Lecture 9 - The Amino Acids II _ Acid-Base Characteristics
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Transcript of Lecture 9 - The Amino Acids II _ Acid-Base Characteristics
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8/18/2019 Lecture 9 - The Amino Acids II _ Acid-Base Characteristics
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The Amino Acids
II. Acid-Base Characteristics
The Rosy Maple Moth(Dryocampa rubicunda)
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H3N – C – C
O
O-H
R
–
–+
H2N – C – C
O
OHH
R
–
–
non-ionized form of amino acid
does not exist in appreciable amounts
fully ionized form of amino acid
-major form at physio pH (6.8-7.2)
note all amino acids
have ionized carboxyl
group above pH 2
and ionized aminogroup below
pH 9-10
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Before we talk about the ionization of amino acids
lets review the concept of pKa and the acid dissociation constant
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Because Ka values vary over a wide range, they are expressed using a log scale:
pKa = -log10KaThe lower the pKa the stronger the acid.
[HA] Ka
[A-] [H+]
=where Ka is the acid dissociation constant. The larger
the Ka, the stronger the acid.
HA A-H+ +
weak acid conjugate base of HA
The dissociation of a weak acid can be represented by the following equation:
in terms of an actual acid, acetic acid
we could write
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H+ +
weak acid, acetic acid
(CH3COOH)
conjugate base, acetate ion
(CH3COO- )
-
CH3COOH CH3COO-H+ +
or
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[CH3COOH]
Ka [CH3OO
-]
[H+
]= 1.76 x 10-5=
CH3COOH CH3COO-H+ +
so Ka is the acid dissociation constant and indicates the extant to which an
acid is dissociated in an aqueous medium when at equilibrium.
Let’s put this number another way - the percent of the acetic acid molecules
that are dissociated when at equilibrium:
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[CH3COOH] K
a
[CH3COO-] [H+]
=1.76 x 10-5
=
[CH3COOH]
[H+]1.76 x 10-5 =
0.1 M
x2
=
x = 1.33 x 10-3 M =
we can solve for the concentrations of the dissociated products in a 0.1 M solution:
[CH3COO-]
[CH3COO-]
% dissociation = 1.33 x 10-3
M
0.1 M = 0.0133 x 100 = 1.33 %
CH3COOH CH3COO-H+ +
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so, the pKa is just a value that indicates the extent to which an acid dissociates
in aqueous solution at equilibrium
the lower the pKa, the more dissociation and the stronger the acid
H+ +
weak acid, acetic acid
(CH3COOH)
conjugate base, acetate ion
(CH3COO- )
-
H+ +
weak acid, formic acid
(HCOOH)
conjugate base, formate ion
(HCOO- )
- H H
pKa =4.76
pKa =3.75
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conjugate base, acetate ion
(CH3COO- )
- H+ +
H+ + incorrect
correct
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-
H H+
+
H+ +
acetate ion
formate ion
δ+ δ-
pKa =4.76
pKa =3.75
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conjugate base, acetate ion
(CH3COO- )
-
conjugate base, formate ion
(HCOO- )
- H
pKa =4.76
pKa =3.75
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H+ +
weak acid, acetic acid(CH3COOH)
conjugate base, acetate ion(CH3COO- )
-
H+ +
weak acid, formic acid
(HCOOH)
conjugate base, formate ion
(HCOO- )
- H H
pKa =4.76
pKa =3.75
H+ +
HH
HH
- pKa =15
conjugate base, ethoxide ion(CH3CH2O- )
ethanol(CH3CH2OH )
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H3N – C – C
O
O-H
R
–
–+
H2N – C – C
O
OHH
R
–
–
non-ionized form of amino acid
does not exist in appreciable amounts
fully ionized form of amino acid
-major form at physio pH (6.8-7.2)
note all amino acids
have ionized carboxyl
group above pH 2
and ionized aminogroup below
pH 9-10
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pKa 2.21 9.15 -- pKa 2.20 9.11 10.07
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end of pKa review
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The Ionization of Amino acids
• All amino acids possess two ionizable groups, the amino group and the carboxyl
group.
• seven of the amino acids also possess ionizable side chains as well.
• the ionization of these groups follow simple acid-base chemistry rules
• since amino acids (and proteins) are most relevant at physiological pH (6.8-7.4)
we are most interested in their ionic states around this pH
• the ionic state of an amino acid influences its reactivity and its ability to participatein chemical reactions
f ll i i d f f i id
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H3N – C – C
O
O-H
R
–
–+
H2N – C – C
O
OHH
R
–
–
non-ionized form of amino acid
does not exist in appreciable amounts
fully ionized form of amino acid
-major form at physio pH
note all amino acids
have ionized carboxyl
group above pH 2
and ionized aminogroup below
pH 9-10
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recall,
that for any acid, say, the amino acid alanine:
NH3CHCH3COOH H+ + NH3CHCH3COO-
and
[NH3CHCH3COOH]
Ka [NH3CHCH3COO-][H
+
]=
pKa = -log10Ka
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H3N – C – C
O
O-H
CH3
–
–+
H2N – C – C
O
OHH
CH3
–
–
– C
O
OH – C
O
O-
pKa = 2.3
at pH < pKa the protonated form (weak acid) predominates
at pH > pKa, the ionized (conjugate base) form predominatessince physiological pH is 7, the carboxylate group is always
unprotonated
when the pH = pKa, both forms have equal concentrations
Veri ication using t e Hen erson Hasse a c equation
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pH = pKa[COOH]
[COO-]
+ log
Veri ication using t e Hen erson-Hasse a c equation
When the environmental pH is equal to the pKa:
2.3 = 2.3
[COOH]
[COO-]
+ log
0 =[COOH]
[COO-]
log
100 =[COOH]
[COO-]
solve for the ratio of
conjugate base/weak acidby taking antilog of 0
1 =[COOH]
[COO-]
thus when pH = pKa, the
concentrations of both
species are equal
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Practice Questions (Solutions will be posted on D2L).
