27.3 Acid-Base Behavior of Amino Acids - PowerPoint PPT Presentation

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27.3 Acid-Base Behavior of Amino Acids

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called a zwitterion or. dipolar ion. Acid-Base Properties of Glycine ... Therefore, the more stable neutral form of glycine is the zwitterion. O. OH. H3NCH2C ... – PowerPoint PPT presentation

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Title: 27.3 Acid-Base Behavior of Amino Acids


1
27.3Acid-Base Behavior of Amino Acids
2
Recall
  • While their name implies that amino acids are
    compounds that contain an NH2 group and a CO2H
    group, these groups are actually present as NH3
    and CO2 respectively.
  • How do we know this?

3
Properties of Glycine
  • The properties of glycine
  • high melting point (when heated to 233C it
    decomposes before it melts)solubility soluble
    in water not soluble in nonpolar solvent

more consistent with this
than this
4
Properties of Glycine
  • The properties of glycine
  • high melting point (when heated to 233C it
    decomposes before it melts)solubility soluble
    in water not soluble in nonpolar solvent

more consistent with this
called a zwitterion or dipolar ion
5
Acid-Base Properties of Glycine
  • The zwitterionic structure of glycine also
    follows from considering its acid-base
    properties.
  • A good way to think about this is to start with
    the structure of glycine in strongly acidic
    solution, say pH 1.
  • At pH 1, glycine exists in its protonated form
    (a monocation).

6
Acid-Base Properties of Glycine
  • The zwitterionic structure of glycine also
    follows from considering its acid-base
    properties.
  • A good way to think about this is to start with
    the structure of glycine in strongly acidic
    solution, say pH 1.
  • At pH 1, glycine exists in its protonated form
    (a monocation).

7
Acid-Base Properties of Glycine
  • Now ask yourself "As the pH is raised, which is
    the first proton to be removed? Is it the proton
    attached to the positively charged nitrogen, or
    is it the proton of the carboxyl group?"
  • You can choose between them by estimating their
    respective pKas.

8
Acid-Base Properties of Glycine
  • Now ask yourself "As the pH is raised, which is
    the first proton to be removed? Is it the proton
    attached to the positively charged nitrogen, or
    is it the proton of the carboxyl group?"
  • You can choose between them by estimating their
    respective pKas.

typical ammonium ion pKa 9
typical carboxylic acid pKa 5
9
Acid-Base Properties of Glycine
  • The more acidic proton belongs to the CO2H group.
    It is the first one removed as the pH is raised.

typical carboxylic acid pKa 5
10
Acid-Base Properties of Glycine
  • Therefore, the more stable neutral form of
    glycine is the zwitterion.

typical carboxylic acid pKa 5
11
Acid-Base Properties of Glycine
  • The measured pKa of glycine is 2.34.
  • Glycine is stronger than a typical carboxylic
    acid because the positively charged N acts as an
    electron-withdrawing, acid-strengthening
    substituent on the a carbon.

typical carboxylic acid pKa 5
12
Acid-Base Properties of Glycine
A proton attached to N in the zwitterionic form
of nitrogen can be removed as the pH is increased
further.
  • The pKa for removal of this proton is 9.60.This
    value is about the same as that for NH4 (9.3).

13
Isoelectric Point pI
  • The pH at which the concentration of the
    zwitterion is a maximum is called the isoelectric
    point. Its numerical value is the average of the
    two pKas.
  • The pI of glycine is 5.97.

pKa 2.34
pKa 9.60
14
Acid-Base Properties of Amino Acids
  • One way in which amino acids differ is in respect
    to their acid-base properties. This is the basis
    for certain experimental methods for separating
    and identifying them.
  • Just as important, the difference in acid-base
    properties among various side chains affects the
    properties of the proteins that contain them.
  • Table 27.2 gives pKa and pI values for amino
    acids with neutral side chains.

15
Table 27.2 Amino Acids with Neutral Side Chains
pKa1 2.34pKa2 9.60pI 5.97
Glycine
16
Table 27.2 Amino Acids with Neutral Side Chains
pKa1 2.34pKa2 9.69pI 6.00
Alanine
17
Table 27.2 Amino Acids with Neutral Side Chains
pKa1 2.32pKa2 9.62pI 5.96
Valine
18
Table 27.2 Amino Acids with Neutral Side Chains
pKa1 2.36pKa2 9.60pI 5.98
Leucine
19
Table 27.2 Amino Acids with Neutral Side Chains
O
H
pKa1 2.36pKa2 9.60pI 5.98


H3N
O
Isoleucine
C
C
CH3CHCH2CH3
20
Table 27.2 Amino Acids with Neutral Side Chains
O
H
pKa1 2.28pKa2 9.21pI 5.74


H3N
O
Methionine
C
C
CH3SCH2CH2
21
Table 27.2 Amino Acids with Neutral Side Chains
pKa1 1.99pKa2 10.60pI 6.30
Proline
22
Table 27.2 Amino Acids with Neutral Side Chains
pKa1 1.83pKa2 9.13pI 5.48
Phenylalanine
23
Table 27.2 Amino Acids with Neutral Side Chains
pKa1 2.83pKa2 9.39pI 5.89
Tryptophan
24
Table 27.2 Amino Acids with Neutral Side Chains
O
H
pKa1 2.02pKa2 8.80pI 5.41


H3N
O
Asparagine
C
C
25
Table 27.2 Amino Acids with Neutral Side Chains
pKa1 2.17pKa2 9.13pI 5.65
Glutamine
26
Table 27.2 Amino Acids with Neutral Side Chains
pKa1 2.21pKa2 9.15pI 5.68
Serine
27
Table 27.2 Amino Acids with Neutral Side Chains
O
H
pKa1 2.09pKa2 9.10pI 5.60


H3N
O
Threonine
C
C
CH3CHOH
28
Table 27.3 Amino Acids with Ionizable Side Chains
O
H
pKa1 1.88pKa2 3.65pKa3 9.60 pI 2.77


H3N
O
Aspartic acid
C
C
  • For amino acids with acidic side chains, pI is
    the average of pKa1 and pKa2.

29
Table 27.3 Amino Acids with Ionizable Side Chains
O
H
pKa1 2.19pKa2 4.25pKa3 9.67 pI 3.22


H3N
O
Glutamic acid
C
C
30
Table 27.3 Amino Acids with Ionizable Side Chains
pKa1 2.20pKa2 9.11pKa3 10.07 pI 5.66
Tyrosine
31
Table 27.3 Amino Acids with Ionizable Side Chains
pKa1 1.96pKa2 8.18pKa3 10.28 pI 5.07
Cysteine
32
Table 27.3 Amino Acids with Ionizable Side Chains
pKa1 2.18pKa2 8.95pKa3 10.53pI 9.74
Lysine
  • For amino acids with basic side chains, pI is the
    average of pKa2 and pKa3.

33
Table 27.3 Amino Acids with Ionizable Side Chains
pKa1 2.17pKa2 9.04pKa3 12.48pI 10.76
Arginine
34
Table 27.3 Amino Acids with Ionizable Side Chains
pKa1 1.82pKa2 6.00pKa3 9.17 pI 7.59
Histidine
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