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Chapter 6 Carbohydrates.pdf

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University of Florida
CHM 2210

Chapter 6 Carbohydrates Wednesday, July 17, 2013 4:20 PM 1. Straight Chain Monosaccharides a. Simple sugars are called monosaccharides b. They usually contain multiple chirality centers, and Fischer projectionsare used to indicate the configuration at each chirality center c. The suffix -ose is used to signify a carbohydrate d. Monosaccharidescan be classified as either aldose or a ketose i. Aldoses contain an aldehyde moiety, while ketoses contain a ketone moiety e. Aldoses and ketoses can be further classified based on the number of carbon atoms they contain i. This is accomplished by inserting a term (tri, tetra, penta…) immediately before the suffix-ose ii. 2. D and L sugars a. Glyceraldehyde is one of the smallest compounds considered to be a carbohydrate b. It has only 1 chirality center so it can exist as a pair of enantiomers i. c. Only + glyceraldehyde (rotates plane-polarized light in a clockwise fashion) is abundant in nature, so glyceraldehyde obtained from natural resources is generally referred to as D-Glyceraldehyde d. Most naturally occurring carbs can be broken down into D-Glyc i. e. In contrast, when synthetic sugars are made, they produce a mixture of D and L f. When a fisher projection of a carb is drawn, D-sugars have OH on the right hand side on the chirality center farthest from the carbonyl group i. g. Just because it is a D sugar doesn’t make the sugar dextrorotatory! i. It means that the lowest chirality center has the R configuration just as it does in + Glyc 3. Drawing fisher projections of carbs 3. Drawing fisher projections of carbs a. b. Basically, turn the chain so all the substituents are on the outside c. If you rotate one of the substituents, flip the dash/wedge d. All wedges go on one side, all dashes on the other 4. Number of possibilities a. The number of sugars possible is the number of chiral 2^ the number of chiral centers b. If it is just a regular sugar, like aldohexose, the number of chiral centers is the carbon chain length -2 c. If there is a substituent, like 2-ketohexose,it is the carbon chain length -2 - the number of substituents d. If it the question asks for the number of D or L sugars, is it the number total/2 5. Commonmonosaccharides a. To rememberthe position of the hydroxyl groups of D glucose, flip the page off with your right hand and your middle finger pointing to the left i. The curled fingers are OH's on the right hand side and the middle finger OH is the left hand side ii. b. To rememberD-ribose, think "Ribose is all right!" i. c. To rememberD-mannose, make a gun with your pointer and middle finger. These are the OH's on the left i. 6. Stereoisomerismis Sugars a. Epimers vary in chirality in the less oxidized carbon at one stereocenter i. Two sugars cannot be epimers at the last carbon or the carbonyl carbon in the straight chain form! b. Anomers vary in chirality at the most oxidized carbon i. Form when a hydroxyl group attacks the carbonyl group ii. Because the attack can occur from either top or bottom,there are two diastereomersformed iii. iv. The two diastereomers that result when the ring structure are referred to as anomers and the new chiral carbon is the anomeric carbon v. The two anomers are referred to as theα and the β anomers vi. vii. The α anomer has the hydroxyl group at the anomeric position in trans to the CH2OH group viii. The β anomher has the hydroxyl group cis 1) ix. x. "β is better!" 7. Mutarotation a. The two anomers of a sugar exist in equilibrium in vivo and can interchange via a straight chain intermediate b. Conversionfrom one anomer to the other is known as mutarotation c. 8. Haworth Projections a. Simplified representation of cyclic sugars b. They emphasize positions above and below the ring, which is the major focus when considering sugars c. To go from the straight chain to ring form ,remember"downright, uplifting" i. Any hydroxyl group on the right side in the fisher projection ends up below the ring in its cyclic structure ii. Any hydroxyl group on the left side ends up above the structure d. e. Six memberedrings are referred to as pyrans while five memberedrings are referred to as furans (one of the carbons is replaced with oxygen)
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