Chemistry 142 Lab 10
Molecules of Life
Introduction
Biochemistry is the study of the molecules responsible for the sustenance and elaboration
of life, and of the chemical and physical processes that they undergo.
We are interested in four major classes of biochemical molecules:
• Sugars
• Lipids
• Proteins
• Nucleic acids
Sugars are the major currency of energy--plants store the energy garnered from the sun in
sugars, and animals harvest these sugars, either directly from the plants (herbivores) or from an
animal that has eaten a plant (carnivores).
Lipids are, broadly speaking, molecules that are insoluble in water. There are many
varieties of lipids, but the major classes that we are interested in are the triglycerides (as in
human fatty tissue), the phospholipids which are used in, among other places, the construction of
cell walls. Myelin, for example, the primary component of the insulation surrounding our nerve
fibers, is composed, in large part, of phospholipids.
Proteins are polymeric biomolecules formed from amino acid monomers. Since there are
many different kinds off amino acids (human beings use 20 in all), there are limitless
combinations available to nature, and therefore there is available to nature a virtually unlimited
supply of differentiated molecules, each performing its own specific biological task.
Nucleic acids are polymers formed from the reaction of certain weak organic bases with
ribose (a simple sugar) and the phosphate ion. The ordering of the segments of the nucleic acids
in our bodies is what we inherent. It is our genetic structure and determines what species we
belong to, as well as our sex, our skin color, the shape of our ear lobes, and a whole host of other
inherited traits.
In today’s lab we will examine each of these families of compounds.
Carbohydrates and Sugars
A carbohydrate is a compound composed of carbon, hydrogen, and oxygen, in which the
hydrogen and oxygen exist in the same ratio as they do in water (2 hydrogen atoms for every
oxygen atom). Carbohydrates are also referred to as saccharides. Carbohydrates can react with
each other forming longer and longer carbohydrate chains in much the same way that polymers
form. Usually the short chain compounds are referred to as sugars, and the longer chain
compounds are referred to as starches.
There are many ways of classifying carbohydrates. One method is to simply cataegorize
them according to the number of sugar sub-units each molecule contains. Under this scheme, a
monosaccharide is a sugar containing a single sugar monomer, a disaccharide would contain 2
monomers, an oligosaccharide would contain, roughly, from 3 to 6 monomers, and lengthier
molecules would be classified as polysaccharides.
An example off a monosaccharide would be glucose, the basic fuel utilized by all living
things. Sucrose, common table sugar, is an example of a disaccharide, being composed of a
single monomeric glucose molecule and a single monomeric fructose molecule.
Oligosaccharides are frequently found in beans and legumes, and are indigestible by
human beings. We rely on intestinal parasites to metabolize these sugars and, inasmuch as these
parasites secrete gaseous byproducts during their metaboplic activities, they are the cause of a
certain degree of distress in social situations.
Polysaccharides are the basis for many of the structural achievements in nature, ranging
from the shell of an insect to the stability of a sequoia. Cellulose is a polymer of glucose, as is
starch, the two differing because of the location on the glucose monomers of the bonds that hold
the polymer together.
Glucose is classified as an HEXOSE becouse it contains 6 carbon atoms, and as a
PYRANOSE because it can fold into a six membered ring (the pyran ring being a common 6
membered ring in organic chemistry)
Glucose is also classified as an ALDOSE. This is because there is an aldehyde group at
one end of the six-member. In fact, in aqueous solution, there is an equilibrium between the ringform of glucose, 99% of the molecules being in the ring form at any given time. Because
aldehydes are easily oxidized, they readily cause reduction in other subtances that they come in
contact with. For this reason, glucose is sometimes referred to as a reducing sugar.
So, glucose is a sugar, a monosaccharide, a hexose and an aldose, which can occur in
either an open-chain or ring structure in aqueous solutions. Examine the structures of glucose on
the next page to see these points illustrated.
