Proteins, Amino Acids, and pH

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Proteins, Amino Acids, and pH
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Weak Base
Weak Acid
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Amino acid as a zwitterion
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If tyrosine is buried deep inside a protein, it
is not readily accessible and a high pH is
required to remove it. From the shape of the
titration curve it is possible to deduce the
approximate pKa of each amino acid. The pKa
values tells us about its environment.
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Isoionic point is pH of pure protein.
We will derive expressions for these two points.
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Comparison of Amino Acids and
Diprotic Acids
Ka1
Ka1Ka2
Kw
The MB and CB equations are
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Rewrite CB Eqn as a function of H to calculate
the pH of a diprotic acid
If we make the approximation
then
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If CH2A.100M then
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How does this compare to the pH of an amino acid
in its intermediate form? This is the so-called
isoionic point.
I will now derive this expression.
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Rewrite CB Eqn as a function of H to calculate
the pH of an amino acid in its intermediate form
to obtain one equation as a function of H.
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Rewrite CB Eqn as a function of H
Insert expressions for the ai to obtain
Multiply both sides of the equation by HKa1 to
obtain
Add CHA H2 to both sides and then divide by
CHAKa1 to get
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The Isoelectric Point
At its isoelectric point that average charge of
all forms of a protein is 0. Therefore at this
point it wont migrate in an electric field. The
isoelectric point is a function of
pH. Isoelectric focusing is a technique where a
mixture of proteins are subject to a strong
electric field in a medium designed to have a pH
gradient
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Finding the Isoelectric Point
Definition of isoelectric point is
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At its isoelectric point that average charge of
all forms of a protein is 0. Here
How does this compare to the pH of an amino acid
in its intermediate form? This is the so-called
isoionic point.
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The pH of an amino acid in its intermediate form
simplifies. For example if Ka110-4 and
Ka210-10 and CHA10-1 then
Taking the log gives
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Is the pH of an amino acid in its intermediate
form greater than or less than 7?
or
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We can understand this result based on the
following rxtns
Ka1 Ka2
Or if we rewrite the first reaction so that we
have
Kb2 Ka2
HA is both an acid and a base. If Kb2gt Ka2 then
HA is a stronger base than it is an acid. This
is consistent with
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A question from previous years exam
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Here are the modified equations for this problem
Here are the equations for pure amino acid
The MB and CB equations are
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Using mass balance write in terms of the ais.
Now include the dilution factors, i.e.,