BCEM 393 Lecture Notes - Lecture 6: Chitin, Starch, Plant Cell

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Polysaccharides
OLIGOSACCHARIDES CONTAIN A GLYCOSIDIC BOND
- Oligosaccharides is the general term for sugars that contain two or more monosaccharides
linked by O-glycosidic bonds
- Disaccharides consist of two monosaccharides joined covalently by an O-glycosidic bond,
which is formed when a hydroxyl group of one sugar reacts with the anomeric carbon of the
other
- A glycosidic bond forms by a condensation reaction, in which one water molecule is produced
- The breakage of a glycosidic bond, is a hydrolysis reaction, in which one water molecule is
used up
DISACCHARIDES
- Since monosaccharides have multiple hydroxyl groups, many different glycosidic linkages are
possible
- Example: glucose, mannose, and galactose: these molecules can be linked together in the
lab to form more than 12,000 different structures
- osyl means anomeric C1in glycosidic linkage
- oside means anomeric C2 in glycosidic linkage
- ose means anomeric C not in glycosidic linkage
REDUCING SUGARS ABLE TO OPEN UP TO THE OPEN-CHAIN FORM
- A reducing sugar is any sugar that is capable of acting as a reducing agent because it has a
free aldehyde group or a free ketone group
- The carbonyl carbon is oxidized to a carboxyl group in mild oxidizing agents such as ferric
(Fe3+) or cupric (Cu2+) ion (ie. base metals)
- In the hemiacetal (ring) form, C-1 of glucose cannot be oxidized by Cu2+
- However, the open-chain form is in equilibrium with the ring form, and eventually the
oxidation reaction goes to completion
- Fehling’s test for reducing sugars
- Red copper (I) oxide then precipitates out of the reaction mixture, which indicates a positive
result
- Conditions matter
- In acid, every carbohydrate would be a reducing sugar, so a reducing sugars test would
not be so useful
- All monosaccharides are reducing sugars
- Ketoses must first tautomerize (convert) to aldoses before they can act as reducing sugars
ene-diol reactions
- Ene-diol reactions are catalyzed by bases
REDUCING SUGARS
- Disaccharides are classified as either reducing or non reducing
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