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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 NON-

POLAR 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 phospho-lipids, water and many water soluble compounds. The phospho-lipids 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