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Triacylglycerols (TAGs), Soaps and Detergents at the
molecular level
Jeff Corkill, Department of Chemistry, Eastern Washington University
RECALL: chemical structure->reactivity->function
NON COVALENT INTERACTIONS (NCIs) Bonding:
1. ionic ccomplete transfer of electrons to achieve "filled shells" ("octets")
METALS AND NON-METALS
2. Covalent : sharing of electrons
NON-METALS ONLY
(a) polar covalent: unequal sharing leading to polar bonds
i.e C-O, N-H, C-F, C-N, N-O, O-H, H-Cl , etc
(b) non-polar covalent : equal sharing leading to non-polar bonnds
i.e O-O, N-N, F-F, H-H , C-C, C-H
But what about molecules that contain both ionic AND covalent bonds?
Many such molecules exist and many compounds in nature have such
structures. Some interesting examples are compounds that act as "soaps and
detergents" and those that form the cell membranes of ALL living species.
TRIACYLGLYCEROLS: 3 fatty acids residues covalently bonded to a
glycerol molecule :see below
The structure or fatty acids: long hydrocarbon chains without ('saturated') OR with
("unsaturated') C=C bonds
TAGs are hydrolysed in the intestines by a reaction catalysed by lipase to the 3 fatty
acids and glycerol. Only in the latter form can these compound be absorbed into the
blood stream
initially
and then two more
steps so the overall reaction is =>
TAGs are hydrolysed in the soap industry by a reaction catalysed by aqueous sodium
hydroxide ('lye') to the 3 fatty acids and glycerol (as above) except the products are
~~~~~~~~~~~~~~~~CO2- Na+
Structures:
Quick review of non-covalent interactions:
1. Ionic and ion-dipole (think solvation of ions in water)
2. Hydrogen bonding (only Hs on O, N and F can do this. [think of as a weak bridge]
3. Dipolar attraction [think of polar bonds as little magnets]
4. Van der Waals' force [very weak between non-polar groups]
For a review see: http://chemistry.ewu.edu/jcorkill/org351/NCI.htm
When molecules of (1) water, (2) soap/detergent and (3) a water insoluble non-polar
compound (e.g. fats/oils on dishes and pans when washing up or grease/engine oil on
clothes or hands) are mixed together, they INTERACT (Note: NOT chemically REACT)
to align themselves IN THE MOST THERMODYNAMICALLY STABLE STATE.
This is done so as the bring the ionic and polar molecules close together (ie the ionic
head of the soap and the water molecules) and put the non-polar sections close to one
another (i.e the fat/oil/grease and the hydrophobic tail of the soap).
One arrangement that meets these criteria are the formation of "MICELLES". These have
hydrophilic exteriors and hydrophobic interiors, see below:
EXPERIMENT
This effect can be demonstrated by putting several drops of olive oil in hot water in a
glass container sitting on top of an overhead projector. The oily drops on top of the
water should be visible. Now several drops of dishwashing liquid are added and the
mixture stirred. The oily drops appear to disappear are micelle formation has taken
place. Actually the micelles can just be seen on the projector due to light scattering by
the micelles. It may be possible to "see" these micelles under the microscope.
To make this experiment more visual, one can add a colored dye to the olive oil and
repeat. In this case, the compound, bilirubin (a breakdown product of hemoglobin) that
is non-polar and so is soluble in the oil but is relatively insoluble in water- see structure
below). In this case, it is easy to see the formation of the micelles as very small pink
droplets that can be observed under the microscope.
Another example of amphiphilic compounds that acts on fats and oils in the same way
as soaps are 'bile salts". [These compounds are formed from cholesterol in the liver and
are stored in the gall bladder until needed.] When fatty/oily foods are consumed, the
large fat/oil droplets must be broken up into millions of tiny droplets in the intestines
before they can be broken down prior to absorption into the blood stream. The bile salts
also have a ionic head and long non-polar hydrophobic tail (see below) like soaps and
cause the fat/oil to be converted into MICELLES. This micelle formation increases the
surface area dramatically so then the intestinal lipase enzyme can act on the surface to
help break down the fat/oil molecules into fatty acids and glycerol. (This is the same
reaction as described earlier in the soap experiment.)
OTHER WAYS OF REPRESENTING POLAR AND NONPOLAR MOLECULES:
Octane (C8H18) non-polar
covalent bonding (van der
Waals's forces only)
Green color indicates low
UNIFORM polarity regions
Water : polar covalent
Hydrogen bonding
capability
Red end (oxygen) has high
electron density and blue
ends are hydrogen with low
electron density
HYDROPHILIC
Tri-glyceride ("fat/oil): mainly non-polar covalent bonding but with a few polar
covalent bonds shown in red and yellow.
See (or click=>) accompanying soap experiment handout for chemical structure
After "saponification", the tri-glyceride becomes 3 fatty acid
anions and glycerol.The blue regions indicate OH bonds, the red
& yellow show the -CO2B hydrophilic ends. See (or click=>)
accompanying soap experiment handout for chemical structure
Edge of micelle on molecular scale: compare the three images
Biological membranes use a different types of molecules
("phospho-lipids") from soaps but the idea ids the same. The
achievement of thermodynamic stability of a mixture of phospholipids, water and many water soluble compounds. The phospholipids have two hydrophobic tails for each ionic head (see below).
Lipid bilayer in biological membrane: Note the lipid membrane
components (in blue -hydrophilic heads and yellow hydrophobic
tails) have two tails per head membrane yellow/blue
Although not shown in the diagram above, cholesterol is found in
membranes of animals and is thought to "modulate" membrane
fluidity. The cholesterol molecule (see below) will align itself in
the membrane so as the polar head will face the water molecules
in the 2 surfaces of the membrane and the non-polar tail will fit
parallel to the hydrophobic tails of the phospho-lipids.
Another examples of micelles are found in the human
lipoproteins HDL and LDL that are involved in the transport of
very non-polar (and thus very water insoluble) cholesterol esters
and tri-glycerides around the body. The structure of a LDL
particle is shown below. The surface is made up from hydrophilic
proteins and the ionic heads of phospho-lipids. The interios
contains the non-polar tails of the phospho-lipids and cholesterol
esters and tri-glycerides. The formation of LDL( in the liver) is
driven, again, by the achievement of thermodynamic stability, i.e
"like interacts with like"
Thumb Tack experiment
-a demonstration of surface tension/ hydrogen bonding in water
and soap
1. Very carefully, "float" a thumb tack (the plastic covered ones
work best!) on the surface of some water in a beaker or glass. It
is supported on the surface by surface tension effects (see left
hand diagram below- those "sticky" water molecules again!)
2. Now use a clean tooth pick to poke the surface of the water
gently near the tack. If done carefully, the tack will remain on
the surface. (The cylindrical tooth picks with a fine point work
better than the flat type.)
3. This is the sneaky part. Give someone else a similar tooth
pick EXCEPT you have prepared this one by putting a little
washing-up liquid on the ends of the tooth pick. You don't need
much!
4. Challenge this person to repeat your action in #2 above. No
matter how gentle and careful they are, the tack will sink
instantly! The soap molecules introduced by the soapy tooth
pick disrupts the hydrogen bonding in the pure water and
lowers the surface tension, so the tack sinks (see right hand
diagram below)
5. You can also do this by coating ONE end of a tooth pick with
soap. Use the dry end yourself for #2 and then hand the tooth
pick to the other person so that they use the "soapy" end for #4.
Vchemistry.ewu.edu/jcorkill/biochem/soap2000.html