Glucose Structures: open and ring
O
C
H
H
OH
HO
H
H
OH
H
OH
CH2OH
H OH
H
O
HO
H
HO
H
H
Lipids
OH
OH
By far the most common lipid found in the human body is the triglyceride. Triglycerides
are esters which form from the reaction of long-chain fatty acids and the glycerol molecule
according to the scheme we learned earlier when we studied alcohols:
Alcohol + Carboxylic Acid  Ester + Water
Shown below is the myristic acid molecule and the glycerol molecule, as well as the
triglyceride formed from them, trimyristin. This is the majopr triglyceride in cows milk.
O
HO
myris tic acid
OH
HO
OH
glycerol
O
H 2C
O
O
HC
O
O
H 2C
O
trimyris tin
A second class of lipids is called phospholipids. These compounds again use glycerol as
a backbone, but contain only 2 conventional esters, which, like triglycerides utilize fatty acids.
The third carbon, however, forms what is known as a phosphate ester, usually with a nitrogenous
base. An example of a phospholipid is a class of compounds, known as cephalins. These are
characgterized by the ethanolamine groups attached to the phosphate group as shown below:
O
H 2C
O
O
HC
O
O
H 2C
O
P
O
CH2
CH2
NH3 +
O–
A Cephalin Molecule (P hosphatidyl Ethanolamine)
Cephalins are found in nervous tissue and are also essential to the clotting of blood. We
have arbitrarily depicted a cephalin based on myristic acid.
Steroids
Steroids are molecules which contain the steroid nucleus, a characteristic arrangement of
rings as shown below:
Steroids are utilized in a variety of roles in the body. Steroids are precursors of Vitamin
D, hormones, bile salts, and structural elements in cell membranes. A few famous steroids are
shown blow.
OH
H
H
H
O
testosterone
O
H
H
H
HO
es trone
H
H
H
HO
cholesterol
Proteins and Amino Acids
Proteins are the building blocks of the human body, and amino acids are, in turn, the
building blocks of proteins. An amino acid consists of a central carbon atom with four groups
arranged arround it:
• a hydrogen atom
• an amine group
• a carboxylic acid
• some other variable group, called the amino acid residue
Amino acids differ from each other according to their residues. The two simplest amino
acids, glycine (hydrogen atom residue), and alanine (methyl group residue) are shown below:
H2N
H
O
C
C
OH
H2N
H
O
C
C
Alanine
CH3
OH
Glycine
H
When amino acids combine with one another, a bond forms between the amine group of
one, and the carboxylic acid group of another. When many amino acids combine, they form a
protein. When only a few combine, the compound is known as a peptide. A simple dipeptide,
made from alanine and glycine, is shown below:
H2N
H
O
H
H
O
C
C
N
C
C
CH3
A dipeptide
OH
H
As noted above, the residues of amino acids can be made of pretty much any organic
functional group. This meaqns that they can consist of a carboxylic acid, or a nitrogenous base,
such as an amine. The presence of one of these groups will alter the acidity of an amino acid. An
acidic amino acid is one that contains a carboxylic acid as a residue, while a basic amino acid
has a residue which is a nitrogenous base.
Aspartic acid is an example of an acidic amino acid, and histidine is an example of a
basic amino acid.
H
C
O
C
C
O
H2 N
H2N
H
C
OH
OH
CH2
CH2
C
Aspartic
Acid
O
Histidine
N
OH
NH
Because an amino acid contains both an acid an base group, it can neutralize itself, that
is, the proton from the carboxylic acid can attach to the basic amine group. Amino acids so
configured have a positive charge at the amino end and a negative charge at the acid end. They
are called zwitterions (German for double ions). A picture of zwitterionic alanine appears on the
next page.
Proteins can be composed of thousands, or more, amino acids. These amino acids can
have acidic or basic residues, which can “neutralize” one another. Since there may be more
acidic than basic residues, or vice versa, it may be necessary to alter the pH of a solution
containing such a protein to achieve precise neutralization. The pH at which this occurs is called
the isolelectric pH.
Proteins generally are least soluble at their isoelectric pH.
ALANINE ZWITTERION
+
H3N
H
O
C
C
O–
CH3
In today’s lab we will undertake the isoelectric precipitation of casein, the principal
protein found in milk. This precipitation takes place naturally when bacteria manufacture lactic
acid, producing sour milk. We will use another acid, namely acetic acid, instead, in order to
induce this isolecrric precipitation.
