Amino acids as amphoteric
compounds
•
•
•
•
Acidity
Basicity
pKa
Electronic and structural features that
influence acidity and basicity
General Structure of Amino
Acidblocks of proteins
Building
•
• Carboxylic acid group
• Amino group
• Side group R gives unique characteristics
Amino acids are polar
• Due to presence
– polar covalent bonds
– N, O and H atoms - are capable to form
hydrogen bonds with water
– Carry charges COO- and NH3+
The water solubility of amino acids vary to
some extend, depending of side chain
Carries positive charge
when pH<6
Learning Check
•
Classify the following amino acids as hydrophobic
(nonpolar), hydrophilic (polar, neutral), acidic, or basic:
A.
Lysine (polar
basic)
C.
Serine (polar
neutral)
B.
D.
Leucine
(nonpolar)
Aspartate (polar
acidic)
Can act as acid (proton donor)
and base (proton acceptor)
The structure is dependent on pH – due to
presence -COOH and -NH2
• R – COOH  R – COO- + H+
acid
conjugate base
+  R – NH + H+
• conjugate
R – NH
3
2
acid
base
pKa of –COOH [1.8-4.3], therefore at pH 7 is COOpKa of –NH2 [9.1-12.5], therefore at pH 7 is NH3+
Zwitterion
• At a particular pH, the amino acid carries no
net charge and is called a zwitterion.
• Zwitterion …. dipolar ion –
has 1 positive and 1
negative charge
• Amphoteric (ampholytes)
• pH, at which the amino acid has a net charge
of zero is called the isoelectric point (pI),
• At the isoelectric point (pI), the + and –
charges are equal.
pH and ionization (1)
In solutions more basic than the pI, the —NH3+ in the
amino acid donates a proton and become (-NH2) .
In solution more acidic than the pI, the COO- in the amino
acid accepts a proton and become (-COOH).
OH–
H+
+
H3N–CH2–COOH
+
H3N–CH2–COO–
H2N–CH2–COO–
Positive ion
zwitterion
Negative ion
Low pH
neutral pH
High pH
By rearranging the above equation we arrive
at the Henderson-Hasselbalch equation:
pH = pKa + log[A-]/[HA]
The Henderson-Hasselbalch Equation
At the point of the dissociation where the
concentration of the conjugate base [A-] = to that
of the acid [HA]:
pH = pKa + log[1]
The log of 1 = 0. Thus, at the mid-point of a titration of a weak acid:
pKa = pH
The term pKa is that pH at which an equivalent
distribution of acid and conjugate base (or base and
conjugate acid) exists in solution.
[R - COO-] [H+]
;
KCOOH=
[R - COOH]
KNH3+
[R - NH2] [H+]
=
[R - NH3+]
pKa of –NH2 [9.1-12.5]
pKa of –COOH [1.8-4.3]
• For an amino acid with only one amine and one
carboxyl group, the pI can be calculated from
the mean of the pKa of this molecule:
pI = (pKa1 + pKa2)/2
Leucine:
pI =
pKCOOH + pK NH3+
2
2,36 + 9,6
=
= 5,98
2
pH and Ionization (2)
• Acidic amino acids such as aspartic acid have a second
carboxyl group that can donate and accept protons.
• If there were three titratable groups or other dissociating
side chain groups, the pI equation would involve all three
pKa's and the denominator would be "3“
pI = (pKa1 + pKa2 + pKa3)/3
• The pI for aspartic acid occurs at a pH of 2.8
Learning Check
Glu ionization in water.
•
Indicate ionizable
groups.
–
–
–
Predict ionization of this
amino acid at pH=1.0
Predict ionization of this
amino acid at pH=10.0
Predict ionization of this
amino acid at pH=7.0
Glutamic Glu
Acid
pKa(COOH) pKa(NH2)
pKa(R)
2.19
4.25
9.67
Peptides and Proteins
Amino terminal-
Carboxyl terminal-
N-terminal-
C-terminal
Oligopeptide :a few amino acids
Polypeptide : many amino acids
Tetrapeptide
1.
Acid-base behavior of a peptide:
N-terminal, C-terminal, R-groups
2.
Peptides have a characteristic
titration curve and a characteristic
pI value
Acidity of organic compounds
• Proton can be formed during break of
C-H, N-H, O-H or S-H bonds.
• Acidity of organic compounds increases in the
following way:
C-H acids < N-H acids < O-H acids < S-H acids
Acidic properties
• Strength of an acid depends on the
stability of the formed anion.
• If the formed anion is stable, it does not
form the stable undissociated acid
molecule and therefore there are H+ in
the medium.
Stability of acid anions depends on
• Electronegativity of
the atom to which
hydrogen is
attached.
• Radius of the atom
to which hydrogen
is attached.
• Delocalization of
negative electric
charge.
Acidity and electronegativity
• The more electronegative an
element is, the more it helps to
stabilize the negative charge of
the conjugate base.
• Acidity increases as the atom to
which hydrogen is attached
becomes more electronegative.
Thus, acidity increases:
CH4 < NH3 < H2O < HF
(pKa values are 48, 38, 16 and 3
respectively)
Basicity and and pKa values
• Basicity is related to the ability of a
compound to use its nonbonding
electrons to combine with a proton.
• A strong base has a large pKa.
Basicity and electronegativity
• Basicity will decrease as an atom
becomes more electronegative.
• Oxygen is more electronegative than
nitrogen, therefore its electrons are less
likely to be donated to a proton.
Basicity and electronic properties
• Proton can attach to the free electron pair.
• Basicity increases where electrons are not
delocalizated.
.. increase
..
• Basic properties
in.. the row:
S-H < O-H < N-H
Delocalization effects
• Delocalization of charge in the conjugate base
anion through resonance is a stabilizing factor
and will be reflected by an increase in acidity.