16 - CARBOHYDRATES Flashcards
polyhydroxy aldehydes or polyhydroxy ketones or substances that yield such compounds on hydrolysis
Carbohydrates
What are the three major classes of carbohydrates?
Monosaccharides
Oligosaccharides
Polysaccharides
simple sugars and consists of a single polyhydroxy aldehyde or polyhydroxy ketone unit
Monosaccharides
are classified based on the carbonyl grp.
Monosaccharides
consist of short chains of monosaccharide units, or residues, or substances joined by glycosidic bonds
- most abundant are the disaccharides, with two monosaccharide units
Oligosaccharides
sugar polymers containing more than 20 or so monosaccharide units, and some have hundreds or thousands of units
Polysaccharides
What are the two classifications of monosaccharides?
Aldose and ketose
a monosaccharide containing an aldehyde grp.
Aldose
a monosaccharide containing a ketone grp.
Ketose
naturally occurring monosaccharides contain 3-7/8 carbon atoms per molecule
Triose - monosaccharide w/ 3 carbon atoms
Tetrose - monosaccharide w/ 4 carbon atoms
Pentose - monosaccharide w/ 5 carbon atoms
Hexose - monosaccharide w/ 6 carbon atoms
are the simplest carbohydrate monosaccharides
Triose
Aldotetrose - aldoses w/ 4 carbon atoms
Aldopentose - aldoses w/ 5 carbon atoms
Ketopentose - ketoses w/ 5 carbon atoms
Ketoheptose - ketoses w/ 7 carbon atoms
are a common way to represent the stereochemistry of monosaccharides; help visualize the 3D arrangement of atoms in a 2D plane
Fischer projections
developed the Fischer projections
H. Emil Fischer
use horizontal and vertical lines to indicate the 3D structure of a molecule
horizontal = project forward
vertical = project backward
intersecting lines = carbon
Fischer projections
Monosaccharides can be classified as D- or L- forms based on the configuration of the chiral carbon farthest from the carbonyl grp.; forms are mirror images of each other
the chiral center that is the most distant or farthest from the carbonyl carbon
Penultimate carbon
a monosaccharide that, when written as a Fischer projection, has the -OH on its penultimate carbon on the RIGHT
D-monosaccharide
a monosaccharide that, when written as a Fischer projection, has the -OH on its penultimate carbon on the LEFT
L-monosaccharide
Natural monosaccharides are in the D- form while, synthetically produced monosaccharides are usually in the L-form
D sugars are more abundant in nature; present in all plants and animals.
Animals = use it as a source of energy
L- glucose is not abundant in nature.
stereoisomers that are mirror images
ex. D-erythrose and L-erythrose
Enantiomers
stereoisomers that are not mirror images
ex. D-erythrose and D-threose
Diastereomers
dextrorotatory (+)
levorotatory (-)
labels for monosaccharides refer to the direction in which they rotate plane-polarized light
monosaccharides rotate the plane of polarized light clockwise
Dextrorotatory (+)
monosaccharides rotate the plane of polarized light counterclockwise
Levorotatory (-)
D- does not always mean dextrorotatory (+), and L- does not always mean levorotatory (-)
ex.
D-glucose is D-(+)-glucose (dextrorotatory)
D-fructose is D-(-)-fructose (levorotatory)
Monosaccharides can be classified by the
(a) number of carbon atoms in the molecule (e.g. pentose vs. hexose)
(b) functional group (aldoses vs. ketoses)
(c) configuration (D- and L- configuration)
(d) optical activity ((+) vs. (-) isomers)
(e) ring structure (furanoses vs. pyranoses)
(f) stereochemistry at an anomeric carbon (alpha vs. beta isomers)
What happens if a sugar forms a cyclic molecule?
- cyclization of sugars takes place to make a cyclic hemiacetal
- cyclization can also result in hemiketal formation
- in both cases, the carbonyl carbon becomes the new chiral center called the anomeric carbon
