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Chapter 24: Carbohydrates [Sections: 24.1–24.10]

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Chapter 24: Carbohydrates [Sections: 24.1–24.10] Chapter 24: Carbohydrates [Sections: 24.1–24.10] Carbohydrates definition • naturally occuring compounds derived from carbon, oxygen and hydrogen • the net molecular formula comes from each carbon having an equivalent of water, hence, hydrates of carbon glucose = C6H12O6 = 6 carbons and 6 water molecules (6 x H2O) • commonly referred to as "sugars" or "saccharides" (driving from saccharum, which is latin for sugar) simple carbohydrates complex carbohydrates • carbohydrates that CANNOT be • carbohydrates that CAN be hydrolyzed hydrolyzed to simpler carbohydrates to simpler carbohydrates + H3O "monosaccharide" "monosaccharides" "trisaccharide" "disaccharide" Monosaccharides: polyhydroxy aldehydes and ketones aldose ketose polyhydroxy aldehydes polyhydroxyketones CH2OH group CH2OH at top CHO group O H ketone carbonyl at top group at 2nd O OH bearing carbon chiral carbons OH bearing H OH H OH chiral carbons CH2OH CH2OH CH2OH group CH2OH group at bottom at bottom 3 carbons in length = 4 carbons in length = CH2OH CHO O HO H HO H H OH H OH HO H H OH CH2OH CH2OH • "ose" ending implies a carbohydrate • most common in nature are aldopentose, aldohexose, ketopentose, ketohexose • each of these are examples of monosaccharides The simplest aldoses: D– and L-glyceraldehyde O H O H O H H * OH H OH HO * H CH2OH CH2OH CH2OH Fischer projection • D-gylceraldehyde (the R enantiomer) rotates a plane of polarized light in the dextroratory (+) direction in a polarimeter while L-glyceraldehyde is levoratory (–) • remember: we cannot predict whether any given enantiomer is (+) or (–) without either: i) conducting an experiment with a polarimeter, or ii) knowing the direction that light is rotated by the other enantiomer • D-glyceraldehyde is naturally–occurring, while L-glceraldehyde is not! • All naturally-occurring monosacharrides derive from D-glyceraldehyde Kiliani-Fischer Synthesis CN CHO O D-threose H H2 , Pd/BaSO4 H OH HCN H OH H2O H * OH NaCN CH2OH CH2OH CH2OH CN CHO D-glyceraldehyde (aldotriose) D-erythrose H2 , Pd/BaSO4 H OH H2O H OH CH OH O 2 CH2OH CHO H NH O R R C N R R H H CH2OH L-threose ? • Kiliani-Fischer synthesis produces two new monosaccharides with one additional carbon atom • two stereoisomeric products result. Type of stereoisomers = • monosaccharide stereoisomers of this type are also called "epimers" because they differ ONLY in stereochemistry of a single stereogenic carbon atom • both stereoisomers called "D" NOT because they are (+)-rotating[they are actually both levoratory] but because they derive from D-glyceraldehyde and have the D-stereochemistry at the bottom-most stereogenic carbon • both are naturally occurring. L-sugars are NOT naturally–occurring! • ALL naturally occurring sugars, both aldoses and ketoses, are D-sugars Glucose • Glucose is a naturally-occurring aldohexose. Given this information, draw as much of its structure as possible: • given this limited information, how many possible stereoisomers could be drawn for glucose? Problems: 1, 2 A more complete perspective and summary: • naturally-occurring monosaccharides, whether aldoses or ketoses, share in common the stereochemistry of the CHOH group at the stereogenic carbon nearest the bottom. • D-glucose and D-fructose are particularly important sugars as far as human energy supplies are concerned Problems: 1–4 Acetals and hemi-acetals O + MeO OMe CH3OH, H R H R H acetal O CH3OH R H O CH OH • in the absence of an acid catalyst, the 3 HO OMe reaction of an aldehyde with an alcohol cannot proceed past the hemi-acetal stage R H R H • this is an equilibrium process that in aldehyde (99%) hemi-acetal (1%) ordinary cases heavily favors the starting aldehyde O O O HO H HO H pyran • however, if a molecule contains BOTH the alcohol group for the reaction AND the aldehyde group, formation of the hemi-acetal is strongly favored when a stable 5- or 6-membered ring can be formed Pyranose rings CHO CH2OH CH2OH H OH O H H O HO H H H OH H + OH H H OH OH OH H OH H OH H OH CH2OH • upon closure to form the 6-membered pyranose ring, two different stereoisomers (called anomers) are formed, based on the stereochemistry of the OH group at the hemi-acetal linkage (or anomeric carbon) • when pyranoses are drawn in this conventional manner, the α-anomer has the OH group pointing down (trans to the CH2OH group) and the β-anomer has the OH group pointing up (cis to the CH2OH group) • the stereochemistries of the other OH groups are defined by the stereochemistries in the monosaccharide predicting the structure of, and naming, pyranose rings: CHO H H OH CH OH H H OH H 2 H HO H H OH HOH2C CHO OH H O H OH OH OH OH H OH H OH H