Lecture Ten - Carbohydrates II Flashcards
What are hemiacetals and hemiketals?
Hemiacetals and hemiketals are compounds that are derived from aldehydesand ketones respectively. The Greek word hèmi means half. These compounds are formed by formal addition of an alcohol to the carbonylgroup.
When the alcohol group is replaced by a second alkoxy group, an acetal or a ketal, respectively, is formed.
What are Haworth projections?
Five and six membered hemiacetals are represented as planar pentagons or hexagons, as the case may be, viewed through the edge.
Most commonly drawn with the anomeric carbon on the right and the hemiacetal oxygen to the back right.
Drawn as cyclic carbohydrates, as opposed to Fischer projections, which are linear.
The designated beta means that the OH on the anomeric carbon is cis to the terminal CH2OH (facing up), alpha means it is trans (facing down).
What is an anomeric carbon, and what are anomers?
An anomeric carbon is the new stereocentre resulting from cyclic hemiacetal formation.
Anomers are carbohydrates that differ in configuration at their anomeric carbons.
What is the nomenclature used for heterocycles?
Heterocycles are molecule which have atoms in them other than carbon.
Six membered hemiacetal rings are shown byt he infix -pyran-.
Five membered hemiacetals rings are shown by the infix -furan-.
Can five membered rings be drawn in the Haworth projection?
Five membered rings are so close to being planar that Haworth projections are adequate to represent furanoses.
What are chair projections?
E.g. Pyranoses, the six membered ring is more accurately represented as a strain free chair conformation.
OH groups are in equatorial positions.
Are monosaccharides soluble in water?
Monosaccharides are colourless cyrstalline solids, very soluble in water, but only slightly soluble in ethanol.
Explain glycosides.
A carbohydrate in which the OH group of the anomeric carbon is replaced by OR.
They are common in nature.
Glycoside bonds: The bond from the anomeric carbon of the glycoside to an OR group.
Glycosides are named by listing the name of the alkyl or aryl group attached to the oxygen followed by the name of the carbohydrate with the ending -e replaced by -ide.
E.g. Methyl Beta-D-glucopyranoside.
E.g. Maltose is a disaccharide contining tow monosaccharide units joined by an alpha glycoside bond.
What are Di- and Polysaccharides?
Two or more sugar molecules can be joined together by glycoside bonds.
In nature, this assembly is performed by enzymes which enables the linkages to be formed specifically and reproducibly.
In nature, enzymes are also involved in the hydrolysis of the di- and polysaccharides.
Exaplin the features of starch.
Starch is used for energy storage in plants.
it can be separated into two fractions, amylose and amylopectin. Each on complete hydrolysis by amazase gives only D-glucose.
Amylose: is composed of continuous, unbranched chains of D-glucose units koined by alpha-1,4’-glycoside bonds. It is water soluble.
Amylopectin: is highly branched polymer of D-glucose. Chains of D-glucose are joined by alpha-1,4’-glycoside bonds and branches are created by alpha-1, 4’-glycoside bonds.
Explain the characteristics of glycogen.
Glycogen is the reserve carbohydrate for animals.
Like amylopectin, glycogen is a nonlinear polymer of D-glucose units joined by alpha-1,4’-glycoside and alpha1,6’-glycoside bonds.
Explain the features of cellulose.
Cellulose is a linear polymer of D-glucose units joined by the beta-1,4’-glycoside bonds.
