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Carbohydrates Or Sugars

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Carbohydrates Or Sugars CARBOHYDRATES OR SUGARS • A group of naturally occurring aldehydes / ketones, formally composed of C atoms and H2O molecules in a ratio equal or close to 1 : 1 general formula: (CH2O)n ... or close • Immense biological importance of carbohydrates: blood group determinants nucleic acids, glycoproteins antigens, immune responses materials & fibers cancer cell markers extracellular support matrices energy storage & generation genetic disease 01 SIMPLE SUGARS OR MONOSACCHARIDES • Carbohydrates of general formula (CH2O)n (3 ≤ n ≤ 9) that possess a structure based on a linear polyhydroxy aldehyde (aldoses) or polyhydroxy ketone (ketose) HOCH2–(CHOH)x–CHO HOCH2–(CHOH)x–(CO)–(CHOH)y–CH2OH an aldose a ketose • Triose, tetrose, pentose, hexose, heptose, octose, nonose: a monosaccharide that incorporates a total of 3, 4, 5, 6, 7, 8 or 9 carbon atoms Note: most monosaccharides of biological importance contain 5 or 6 C atoms. Monosaccharides incorporating more that 6 C atoms are known, but they are rare, and rarity increases with an increasing number of C atoms. 02 STEREOISOMERISM IN MONOSACCHARIDES • Monosaccharides as chiral molecules due to the presence of multiple stereogenic centers HOCH2–(CHOH)x–CHO HOCH2–(CHOH)x–(CO)–(CHOH)y–CH2OH an aldose a ketose • Common ketoses of biological interest as compounds of the type HOCH2–(CHOH)x–(CO)–CH2OH • The vast majority of naturally occurring monosaccharides as chiral and enantiomerically pure substances 03 REPRESENTATION OF MONOSACCHARIDES • Frequent use of Fischer projections to represent monosaccharides; e.g.: CHO OH CHO H OH O H OH HO H HO H H OH H OH H OH H OH H OH H OH OH OH OH ribose glucose fructose • Ribose as a pentose; glucose and fructose as hexoses • Ribose and glucose as aldoses; fructose as a ketose 04 GLYCERALDEHYDE: THE SIMPLEST ALDOTRIOSE • D- and L-forms of glyceraldehyde: CHO CHO H OH enantiomers HO H OH HO D-(R)-(+)-glyceraldehyde L-(S)-(–)-glyceraldehyde (OH on the right) (OH on the left) • D-Glyceraldehyde as the formal progenitor of all common monosaccharides 05 STEREOCHEMICAL PROGENY OF COMMON ALDOSES key stereogenic center CHO H OH OH glyceraldehyde CHO (aldotriose) CHO H OH HO H H OH H OH OH (aldotetroses) OH erythrose threose CHO CHO CHO CHO H OH HO H H OH HO H H OH H OH (aldopentoses) HO H HO H H OH H OH H OH H OH OH OH OH OH ribose arabinose xylose lyxose (aldohexoses) CHO CHO CHO CHO CHO CHO CHO CHO H OH HO H H OH HO H H OH HO H H OH HO H H OH H OH HO H HO H H OH H OH HO H HO H H OH H OH H OH H OH HO H HO H HO H HO H H OH H OH H OH H OH H OH H OH H OH H OH OH OH OH OH OH OH OH OH allose altrose glucose mannose gulose idose galactose talose 06 D- AND L-MONOSACCHARIDES enantiomeric compounds! CHO CHO H OH HO H OH HO D-(R)-(+)-glyceraldehyde L-(S)-(–)-glyceraldehyde (OH on the right) (OH on the left) CHO CHO CHO CHO CHO H OH HO H CHO H OH HO H H OH HO H H OH HO H H OH HO H H OH H OH HO H HO H H OH HO H OH H OH HO H HO OH HO L-erythrose D-erythrose OH HO D-ribose L-ribose D-glucose L-glucose • Most natural monosaccharides belong to the D-stereochemical series. The much rarer L-sugars are produced primarily by fungal or microbial organisms for specialized purposes 07 STRUCTURE OF MONOSACCHARIDES: FORMATION OF HEMIACETALS • Molecules incorporating both an alcohol and an aldehyde or ketone functionality tend to exist as cyclic hemiacetals — if a 5- or 6-membered ring can thus be formed; e.g.: aldehyde or ketone OH R O R O OH 5- or 6-membered ring 08 HEMIACETAL FORMS OF D-RIBOSE AND D-GLUCOSE wavy line means that either Ribose (building block of RNA): configuration is possible CHO H H OH O OH H OH HO CHO HO H OH H OH HO OH HO OH OH aldehyde (carbonyl) common hemiacetal form of D-ribose (preferred) Fischer projection form of D-ribose of D-ribose Glucose: CHO H H H OH OH O OH HO CHO HO HO H H OH HO OH HO OH H OH OH OH OH aldehyde (carbonyl) common hemiacetal form of D-glucose (preferred) Fischer projection form of D-glucose of D-glucose 09 FURANOSE AND PYRANOSE FORMS OF MONOSACCHARIDES O O O O furan tetrahydrofuran pyran tetrahydropyran H H O OH O OH HO furanose form HO pyranose form of D-ribose of D-glucose ("D-ribofuranose") HO OH ("D-glucopyranose") HO OH OH Anomeric position or anomeric carbon: the one sustaining the hemiacetal function (arrows above) 10 POSSIBLE EXISTENCE OF TWO DIASTEREOMERS OF THE HEMIACETAL FORM OF A MONOSACCHARIDE alpha- and beta-anomers of a monosaccharide anomeric carbon H H H H O OH O OH O OH O OH HO HO HO HO HO OH HO OH HO OH HO OH OH OH -anomer of !