OH CH2OH glucose H CH OH CH2OH 2 O H O OH H O H H OH H OH H H OH H OH CH2OH H OH OH H OH H H β-anomer H OH H O Name: OH H CH2OH H OH CH2OH O O H H H H H H OH H OH H OH O OH OH H OH H OH α-anomer Name: Shortcut for drawing pyranose rings of D-sugars CHO H OH HO H H OH H OH CH2OH predict the structures, and corresponding names, of the pyranose rings of D-talose: CHO HO H HO H HO H H OH CH2OH Problems: 5 Mutarotation • the two anomers of glucose can be separated and purified • the β-anomer has [α] = +18.7°, the α-anomer has [α] = +112.2° • no matter which isomer is started with, however, upon sitting in aqueous solution the final optical rotation from the polarimeter = +52.6° which is somewhere in between the readings of either pure compound • this results from the fact that each ring is in equilibrium with the "open form" and hence in equilibrium with the other anomer as well • the result is a mixture of the two anomers with a net resulting optical rotation CHO CH2OH CH2OH H OH pure β H O H pure α H O OH HO H H H OH H +18.7° OH H H OH +112.2° H OH OH OH H OH H OH H OH CH2OH β-D-glucopyranose "open form" glucose α-D-glucopyranose 63% < 0.01% 37% • the interconversion of one anomer with the other in this way is known as "mutarotation" Relative stability of the two glucopyranose anomers CH OH 2 CH2OH O H H O H O OH O H H OH H OH H OH OH OH H H OH H OH Glycoside formation O + CH3OH O H HO H H+ OH hemi-acetal CH2OH CH2OH H O OH H H O OCH3 OH H H OH H OH H methyl β-D-glucopyranoside OH H CHO H OH H OH H OH CH3OH CH OH, H+ HO H + 3 H OH H+ x H OH CH2OH CH2OH CH2OH H O H H H O H OH H H D–glucose OH H methyl α-D-glucopyranoside OH OH OH OCH3 H OH H OH • formation of an acetal via a "glycoside" linkage •when formed from a sugar molecule it is called a "pyranoside" since it comes from glucopyranose • in the absence of an acid catalyst, the anomeric carbon of pyranosides is locked in place • pyranosides will not undergo mutarotation under neutral conditions Problems: 6 Reducing sugars: Distinguishing between pyranose and pyranoside molecules. CHO CO2H H OH + H OH HOH2C Ag in NH OH O 4 HO HO H HO H OH + Ag° H OH HO OH Tollen's reagent H OH H OH H OH CH OH pyranose 2 CH2OH + HOH2C Ag in NH4OH HO O OCH3 HO OH Tollen's reagent pyranoside Disaccharides HOH C HOH C 2 O 2 O HO HO HOH2C HO O HO OH HO OH OH OH + HO OH H3O CH2OH α-glucopyranose O O HO OH OH maltose • maltose is a disaccharide and a complex carbohydrate (can be hydrolyzed) • the two monosaccharides are connected via a "glycoside" linkage • in maltose, the α-pyranoside linkage is retained between the two sugars and is fixed • the hemi-acetal linkage, however, is still able to mutatorate • amylose is a polysaccharide made up of thousands of repeating glucose units • all of the units are connected by the α- glycoside link • amylose is a major consituent of starches in food that humans rely upon for food/energy • more complex branched polymers (glycogen) are formed by the body as a way of storing glucose for energy HOH2C HOH2C HOH2C O O O HO HO HO HOH2C O OH O HO OH HO OH + HO OH H3O OH HO OH OH β-glucopyranose cellobiose • cellobiose is also a disaccharide and a complex carbohydrate (can be hydrolyzed) • in cellobiose the β-gylycoside linkage is retained between the two sugars and is fixed • the hemi-acetal linkage is still able to mutatorate • cellulose is a polysaccharide made of ~7,000 units on average of glucose • all of the units are connected by the β- glycoside link • humans have enzymes specific for hydrolysis of the α-glycoside link and can therefore hydrolyze starches like amylose to release the glucose for energy • humans lack the enzymes necessary to hydrolyze the β-glycoside link and cannot, therefore, liberate the glucose from polysaccharides like cellobiose • cows also do not naturally produce enzymes necessary to hydrolyze the β-glycosidee link but their stomachs contain microorganisms that DO produce the required enzymes so that cows are able to digest cellulose (e.g., grasses) Some other interesting polysaccharides... • hydrolyzed in the body by an enzyme called lactase • lactase production begins to decline for children past age 2 and continues • ~75% of adults have lactose intolerance D-Galactose D-Glucose type of glycoside link? D-Fructose 2008 Georgia sugar refinery explosion type of glycoside link to glucose? D-Glucose where is the glycoside link for fructose? 31% glucose 38% fructose 17% water 7% maltose 1% sucrose nectar: 90% water with a mixture of compounds comprised of 55% sucrose, 24% glucose, 21% fructose plus aroma chemicals and proteins • derivatized amino sugar • similar structure to cellulose • NH bonds allow for hydrogen bonding which increases the strength of strands • main constituent of insect exoskeletons Chapter 24 Essential Concepts 1.
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