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Structure of Saccharides

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Structure of Saccharides Structure of saccharides Monosaccharides Carbohydrates exist as aldoses, which are polyhydroxyaldehydes, and ketoses, being polyhydroxyketones. Their open–chain forms can be suitably represented by Fischer pro- jections. Aldoses and ketoses can be of different chain length and are all derived from glyceraldehyde. A formal insertion of a stereogenic (HCOH)–group between the carbonyl carbon atom and the a–C–atom of glyceraldehyde leads to aldotetroses and further successive (HCOH)–group insertions between the carbonyl carbon atom and the adjacent stereogenic center to aldopentoses and aldohexoses. By an analogous sequence the group of ketoses branches out from 1,3–dihydroxyacetone, leading to tetruloses, pentuloses, and hexuloses. O O OH OH OH O HO O OH OH OH OH D-Glyceraldehyde 1,3-Dihydroxyacetone L-Glyceraldehyde 1,3-Dihydroxyacetone OH OH O O OH HO OH OH D-Tetrulose L-Tetrulose O O 1 1 OH 1 1 OH O (HCOH) n 2 (HCOH) n 2 O 2+n OH (HCOH) n HO 2+n (HOCH) n 3+n OH 2+n OH 3+n OH 2+n OH D-Aldoses D-Ketoses L-Aldoses L-Ketoses The group of aldoses is derived from glyceraldehyde, ketoses can be built up from 1,3–dihydroxyacetone. O OH OH (+)-D-Glyceraldehyde O O OH HO OH OH OH OH (-)-D-Erythrose (-)-D-Threose O O O O OH HO OH HO OH OH HO HO OH OH OH OH OH OH OH OH (-)-D-Ribose (-)-D-Arabinose (+)-D-Xylose (-)-D-Lyxose O O O O O O O O OH HO OH HO OH HO OH HO OH OH HO HO OH OH HO HO OH OH OH OH HO HO HO HO OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH (+)-D-Allose (+)-D-Altrose (+)-D-Glucose (+)-D-Mannose (+)-D-Gulose (-)-D-Idose (+)-D-Galactose (+)-D-Talose The acyclic forms of D–aldoses drawn as their Fischer projections. OH O OH OH D-Tetrulose OH OH O O OH HO OH OH OH OH (-)-D-Ribulose (-)-D-Xylulose OH OH OH OH O O O O OH HO OH HO OH OH HO HO OH OH OH OH OH OH OH OH (+)-D-Psicose (-)-D-Fructose (+)-D-Sorbose (-)-D-Tagatose The acyclic forms of D–ketoses drawn as their Fischer projections. Monosaccharides form cyclic hemiacetals Monosaccharides exist preferably as cyclic hemiacetals and hemiketals. These arise from the intramolecular nucleophilic attack of a hydroxyl–oxygen atom at the carbonyl carbon atom of the acyclic species. Depending on which hydroxyl group of the monosaccharide chain reacts with the carbonyl group, 5– or 6–membered rings are formed which are called furanoses and pyranoses, respectively, in analogy to their unfunctionalized heterocycle analogs, tetrahydrofuran and tetrahydropyran. a-D-Glucofuranose a-D-Glucopyranose HO HO OH OH HO HO O OH O OH O OH OH OH 5-membered ring HO 6-membered ring OH OH OH OH OH OH OH HO HO D- Glucose O OH open chain OH form O OH OH b-D-Glucofuranose b-D-Glucopyranose 5 O O 4 1 4 1 3 2 3 2 tetrahydrofuran tetrahydropyran ring ring The formation of 5– and 6–membered monosaccharide rings is exemplified by the aldose D–glucose. Representation of monosaccharide structures O H O OH OH OH HO HO O OH HO O OH OH HO OH O OH OH HO OH HO HO OH HO HO OH HO OH OH HO Fischer Zig-zag Mills Haworth Chair projection projection projection projection conformation Different projections for the structure of the open chain and the cyclic form of b–D–glucose. They are all identical in terms of their absolute configurations, as can easily be shown for each stereogenic center applying the CIP convention. The anomeric configuration The new stereogenic center generated by hemiacetal ring closure is called the anomeric center. The two possible stereoisomers are referred to as anomers, designated as a or b according to the stereochemical relationship between the anomeric center and the configuration of the most distant stereogenic center. If the hydroxyl groups bound to this center point in the same direction (cis), this anomer is called the a–anomer, when they are pointing in opposite directions (trans), it is named b. Anomers are diastereomers. 