Chapter 23: Carbohydrates hydrates of carbon: general formula Cn(H2O)n Plants: photosynthesis hν 6 CO2 + 6 H2O C6H12O6 + 6 O2 Polymers: large molecules made up of repeating smaller units (monomer) Biopolymers: Monomer units: carbohydrates (Chapter 23) monosaccharides peptides and proteins (Chapter 25) amino acids nucleic acids (Chapter 26) nucleotides Synthetic Polymers (Chapter 27) various 261 23.1: Classification of Carbohydrates. I. Number of carbohydrate units monosaccharides: one carbohydrate unit (simple carbohydrates) disaccharides: two carbohydrate units (complex carbohydrates) trisaccharides: three carbohydrate units polysaccharides (oligosaccharides): many carbohydrate units CHO H OH HO HO HO H HO HO O glucose HO O H OH HO HO OH HO H OH OH CH2OH HO HO HO O HO O HO HO O HO HO O HO HO HO O O O O O HO HO O HO HO HO O O HO HO galactose HO OH O 262 glucose glucose 118 II. Position of carbonyl group the carbonyl at C1 is an aldehyde: aldose the carbonyl at any other carbon is a ketone: ketose III. Number of carbons three carbons: triose six carbons: hexose four carbons: tetrose seven carbons: heptose five carbons: pentose etc. IV. Cyclic form (Chapter 23.6 and 23.7) CHO CHO CHO CHO CH2OH H OH HO H H OH H OH O CH2OH H OH H OH HO H HO H CH2OH H OH H OH H OH CH2OH H OH H OH CH OH 2 CH2OH glyceraldehyde threose ribose glucose fructose (pentose) (triose) (tetrose) (hexose) (hexose) (aldohexose) (ketohexose) 263 23.2: Fischer Projections and the D, L Notation. Representation of a three-dimensional molecule as a flat structure (Ch. 7.7). Tetrahedral carbon represented by two crossed lines: horizontal line is coming vertical line is going back out of the plane of the behind the plane of the page (toward you) paper (away from you)! substituent carbon (R)-(+)-glyceraldehyde CHO CHO CHO H C H OH OH H CH2OH HO CH2OH CH2OH (S)-(-)-glyceraldehyde CHO CHO CHO HO H HO H HO C CH2OH H CH2OH CH2OH 264 119 before the R/S convention, stereochemistry was related to (+)-glyceraldehyde CHO CHO H OH HO H CH2OH CH2OH D-glyceraldehyde L-glyceraldehyde R-(+)-glyceraldehyde S-(-)-glyceraldehyde (+)-rotation = dextrorotatory = d (-)-rotation = levorotatory = l D-carbohydrates have the -OH group of the highest numbered chiral carbon pointing to the right in the Fischer projection as in R-(+)-glyceraldehyde. For carbohydrates, the convention is to arrange the Fischer projection with the carbonyl group at the top for aldoses and closest to the top for ketoses. The carbons are numbered from top to bottom. 265 Carbohydrates are designated as D- or L- according to the stereochemistry of the highest numbered chiral carbon of the Fischer projection. If the hydroxyl group of the highest numbered chiral carbon is pointing to the right, the carbohydrate is designated as D (Dextro: Latin for on the right side). If the hydroxyl group is pointing to the left, the carbohydrate is designated as L (Levo: Latin for on the left side). Most naturally occurring carbohydrates are of the D-configuration. 1 CHO 1 H 2 OH CHO 3 2 HO H H OH 3 H 4 OH HO H highest numbered highest numbered 5 HO 4 H "chiral" carbon H OH "chiral" carbon 5 CH2OH 6 CH2OH D-Glucose L-Arabinose CHO HO H CHO H OH HO H HO H H OH highest numbered highest numbered H OH "chiral" carbon HO H "chiral" carbon CH2OH CH2OH 266 L- glucose D-Arabinose 120 23.3: The Aldotetroses. Glyceraldehyde is the simplest carbohydrate (C3, aldotriose, 2,3-dihydroxypropanal). The next carbohydrate are aldotetroses (C4, 2,3,4-trihydroxybutanal). aldotriose CHO CHO H OH HO H CH2OH CH2OH D-glyceraldehyde L-glyceraldehyde aldotetroses 1CHO 1 CHO 2 HO 2 H highest numbered H OH highest numbered 3 3 "chiral" carbon H OH HO H "chiral" carbon 4 4 CH2OH CH2OH D-erythrose L-erythrose CHO CHO HO H H OH highest numbered highest numbered HO H "chiral" carbon H OH "chiral" carbon CH2OH CH2OH D-threose L-threose 267 23.4: Aldopentoses and Aldohexoses. Aldopentoses: C5, three chiral carbons, eight stereoisomers CHO CHO CHO CHO H OH HO H H OH HO H H OH H OH HO H HO H H OH H OH H OH H OH