Why is sugar sweet?
Outline
•
Structure of Sugars
•
Non-covalent Interactions
•
Sugar and Taste Receptors
•
Homework
• Sucrose, or table sugar, is formed from two simple
sugars: glucose and fructose.
•
• All sugars are sweet because they contain OH groups
with a particular orientation that can interact with the
taste receptor for sweetness in our tongues.
Structure of Sugars
Glyceraldehyde
From last time we saw that the product of the Calvin cycle was glyceraldehyde-3-phosphate. This can
react with water to make the simplest sugar, glyceraldehyde, but most couples to form glucose.
The formula for glyceraldehyde is C3H6O3. In general, the formula for all simple sugars has this same
ratio of carbon, hydrogen, and oxygen: CnH2nOn.
We know from VSEPR that the first carbon atom (with only 3 bonds and no lone-pairs) must have its
bonds all in one plane but the other two carbon atoms must be tetrahedral, each with 4 bonds. Using a
dash to indicate a bond going back and a wedge to indicate a bond coming forward, the molecule can be
drawn this way.
There are two ways to draw the molecule. Does it make a difference which one we use?
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Prof. Shapley
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If we focus on the middle carbon atom (C2) as we look down the chain of 3 carbon atoms, we can see that
one structure really is different from the other. We can't rotate the molecule in any way to make the two
structures the same. The only way to convert one into the other is by breaking bonds. These two structures
of glyceraldehyde are mirror images of one another. Molecules that are non-superimposable mirror
images of each other are called enantiomers.
In photosynthesis, only the enantiomer of glyceraldehyde on the right is formed.
Stereochemistry
Molecules like glyceraldehyde that have the same atoms bonded in the same way but have a different 3dimensional shape are called chiral molecules.
Many chemistry reactions in living things produce chiral molecules because the enzymes that bring the
reactants together are themselves chiral. Enzymes can bring together the reactants in such a way as to
form molecules of only one shape.
In organic molecules, the most common cause of a molecule being chiral is that it has one or more
asymmetric carbons. An asymmetric carbon is a carbon atom that is bonded to 4 different atoms or
groups.
The C2 atom of glyceraldehyde is an asymmetric carbon. It is bonded to:
1.
OH
2.
C(O)H
3.
CH2OH
4.
H
Let's rotate the glyceraldehyde C2 so that the smallest group bonded to it (H) goes to the back. Then we
draw a circle beginning from the heaviest atom (OH, with atom O at 16 AMU). The other 2 groups both
begin with carbon but the CHO group is more important because it has 2 bonds to the heavier oxygen
rather than just one.
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When the circle goes in the clockwise direction (to the right), C2 has an R configuration.
When the circle goes in the counter clockwise direction (to the left), C2 has an S configuration. (S stands
for sinister as left was once thought to be the sinister direction.)
Other Simple Sugars
Below are a few other, common sugar molecules. Each of these has alcohol groups and one carbon with a
double bond to oxygen. Where are the asymmetric carbons? Can you identify their configuration as R or
S?
Non-covalent Interactions
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London Forces
We could discount intermolecular interactions between gas-phase molecules because these molecules are
mostly far apart and moving rapidly relative to each other. In the liquid phases, all molecules interact with
one another. The stronger the interaction between a molecule and a pure liquid, the greater will be the
solubility of the molecule in the liquid.
All molecules interact with each other through London dispersion forces, or induced dipole interactions.
In the figure below, a 2-atom molecule collides with a 3-atom molecule. The electron cloud of the first
molecule repels the electron cloud of the molecule it strikes, causing a displacement of some electron
density away from the nucleus. The nucleus is then poorly shielded by its own electrons and attracts the
electron cloud of the first molecule.
Both molecules now have a small dipole moment that was induced by molecular collision.
Dipole-Dipole Interactions
Molecules with permanent dipoles can interact with other polar molecules through dipole-dipole
interactions. Again this is electrostatic in nature. The molecular dipole vector points towards high electron
density.