1-OH: 1-OH: 1-OH: Fischer left Fischer right 1-OH: 1 Fischer Fischer right HO OH HO left OH OH OH HO HO HO HO OH OH OH OH HO HO 5-OH: 5-OH: 5 5-OH: 5-OH: Fischer O Fischer O O Fischer O Fischer right right left left OH OH HO HO b - D a - D a - L b - L 5-OH: 5-OH: OH OH 5-OH: Haworth Haworth 5-OH: Haworth 'up' 'up' Haworth O OH 'down' O O OH O 'down' 5 1-OH: 1-OH: 1-OH: 1-OH: 1 Haworth OH OH Haworth Haworth Haworth up down down OH up OH Corresponding structures of a– and b–anomers of monosaccharide hemiacetals of the D– and L–series, represented as Fischer and Haworth projections; the correct numbering of the carbohydrate ring is indicated. Thus, for D–glucose and all compounds of the D–series, a–anomers have the hydroxyl group at the anomeric center projecting downwards in Haworth formulae; a–L–compounds have this group projecting upwards. The b–anomers have the opposite configurations at the anomeric centers, i.e. the hydroxyl group projects upwards and downwards for b–D– and b–L–compounds, respectively. A wavy line is used for the anomeric bond when the anomeric configuration is not specified. Mutarotation Each crystalline free sugar is a discrete stereoisomer. On dissolution in water however, the hemiacetal ring opens and reforms to give products with different ring sizes and configurations at the anomeric center. This equilibration occurs with all reducing saccharides and is accompanied by a change in optical rotation known as mutarotation. It can be acid– and base–catalyzed. b-D-Glucopyranose a-D-Glucopyranose Haworth projection chair conformation chair conformation Haworth projection OH OH OH OH O OH O O O OH HO HO HO OH HO OH HO HO HO HO OH O OH OH OH OH HO OH OH OH HO HO CH OH CH2OH (+)-D-Glucose 2 HO OH (Fischer projection) HO O HO HO O OH OH OH O O OH HO HO OH OH OH HO HO b-D-Glucofuranose a-D-Glucofuranose Haworth projection envelope conformation envelope conformation Haworth projection Mutarotation of D–glucose in solution leads to a mixture of a– and b–pyranoses as well as a– and b–furanoses. In addition to the structures shown, several other species can be involved in mutarotation including the acyclic hydrate and even septanoses and oxetanoses. Conformations of monosaccharides Like cyclohexane, the 6–membered ring of monosaccharides also exists in two isomeric chair 1 4 conformations, which are specified as C4 and C1, respectively, where the letter C stands for 'chair' and the numbers indicate the carbon atoms located above or below the reference plane of the chair, made up by C–2, C–3, C–5 and the ring oxygen. The conformational shape of a pyranose is mainly governed by the relative stability of the two possible chair conformations which are both free of torsional strain, but one of which, in most cases, is clearly energetically unfavored because of van der Waals 1 interactions of the ring substituents. Thus, the C4 conformation of b–D–glucopyranose is 4 clearly unfavored compared to its C1 conformation because the van der Waals repulsion of the 1,3–diaxially positioned ring substituents result in a free energy difference between the 4 two chairs of approximately 25 kJ/mol. Consequently only one, the C1 conformation of b–D– glucose is observed by NMR spectroscopy. On the other hand, as expected from their configurations, the energy difference between both chair conformations of a–idose and a– altro-se is so small, that consequently both forms can be observed in the NMR spectrum. HO OH OH OH 4 D-Glucopyranose 1 O O HO OH HO 4 1C conformation HO 1 4 HO OH less stable 4 1 C C 1 4 HO OH HO OH D-Idopyranose OH 1 4 O O HO OH OH both conformations OH 1 4 HO similarly stable Other principal conformations of pyranoses are half–chair (H), boat (B), and skew (S) conformation, which are named as indicated. The chair is by far the most stable and only the skew conformation has an energy minimum in a similar range, but this is still some 20 kJ higher than the chair. Principal conformations of the furanose ring are the envelope forms 1 2 3 4 O O 1 1 2 2 3 3 4 4 ( E, E1, E, E2, E, E3, E, E4, E, EO) and the twist forms ( T1, TO, T2, T1, T3, T2, T4, T3, TO, O T4). 5 O 4 1 2 3 1 4 O 4 2 O O 1 4 O 3 O 1 4 1 4 1 5 1 4 C4 C1 1,4B B1,4 5So oH5 2 4 1 3 O O O O O 3 2 4E o 2 1E E T3 3T2 In addition to intramolecular van der Waals interactions, carbohydrate conformations are determined by some other factors, such as electrostatic interactions as well as intramolecular hydrogen bond formation and especially the anomeric effect. The anomeric effect The equatorially positioned substituents of a carbohydrate ring are, for steric reasons, the most energetically favored, compared to their axial counterparts, as is the case in every molecule with a chair conformation.
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