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A2 CHEMISTRY NOTES – AMINO ACIDS
Amino Acids
What are amino acids?
Amino acids are are compounds containing an amino group, -NH2, and a carboxylic acid group,
-COOH. The biologically important amino acids have the amino group attached to the carbon
atom next door to the -COOH group. They are known as ________ ______. The two
simplest of these amino acids are 2-aminoethanoic acid and 2-aminopropanoic acid. Because
of the biological importance of molecules like these, they are normally known by their
traditional biochemical names. 2-aminoethanoic acid, for example, is usually called _______,
and 2-aminopropanoic acid is usually known as ___________.
The general formula for a 2-amino acid is:
. . . where "R" can be -OH, -SH, other amine or carboxylic acid groups.
Physical properties
Melting points
The amino acids are crystalline solids with surprisingly _____ melting
points. It is difficult to pin the melting points down exactly because the
amino acids tend to decompose before they melt. Decomposition and
melting tend to be in the 200 - 300°C range. Because it has both a basic
_______ group and an acidic _________ acid group there is an internal
transfer of a __________ ion from the -COOH group to the -NH2 group
to leave an ____ with both a negative charge and a positive charge. This is
called a ______________.
This is the form that amino acids exist in even in the solid state. Instead of
the weaker _________ bonds and other intermolecular forces that you might
have expected, you actually have much stronger ______ attractions between
one ion and its neighbors. These ionic attractions take more energy to break
and so the amino acids have ______ melting points for the size of the molecules.
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A2 CHEMISTRY NOTES – AMINO ACIDS
Optical activity
Apart from glycine, 2-aminoethanoic acid, the carbon at the centre
of the structure of the general formula for amino acids has
_______ different groups attached.
This is equally true of the __________ instead of this simpler
structure.
Because of these ______ different groups
attached to the same carbon atom, amino acids (apart from
glycine) are _______. The lack of a plane of symmetry means
that there will be two stereoisomers of an amino acid (apart
from glycine) - one the non-superimposable mirror image of the
other. For a general 2-amino acid, the isomers are:
All the naturally occurring amino acids have the right-hand structure in this diagram. This is
known as the "L-" configuration. The other one is known as the "D-" configuration.
Adding an alkali to an amino acid solution
If you increase the pH of a solution of an
amino acid by adding ________ ions, the
hydrogen ion is removed from the ____ group.
Adding an acid to an amino acid solution
If you decrease the pH by adding an ____ ions
to a solution of an amino acid, the _______ part
of the zwitterion picks up a ____ ion.
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A2 CHEMISTRY NOTES – AMINO ACIDS
Peptides and polypeptides
Glycine and alanine can combine together with the elimination of a molecule of ______
to produce a dipeptide.
Either:
Or:
In each case, the linkage shown in the structure of the dipeptide is known as a
_________ link. In chemistry, this would also be known as an ______ link. If you
joined lots and lots together (as in a protein chain), you get a ______________.
A protein chain will have somewhere in the range of __ to ______ amino acid residues.
Hydrolysis of polypeptides
H
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A2 CHEMISTRY NOTES – AMINO ACIDS
The Structure of Proteins
The primary structure of proteins
This is the term is used to describe the order of the ______ acids joined together to
make the protein. This primary structure is usually shown using abbreviations for the
amino acid residues. These abbreviations commonly consist of three letters or one
letter, for example:
Amino acids join together to make polypeptides which in turn make up proteins.
Enzymes and many structues within cells are made up of proteins.
The chemical bonds
in the _______ link
hold the primary
structure together
__________ bonds hold the
secondary, tertiary and
quaternary structures together
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A2 CHEMISTRY NOTES – AMINO ACIDS
Chromatography of amino acids
Amino acids have no ________. Therefore all of these procedures are carried out "blind", and the results will be
seen when a revealing agent (____________) is sprayed on the resulting chromatogram.
1. Chromatography paper is cut and marked with a __________ line and labeled as shown.
2. Different pipette are used to place a drop of each amino acid on the paper.
3. A small amount of chromatography solvent (butanol/ethanoic acid) is poured into the glass chamber (to
about 1 cm depth) and a lid is used to allow the atmosphere to become saturated with vapour.
4. The solvent gradually rises up the paper, passing the line and heading upwards.
After about 3 hours, the liquid should have risen about three-quarters of the height of the paper.
5. The paper is removed from the apparatus, and a pencil is used to mark the position of the solvent front.
6. The chromatograms is placed into an oven at about 45 C to dry.
7. In a fume cupboard __________ is sprayed evenly over the paper.
8. The chromatograms is returned to the oven to develop the _______. Spots should be visible as _______
smears on the paper.
Calculation of Rf values
The distance from the start line to the solvent front and to the front of each spotis
measured. An Rf value for each spot is calculated (Rf means relative to front):
distance moved by spot
distance moved by solvent front
The values obtained can be with reference Rf values. Different solvents and different
types or makes of chromatogaphy papers will give slightly different results.
Amino Acid
alanine
arginine
asparagine
glutamic acid
glycine
histidine
serine
threonine
tryptophan
Rf Value
0.38
0.20
0.5
0.30
0.26
0.11
0.27
0.35
0.66
Amino Acid
aspartic acid
cysteine
glutamine
isoleucine
leucine
lysine
methionine
phenylalanine
Praline not a true amino acid
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Rf Value
0.24
0.4
0.13
0.72
0.73
0.14
0.55
0.68
0.43