-anomer of "-anomer of !-anomer of " D-glucopyranose D-ribofuranose D-ribofuranose D-glucopyranose ( -D-glucopyranose) (!-D-ribofuranose) ("-D-ribofuranose) (!-D-glucopyranose) " alpha-anomer: the OH group of the hemiacetal moiety is trans relative to the CH2OH group on the carbon that defines the D / L series beta-anomer: the OH group of the hemiacetal moiety is cis relative to the CH2OH group on the carbon that defines the D / L series 11 CONFORMATIONS OF PYRANOSES; e.g., GLUCOSE H OH H H OH HO O OH O H HO HO HO H H HO OH H HO OH HO H O H H OH H OH HO all equatorial: A-value of OH < 1.0 kcal/mol "-D-glucopyranose favored A-value of CH2OH ≈ 2 kcal/mol H H OH O OH H OH HO HO O H H HO H HO H H HO OH H HO H O OH OH H HO OH OH !-D-glucopyranose all but 1 OH equatorial: favored under appropriate condions, it is possible to obtain pure α-D-glucopyranose or pure β-D-glucopyranose (crystallizaon). These two anomeric forms of glucopyranose, being diastereomeric, differ in solubility, melng point (146 °C for the α-anomer, 150 °C for the 25 β-anomer), and opcal rotaon ([a]D = +112° for the α-anomer, + 19° for the β-anomer). 12 INTERCONVERSION OF PYRANOSE AND FURANOSE FORMS AND OF α- and β-ANOMERS: RIBOSE H O OH CHO O OH H OH HO HO OH b H OH a HO OH OH H OH OH !-D-ribofuranose !-D-ribopyranose D-ribose H O OH b a O OH HO H OH a HO OH HO HO OH OH CHO HO "-D-ribofuranose "-D-ribopyranose b OH all forms will be present at equilibrium in a solution of ribose, although one especially thermodynamically favorable form may be the dominant (or even the exclusive) species (β-D-ribofuranose in the this case) 13 INTERCONVERSION OF PYRANOSE AND FURANOSE FORMS AND OF α- and β-ANOMERS: GLUCOSE CHO H HO O OH H OH HO H OH O HO H HO HO OH b H OH a OH HO OH H OH OH !-D-glucopyranose (1) !-D-glucofuranose (3) D-glucose H HO O OH b a HO H O OH a HO H OH HO OH HO CHO OH HO OH HO OH "-D-glucopyranose (2) "-D-glucofuranose (4) OH b at equilibrium in H2O: 1 ≈ 64%; 2 ≈ 36%; 3 + 4 < 1% 14 MUTAROTATION OF MONOSACCHARIDES: THE CASE OF GLUCOSE if one prepares an aqueous soluon of either pure anomer of glucose, and one measures the specific opcal rotaon of the soluon over me, one observes that the rotaon of a soluon of pure α-anomer, ini7ally eQual to +112°, drops to a final value of ca. +53°, while that of a solu7on of pure β-anomer, ini7ally eQual to +19°, increases to a final value of ca. +53°. H OH H OH H H OH H [ H+ ] H H OH [ H+ ] H O OH O HO HO HO OH HO HO OH HO O HO H HO H HO H HO H H HO H HO H OH H H H H O H internal "-D-glucopyranose -D-glucopyranose rotation ! m.p. 150 °C m.p. 146 °C [!]D = + 19° [!]D = + 112° This phenomenon is termed mutarotaon (="rota7on change") and it is due to equilibraon of the anomers. At eQuilibrium, the solu7on contains a mixture of ca. 64% of β-anomer (all eQuatorial, more stable) and ca. 36% of α-anomer: (0.64 x 19°) + (0.36 x 112°) = 12.2° + 40.3° = +52.5° 15 CHEMICAL REACTIVITY OF CARBOHYDRATES carbohydrates contain both OH and C=O groups ( oZen “masked” as hemiacetals); therefore, their reac7vity will parallel that of alcohols, hemiacetals, and C=O compounds: hemiacetal aldehyde H H O OH OH O HO HO HO OH HO OH alcohols alcohols OH OH !- or "-glucopyranose open form of glucose 16 FORMATION OF GLYCOSIDES OF MONOSACCHARIDES much like a hemiacetal will react with an alcohol under acidic condions to form an acetal, so a monosaccharide will react under the same condions to form an acetal termed a glycoside (riboside, glucoside, ….) HO HO HO CH OH O 3 O H OCH3 OH + O HCl OCH3 H HO OH – H2O HO OH HO OH !- or "-D-ribofuranose !-methyl-D-ribofuranoside "-methyl-D-ribofuranoside OH OH OH CH3OH O O HO O HO + HO HO H HO OCH HO HCl 3 HO OH HO HO – H2O OCH3 H !- or "-D-glucopyranose !-methyl-D-glucopyranoside "-methyl-D-glucopyranoside note: the mechanism of these reac.ons is analogous to that seen earlier for simpler hemiacetals 17 HYDROLYSIS OF GLYCOSIDES IN AQUEOUS ACID much like an acetal undergoes hydrolysis in aqueous acid, so a glycoside can be hydrolyzed back to a “free” sugar under aQueous acidic condi7ons OH OH O aqueous HO HO O HO HCl HO HO OCH3 HO OH !- or "-methyl-D-glucopyranoside !- and "-D-glucopyranoses note: the mechanism of this reac.on is analogous to that seen earlier for simpler acetals 18 REDUCING AND NON-REDUCING SUGARS: THE TOLLENS TEST hemiacetal: OH OH reducing sugar OH Ag(NH ) NO HO OH O OH 3 2 3 0 HO HO HO O + Ag HO HO CHO aq.
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