CH2OH CH2OH CH2OH CH2OH D-ribose D-arabinose D-xylose D-lyxose Aldohexoses: C6, four chiral carbons, sixteen stereoisomers 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 CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH D-allose D-altrose D- glucose D-mannose D-gulose D-idose D-galactose D-talose 268 121 Manipulation of Fischer Projections 1. Fischer projections can be rotate by 180° (in the plane of the page) only! CHO CH2OH CHO CH2OH 180 ° H OH HO H 180 ° HO H H OH CH2OH CHO CH2OH CHO (R) (R) (S) (S) 180° 180° Valid Valid Fischer Fischer projection projection 269 a 90° rotation inverts the stereochemistry and is illegal! 90 ° CHO OH H OH ≠ OHC CH2OH CH2OH H (R) (S) 90 ° This is not the correct convention 90° for Fischer projections Should be projecting toward you Should be projecting away you This is the correct convention for Fischer projections and is the enantiomer 270 122 2. If one group of a Fischer projection is held steady, the other three groups can be rotated clockwise or counterclockwise. hold steady CHO CHO H OH HO CH2OH CH2OH H (R) (R) CHO H HO H OHC OH hold steady CH2OH CH2OH (S) (S) 120° 120° hold hold steady steady hold steady 120° 120° hold hold hold 271 steady steady steady Assigning R and S Configuration to Fischer Projections 1. Assign priorities to the four substituents according to the Cahn-Ingold-Prelog rules 2. Perform the two allowed manipulations of the Fischer projection to place the lowest priority group at the top or bottom. 3. If the priority of the other groups 1→2→3 is clockwise then assign the carbon as R, if the priority of the other groups 1→2→3 is counterclockwise then assign the center as S. 2 place at the top 4 CO2H H 1 H2N H 4 2 HO2C NH2 1 hold steady CH3 CH3 rotate other 3 3 three groups counterclockwise 1-2-3 counterclockwise = S 2 4 2 CO2H H CO2H 4 1 2 H NH2 1 H2N CO2H H2N CH3 CH3 1 3 CH3 H 3 3 4 1-2-3 clockwise = R 272 123 Fischer projections with more than one chiral center: 1 CHO CHO 2 HO H HO H 3 H OH H OH 4 CH2OH CH2OH Threose 273 23.5: A Mnemonic for Carbohydrate Configuration. (please read) 25.6: Cyclic Forms of Carbohydrates: Furanose Forms. O H+ + HO OR2 H , R2OH R2O OR2 + R2OH R1 H (Ch. 17.8) R1 H R1 H hemiacetal acetal O OH OR H + H H H , ROH OH O Ch. 23.1425.13 O cyclic hemiacetal mixed acetal (glycoside) 274 124 Cyclization of carbohydrates to the hemiacetal creates a new chiral center. The hemiacetal or hemiketal carbon of the cyclic form of carbohydrates is the anomeric carbon. Carbohydrate isomers that differ only in the stereochemistry of the anomeric carbon are called anomers and designated as α and β. CHO * H OH H H O O H OH H H * + H H * H OH H H H OH CH2OH OH OH OH OH D-erythrose Converting Fischer Projections to Haworth formulas 275 23.7: Cyclic Forms of Carbohydrates: Pyranose Forms. H H 5 OH 5 H O H O OH H H 4 4 1 H H 1 H H 1 CHO HO HO H 2 2 H 3 H OH 3 3 HO OH OH OH H OH H H H H 1 new chiral 4 5 4 3 2 H OH HO CHO H center 5 H 5 H H H OH OH OH OH 5 H H O H OH H H H 4 H H 1 4 H H 1 D-ribose HO HO 2 OH 3 2 O 3 HO OH OH OH ribopyranose 6 CH OH 2 6 CH OH 5 2 OH 5 H O H O OH H H 4 4 1 1 OH H 1 OH H CHO HO HO H 2 3 2 H 3 H OH 6 H OH 3 H OH HO H HOH C H OH H 2 1 new chiral 5 2 H 4 OH HO 4 3 CHO center 6 CH2OH 6 5 CH2OH 5 HOH2C H H OH H OH OH 5 6 H H O H OH H H H 4 OH H 1 4 OH H 1 D-glucose HO HO 2 OH 3 2 O 3 H OH H OH glucopyranose 276 Note: the pyranose forms of carbohydrates adopt chair conformations. 125 23.8: Mutarotation. The α- and β-anomers are in equilibrium, and interconvert through the open form. The pure anomers can be isolated by crystallization. When the pure anomers are dissolved in water they undergo mutarotation, the process by which they return to an equilibrium mixture of the anomer. HOH C 2 HOH C cis OH 2 HO HO O O HO HO OH HO HO CHO H H OH H β-D-Glucopyranose (64%) HO H [ ] +18.7° H OH (β-anomer: C1-OH and α D H OH CH2OH are cis) CH2OH HOH C 2 HOH C OH 2 trans HO HO O HO H HO H HO HO O OH α-D-Glucopyranose (36%) [α]D +112.2° ( -anomer: C1-OH and α 277 acid-catalyzed mechanism: p.