbon atom is C-2, and the CH2OH is either above the anomeric carbon (alpha anomer), as in sucrose or below as in FRUCTANS (beta anomer). D-Galactose – an aldohexose, not found free, most important as a component of the disaccharide lactose, milk sugar (with glucose). It also occurs in other complex CHO’s and complex lipids in the brain and nervous tissue (galactoglycerides and cerebrosides). D-Mannose – Mainly as mannans in yeasts, molds, and bacteria. Back to top Hexose Reactions Hexose compounds can undergo a number of chemical reactions.
Glycoside Formation Combining the H of a hydroxyl group on a sugar with an alcohol group or another hydroxyl group causes an Esterification or Condensation reaction to yield a glycoside. This occurs at C atom one, the anomeric C atom. Since sugars contain alcohol groups and hydroxyl groups, they can combine with other sugars to form disaccharides, tri, tetra, etc. and polysaccharides, all joined by glycosidic linkages. Examples:
Back to top Does glucose have 4 carbon atoms?Glucose is a hexose, with six carbon atoms (Fig. 5.41), and an aldehyde-aldose monosaccharide. Fructose is also a hexose, but a ketose. Fig.
How many asymmetric carbons are there?Just checking in. Are you still watching?. How many asymmetric carbons are there in sugar galactose?Hence Galactose has 4 chiral carbon atoms.
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Does glucose have 4 asymmetric carbon atoms?
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