Download Peptides and Amino Acids Amide Bonds and Peptides

11/30/2012
Peptides and Amino Acids
Hydrolysis of Peptide Bonds
and
Chromatographic Analysis
of Amino Acids
Amide Bonds and Peptides
The amide bond is a strong, resonance
stabilized functional group.
O
O
C
C
N
N
H
H
When amino acids are linked together by
amide bonds, we refer to the products as
peptides and the bond as a peptide bond.
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Example of a Tetrapeptide
Peptides are sometimes classified by the
number of amino acid components. The
structure below is a tetrapeptide with three
amide or peptide bonds.
O
H2N
C
O
NH
C
CH3
NH
C
OH
NH
O
O
Peptides in Nature
Although polypeptides such as enzymes contain
hundreds of amino acid units, many small naturally
occurring peptides have been discovered. An
example is the octapeptide angiotensin II.
H2N
NH
OH
NH
O
H2N
O
NH
NH
CH2CO2H
O
N
NH
O
NH
NH
O
N
NH
O
NH
CO2H
O
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Structure of Peptides
Polypeptides are polyamides, but the
monomeric units are not identical as they
are in polyamides such as nylon.
In order to deduce the structure of a
polypeptide, we need to know which amino
acids are present, and the order in which
they are linked together.
We can convert a polypeptide into a mixture
of amino acids by hydrolysis.
Hydrolysis of a Tetrapeptide
O
H2N
C
CH3
O
NH
C
O
NH
C
OH
NH
O
hydrolysis
mixture of alanine,
leucine, valine, and
phenylalanine
The amino acids present can be determined
by chromatographic analysis.
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Analysis of Amino Acid Mixture
Commercial amino acid analyzers use
column chromatography to determine which
amino acids are present and the relative
amounts of each one.
In today’s experiment, you will use thin
layer chromatography (TLC) to separate
and identify amino acids.
Ionic Nature of Amino Acids
Because of the presence of both acidic and
basic functional groups, an amino acid will
always have a charge on at least one
functional group!
O
H2N
C
CH3
more basic
O
O
H3N
C
O
O
CH3
isolelectric point
H3N
C
OH
CH3
more acidic
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TLC of Aspartic Acid
One of the amino acids you will analyze today is
aspartic acid. The solvent system is 20% acetic
acid. Draw the structure of aspartic acid that
should be the one actually traveling up the TLC
plate in this solution. (Write down your answer.)
O
H2N
OH
aspartic acid
CH2CO2H
TLC of Aspartic Acid
One of the amino acids you will analyze today is
aspartic acid. The solvent system is 20% acetic
acid. Draw the structure of aspartic acid that
should be the one actually traveling up the TLC
plate in this solution. (Write down your answer.)
O
H3N
OH
CH2CO2H
structure of
aspartic acid
in acidic
conditions
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Aspartame
O
H2N
C
CH2
CO2H
OCH3
N
H
O
Aspartame®
Methyl aspartylphenylalaninate
Experimental Procedure
Today you will hydrolyze (under basic
conditions) a sample of Aspartame and
analyze the mixture of amino acids by TLC.
You will also perform TLC analysis of some
soft drinks containing aspartame to look for
Aspartame and for the constituent amino
acids.
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Procedure (Cont.)
Since Aspartame® is a methyl ester of
phenylalanine, hydrolysis of Aspartame®
should liberate 1 equiv. of methanol.
You will use qualitative tests with KMnO4
and with Cerium(IV) salts to test for the
presence of methanol or other alcohols in all
samples.
Procedure (Cont.)
Be sure to not contaminate your test
samples from materials from another
sample or your results will be incorrect.
Your instructor will now answer any
questions.
This is your last experiment of the semester!
Congratulations.
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