One interesting dipeptide iks aspartame, a sweetener. It consists of two amino acids,
phenylalanine and aspartic acid. It is commonly referred to as nutra-sweet, and its structure is
shopwn below:
H 2N
H
O
H
H
O
C
C
N
C
C
CH2
C
OH
CH2
O
OH
As partame
Experimental
Carbohydrates
Sucrose is a carbohydrate and concentrated sulfuric acid is a powerful dehydrating agent.
It is so powerful that it can literally rip the water off of a carbohydrate, leaving only the
elemental carbon behind. It does so very exothermically producing a considerable amount of
steam.
Your instructor will add 70 ml of concentrated sulfuric acid to 70 grams of sugar in a 200
ml tall form beaker under a hood. Observe the reaction that takes place and explain your
observations.
Glucose is a reducing sugar. Your instructor will mix a solution of glucose with silver
nitrate in a large flask. Observe any changes that occur and explain them in terms of the ability
of sugars to cause reduction.
Proteins
You will perform an isoelectric precipitation of casein from milk. Place 125 ml of skim
milk into a 250 ml flask. Have ready 25 ml of vinegar and 10 ml of distilled water and 1.5 grams
of sodium bicarbonate.
Heat the milk on a hot plate (med low setting = 4). Swirl the flask and do not let the milk
scorch. As soon as the temperature has reached 35 degrees centigrade, remove the milk from the
burner. Add the vinegar and swirl. You should see the milk protein (the curd) separating from
the aqueous solution containing lactose and other water soluble substances (the whey). Carefully
pour off the whey, keeping as much solid as possible in the flask.
Place the curd on a pile of paper towels and dry it. Then crumble it in small pieces back
into a small beaker. At the water and the bicarbonate of soda an stir with a stick. he mixture will
froth a bit.
Congratulations! You have made carpenter’s glue. In fact, Elmer’s glue (ffrom Borden’s)
is casein glue, with some preservatives added.
Lipids
We know that we can run the esterification of an ester in reverse and use this to make
soap. The process is called saponification of an ester. To do this we need to heat a fat in the
presence of base and water. the products will be glycerine and the sodium salt of the carboxylic
acids that make up the triglycerides.
We will use crisco as our triglyceride and sodium hydroxide as our base. You need to
prepare for this ahead of time and so here is a checklist of what should be present under your
hood.
10 ml of ethanol
10 ml of 6M NaOH
10 grams of crisco (a rounded teaspoon)
a 600 ml beaker, half filled with saturated NaCl solution
a hotplate set to “2”
a thermometer
a 150 ml beaker
a pile of paper towels
a ring stand with a clamp for your thermometer.
a glass stirring rod.
plastic spoons
SAFETY GLASSES MUST BE WORN AT ALL TIMES!!!!!
When you are sure that all is ready, place the crisco into the 150 ml beaker, put this on the
hotplate and melt it. The temperature should be 45 degrees--try not to let it get much hotter.
remove the crisco and let it sit 5 minutes, and lower the setting on the hotplate to “1”.
Replace the crisco on the hotplate and ad all at once the ethanol and the sodium hydroxide.
Monitor the temperature keeping it below 60 degrees at all times. Keep stirring the mixture.
After 10 or 15 minutes the mixture will start to foam and thicken. Keep stirring and heating for 5
minutes after this.
Then remove the beaker and scoop your soap into the saturated NaCL in the 600 ml beaker. You
should get a semisolid soap floating on the surface. Scoop this off the surface with a spoon(wear
gloves!!) onto the paper towels and pat dry. This is your soap.
Put ta few flakes of the soap into 2 small beakers of distilled water water. Put some ordinary
hand soap into 2 other beakers. Test the response of you soap solutions to magnesium ions
(epsom salts) and calcium ions.
Soap scum forms when disoolved carboxylic acid anions are precipitated as an insoluble ionic
salts from trace ions, such as calcium and magnesium, in our water supply.
Finally, use universal pH paper to test the pH of a sdolution of your soap as well as that of a
solution of commercial soap.