1. When the pH = pKa of the ionization of the carboxyl group on an amino acid,
the protonated and unprotonated forms of the carboxyl group are at
equal concentrations in a population of amino acid molecules.
Calculate the ratio of protonated to unprotonated carboxyl groups (pKa 2.3)
when the environmental pH is pH 1.0 (i.e less than the pKa).
Calculate the ratio at pH 5.
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We can do the same thing for the amino group on alanine
H3N – C – C
O
O-H
CH3
–
–+
H2N – C – C
O
OHH
CH3
–
–
+NH3 – pKa = 9.7
NH2 –
at pH < pKa the protonated form predominates
at pH > pka, the unprotonated form predominatessince physiological pH is 7, the amino group is always
protonated and this gives it a positive charge
when the pH = pKa, both forms have equal concentrations
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1. At pH less than 2.3
both groups are protonated
since pH is below both their
pKa’s.
2. When pH = pKa (2.3), protonated
and non-protonated carboxyl
groups at equal concentrations.
3. At physiological pH, both
groups are ionized.
4. At pH above 9.7, the amino
group loses its proton.
The changing ionic (charge) state of an amino acid as pH increases
( )
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so, at any pH between 2.3 and 9.7 (which includes physiological pH range of 6.8-7.4)
both the amino and carboxylate groups are ionized
notice this graph introduces
a new quantity, pI
pI means isoelectric point
pI for an amino acid is
the pH at which the
predominant form of the
amino acid is neutral.
i.e no net charge
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so, the pI is the pH at which the amino acid carries no net charge (is neutral)
Calculating the Isoelectric point (pI) for the amino acid alanine:
pI =2
(pK1 + pK2)
pI =2
(2.3 + 9.7)
pI = 6therefore, alanine is a neutral
chemical at pH of 6
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All amino acids possess ionizable amino and carboxyl groups, but seven of the
amino acids also have ionizable side chains as well.
Predict the form of the side chain at physiological pH (pH 6 8-7 4)
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Predict the form of the side chain at physiological pH (pH 6.8 7.4)
Remember: pH < pKa → group is protonated
pH > pKa → group is unprotonated
pH > pKa
pH > pKa
pH < pKa
pH < pKa
pH < pKa
pH < pKa
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Calculating the pI for amino acids with ionizable side chains is straight-forward
but a little different than for amino acids without ionizable side chains.
C id th ti l h d i id l t t
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Consider the negatively charged amino acid glutamate
pI =2
(pK1 + pK2)
=2
(2.19 + 4.25)
= 3.22
Nelson p85
2.19
4.25
4.252.19
9.67
9.67
Notice that the calculation of pIalways involves the pKa’s that
bracket the neutral charge
Glutamate:
pKacarboxyl = 2.19pKaamino = 9.67
pKasidechain = 4.25
Notice that at pH above its
pI, glutamate is predominantly
negatively charged
Consider the positively charged amino acid lysine
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2.18
8.95
10.79
10.798.952.18
Nelson p85
Lysine:
pKacarboxyl = 2.18
pKaamino = 8.95
pKasidechain = 10.79
pI =
2
(pK2 + pK3)
=
2
(10.79 + 8.95)
= 9.87
Consider the positively charged amino acid lysine
Notice that at pH below its
pI, lysine is predominantlypositively charged
Practice Questions (Solutions will be posted on D2L)
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Practice Questions (Solutions will be posted on D2L).
2. Given the above data, calculate the pI for the amino acid isoleucine.
3. Calculate the pI for the amino acid tyrosine.
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Proteins are of course long polymers of amino acids
To understand the structure and function of proteins, we need to understand
the structure and function of amino acids.
… the pentapeptide Tyrosine-Glycine-Glycine-Phenylalanine-Leucine
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Essential vs non-essential amino acids for humans
Essential in this context means “required in diet”
-all amino acids are actually essential for life
Essential Non-essential
Histidine Cysteine
Isoleucine AlanineLeucine Serine
Lysine Tyrosine
Methionine Proline
Phenylalanine Glutamate
Threonine Aspartate
Tryptophan Arginine
Valine Glutamine
Asparagine
Glycine
Structure of
glucose
CHNOPS