Monosaccharides, Oligosaccharides, and Polysaccharides.docx

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R.Roy Baker

Classes of Carbohydrates  3 major classes 1. Monosaccharides: - simple sugars D-glucose - consist of a single polyhydroxy aldehyde or ketone unit - Most abundant monosaccharide in nature is the 6-C sugar D-glucose (dextrose) D-glucose 2. Oligosaccharides: - Consist of short chains of monosaccharide units (or residues) joined by characteristic linkages called glycosidic bonds - Most abundant form is the disaccharides, with two monosaccharide units - Example: sucrose, or cane sugar, which consists of the 6-C sugars D-glucose and D-fructose. - In cells, most oligosaccharides have 3 or more units that do not occur as free entities but are joined to nonsugar molecules (lipid or proteins) in glycoconjugates (glycoprotein/ glycolipid)  All common monosaccharides and disaccharides have names ending with the suffix “-ose” . 3. Polysaccharides: - Contain more than about 20 monosaccharide units - May have hundreds or thousands of monosaccharide units. - Some are linear chains (eg: cellulose) - Some are branched chains (eg: glycogen)  starch and cellulose both consist of recurring units of D-glucose, but they differ in the type of glycosidic linkage  have strikingly different properties and biological roles. Functions of Carbohydrate 1. Energy- simple sugars (eg. Glucose) 2. Energy storage- complex carbohydrates (eg. Glycogen, starch) 3. Source of carbon for anabolism (eg- making of fats) 4. Structural function: cellulose in plants, cell wall of bacteria 5. Component of other molecules: nucleotides, glycoproteins, glycolipids, etc Monosaccharides  The simplest of the carbohyrates  General Formula Monosacchrides: (CH 2O) n n=3-7  Physical properties: - Colorless - Crystalline solids - Freely soluble in water but insoluble in nonpolar solvents - Have a sweet taste  Structure: - The backbone of common monosaccharide molecules are unbranched carbon chains in which all the carbon atoms are linked by single bonds. - In the open-chain form, one of the carbon atoms is double-bonded to an oxygen atom to form a carbonyl group: + If the carbonyl group is at an end of the carbon chain, it is in an aldehyde group.  The monosaccharide is an aldose . + If the carbonyl group is at any other position, it is in a ketone group  The monosaccharide is a ketose . - Each of the other carbon atoms has a hydroxyl group.  These atoms are often chiral centers => give rise to the many sugar stereoisomers found in nature. - The simplest monosaccharides are the two 3C trioses: + Glyceraldehyde- an aldotriose + Dihydroxyacetone- a ketotriose  Nomenclature: - Monosaccharides with 4,5,6 and 7 carbon atoms in their backbones are called tetroses, pentoses, hexoses, and heptoses. - Examples: + The hexoses: most common in nature Aldohexose D-glucose ketohexose D-fructose + The pentose: components of nucleotides and nucleic acids Aldopentoses D-ribose Aldopentose 2-deoxy-D-ribose Monosaccharides Have Asymmetric centers: Stereoisomers in Sugar  All the monosaccharides except dihydroxylacetone (which is a symmetric molecule) contain one or two asymmetric (chiral) carbon atoms.  Occur in optically active isomeric forms  The simplest aldose, glyceraldehyde, contains one chiral center (The middle carbon atom)  Has two different optical isomers, or enantionmers .  By convention, one of these two forms is designated the D isomer, the other the L isomer.  Two way of representing 3-D sugar structures on paper: 1. Fischer projection formulas: - C-1 atom is always at the top of the figure - Horizontal bonds project out of the plane of the paper, toward the reader. - Vertical bonds project behind the plane of the paper, away from the reader. 2. Perspective formulas - Solid wedge-shaped bonds point toward the reader. - Dashed wedge-shaped bonds point away from the reader.  Stereoisomers: have same numbers of functional groups,but position of these around an asymmetric C is different.  Stereoisomers cannot be superimposed and are distinct  A molecule with n chiral centers can have 2 stereoisomers. Ex: Aldohexoses, with 4 chiral centers, have 2 = 16 stereoisomers.  A ketose has one fewer chiral carbon atomthan the aldose of the same empirical formular. Ex: Keto tetrose with 1 chiral centerhas only 2 stereoisomers. Aldo tetrose with 2 chiral centers has up to 4 stereoisomers.  When the hydroxyl group on the C chiral center most distant from the carbonyl carbon ison the right in the projection formula, the sugarD isomer .  When the hydroxyl group on the C chiral center most distant from the carbonyl carbon ison the left in the projection formula, the sugarL isomer .  Of the 16 possible aldohexoses, 8 are D forms and 8 are L: ALL D ALDOHEXOSES  Most of hexoses of living organisms are D isomers.  Epimers : two sugars that differ only in the configuration around one carbon atoms . Ex: D-glucose and D-mannose differ only in the stereochemistry at C-2.  Some sugars occur naturally in their L form. Ex: L-arabinose and the L isomers of some sugar derivatives that are common components of glycoconjugates. L-arabinose  Chiral molecules are optically active.  They rotate the plane of polarized light. Optically active substances Light is also made up of vibrations - electromagnetic ones. Some materials have the ability to screen out all the vibrations apart from those in one plane and so produce plane polarised light. An optically active substance is one which can rotate the plane of polarisation of plane polarised light. if you shine a beam of polarised monochromatic light (light of only a single frequency - in other words a single colour) through a solution of an optically active substance, when the light emerges, its plane of polarisation is found to have rotated. The rotation may be either clockwise or anti-clockwise. Assuming the original plane of polarisation was vertical, you might get either of these results. Cyclization of Aldoses and Ketoses  In aqueous solution, aldotetrosesand all monosaccharides with five or more carbon atoms in the backbone occur predominantlycyclic (ring) strucutres .  In this ring structure, the carbonyl group has focovalent bond with the oxygen of a hydroxyl group along the chain.  The formation of these ring structure is the result of a general reaction between aldehydes or ketones and alcohols to formderivatives called HEMIACETALS or HEMIKETALS.  HEMIACETALS and HEMIKETALS both contain an additional asymmetric carbon atom.  They have one more chiral carbon atom than the regular non-cyclic form.  This chiral center is at the carbon atom of the carbonyl group.  The oxygen atom from the reacting hydroxyl groupbecomes a member of the five- or six-membered ring structures. There can be two ring structures depending on how the ring is formed.  Example of cyclization: D-glucose exists in solution as an intramolecularhemiacetal in which the free hydroxyl group at C-5 has reacted with the aldehydic C-1 .  Rendering the aldehydic C-1 asymmetric and producing two stereoisomers, designatedalpha and beta . - OH below the ring is for alpha, OH above the ring is for beta  Because it resembles six -membered heterocyclic compopyran , the six-membered ring of a monosaccharide pyranosed a.  Similarly, because tfive -membered ring of a monosaccharide resembles furan , it is calfuranose .  Unlike pyran and furan, the rings of carbohydrates DO NOT contain double bonds.  The 6-membered aldopyranose ringis much more stable than the aldofuranose ring.  Aldopyranose ring predominates in aldohexose solutions.  Only aldoses having five or more carbon atoms can form pyranose rings.  ANOMERS: Isomeric formofmonosaccharidesthat differ only in their configuration about the hemiacetal or hemiketal carbon atom.  The hemiacetal or carbonyl carbon
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