Chapter 16_Printed

Chapter 16_Printed - Chapter 16 Carbohydrates 1 Sugars...

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Chapter 16: Carbohydrates 1. Sugars: Their Structures and Stereochemistry a. Monosaccharides i. The building blocks of all carbohydrates are simple sugars called monosaccharides ii. A monosaccharide can be either a: 1. Aldose a. Polyhydroxy aldehyde b. Glyceraldehyde – an aldotriose (aldose with three carbons) c. Aldoses with more than 3 carbons: i. 4: aldotetroses ii. 5: aldopentoses iii. 6: aldoheptoses 2. Ketose a. Polyhydroxy ketone b. Dihydroxyacetone – an ketotriose (ketose with three carbons) c. Ketoses with more than 3 carbons: i. 4: ketotetroses ii. 5: ketopentoses iii. 6: ketohexoses iii. Six-carbon sugars are the most abundant in nature iv. Two five-carbon sugars, ribose and deoxyribose, occur in structures of RNA and DNA v. Four-carbon and seven-carbon sugars play roles in photosynthesis and other metabolisms b. Isomerism i. The most highly oxidized carbon is designated as C-1 ii. The designation of D or L depends on the arrangement at the chiral carbon with the highest number 1. D conformation: hydroxyl group is on the right of highest chiral carbon 2. L conformation: hydroxyl group is on the left of the highest chiral carbon iii. Diastereomer – nonsuperimposible, non mirror image
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iv. Entantiomer- nonsuperimposible, mirror image v. 2 N rule 1. To determine the number of possible stereoisomers, raise 2 to the number of chiral centers vi. Most sugars have the D configuration, based on the D-glyceraldehyde c. Cyclic Sugar Molecules i. Most 5 or 6 carbon sugars exist as cyclic molecules ii. The cyclization takes place as a result of interaction between the functional groups on distant carbons: 1. C-1 and C-5 to form a cyclic hemiacetal (in aldohexoses) 2. C-2 and C-5 to form a cyclic hemiketal (in ketohexoses) iii. The carbonyl carbon becomes a new chiral center called the anomeric carbon
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iv. Anomers 1. The cyclic sugar can take two forms which are anomers of each other: a. α i. The Fischer projection of the α-anomer of a D sugar has the anomeric hydroxyl group to the right of the anomeric carbon (C-OH) b. β i. The Fisher projection of the β-anomer of a D sugar has the anomeric hydroxyl group to the left of the anomeric carbon (HO-C) 2. The free carbonyl species can readily form either the α-anomer or the β-anomer, and the anomers can be converted from one form to another v. Haworth Projection Formulas 1. Furanose – 5 membered ring 2. Pyranose – 6 membered ring 3. Fisher vs. Haworth a. For a D sugar, any group that is written to the right of the carbon in a Fisher projection has a downward
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Chapter 16_Printed - Chapter 16 Carbohydrates 1 Sugars...

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