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Note that polar molecules also interact with each other through London forces. The dipolar interactions
add to this force.
Hydrogen Bonding Interactions
Hydrogen that is bonded to very electronegative elements (F, O, or N) is highly electron deficient. It acts
as a Lewis acid and interacts with basic sites in other molecules. The hydrogen bonding interaction is
stronger than dipole-dipole interactions. Again, it adds to the existing London dispersion forces to
stabilize molecules in solution.
Alcohol Functional Group
Functional groups are parts of molecules that have a specific type of reactivity. There are two kinds of
functional groups in sugars. One of these is the alcohol functional group.
We've talked about simple alcohols before. Alcohols are formed from hydrocarbons in the reactions that
form smog.
The O-H unit in an alcohol behaves like the O-H unit in water. The hydrogen and oxygen can hydrogen
bond to other, similar molecules. Hydrogen bonds are stronger than other intermolecular interactions but
weaker than covalent bonds.
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Alcohols, like water, can act as weak acids. They can lose a proton to form the
strong alkoxide base.
Typically, all but one of the carbon atoms in a sugar are connect to OH
groups. Sugars are poly-alcohols or polyols.
Aldehyde Functional Group
The aldehyde functional group consists of a carbon that is bonded to a hydrogen atom and doubly bonded
to an alcohol, RC(O)H.
Like carbon dioxide, the carbon end of an aldehyde C=O bond
is electron-poor or electrophilic while the oxygen end is more
electron-rich.
The oxygen end of the aldehyde is a hydrogen bond acceptor.
It can interact with the hydrogen atoms of water or of
alcohols. Hydrogen atoms in C-H bonds, like the hydrogen in
an aldehyde group, can't hydrogen bond.
Sugars contain an aldehyde group or a similar C=O unit
called a ketone.
Solubility of Sugars
In a crystal of solid sugar, there are extensive hydrogen bonds between the hydroxyl groups of different
sugar molecules. This interaction stabilizes the solid state structure. In order to dissolve the crystal, a
solvent must replace the inter-sugar hydrogen bonds with solvent-sugar hydrogen bonds. Water is the best
solvent for this because each molecule of water can donate 2 hydrogen atoms to form hydrogen bonds
with other molecules and it can accept 2 hydrogen bonds with its 2 lone pairs.
In general, solute molecules are most soluble in pure liquid solvents that provide the same interactions
that the solute molecules have between them in the solid state. That results in the commonly known
principle that:
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LIKE DISSOLVES LIKE
Taste Receptors
Sweetness is related to a substance's ability to hydrogen bond to a
protein-based receptor in taste buds at the tip of the tongue.
In the figure at right, you see that molecules with a hydrogen
bond donor (an OH group in a sugar) about 3 x 10-8 cm away
from a hydrogen bond acceptor (another OH group in the
sugar) are able to strongly interact with the sugar receptor
protein in the taste bud. It is also important that there is a
relatively non-polar part between the hydrogen bond donor and
acceptor. In sugars, the non-polar part consists of the C-H bonds adjacent to
the OH groups.
Sweetness varies with structure. In the table below, table sugar (sucrose) is
given an arbitrary sweetness value of 1.0 and other sweet substances are rated relative to that. All sugars
are sweet as are many other organic and even inorganic molecules.
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Notice that sugars in the table above are classified as monosaccharides (one sugar unit) or disaccharides
(two sugar units). Next time we'll talk about how monosaccharides are converted to disaccharides,
trisaccharides, and polysaccharides.
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Several artificial sweeteners have been used in foods and drink. These were all discovered accidentally by
sloppy chemists who didn't wash the chemicals off their hands before leaving the laboratory.
The use of cyclamate and saccharin has been restricted because they can cause bladder cancer in rats at
high dosage levels. These compounds are not metabolized by the body and so provide no calories to food.
Aspartame is made from two amino acids: aspartic acid (tasteless) and phenylalanine (bitter) that give a
sweet compound as the dipeptide. It is metabolized like other peptides and proteins in the body but it has
far fewer calories than sugars.
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