Amino Acids
They are classified as , , , etc. amino acids
according the carbon that bears the nitrogen.
+
an -amino acid that is an
NH3

CO2–
+
–
H3NCH2CH2CO2

+
–
H3NCH2CH2CH2CO2

intermediate in the biosynthesis
of ethylene
a -amino acid that is one of
the structural units present in
coenzyme A
a -amino acid involved in
the transmission of nerve
impulses
Table 27.1
Glycine is achiral. All of the other amino acids in
proteins have the L-configuration at their carbon.
H O
+
–
H3N C C O
R
The major differences among the side chains:
Size and shape
Electronic characteristics
Table 27.1
General categories of -amino acids
nonpolar side chains
polar but nonionized side chains
acidic side chains
basic side chains
Properties of Glycine
The properties of glycine:
high melting point: (when heated to 233°C
it decomposes before it melts)
solubility: soluble in water; not soluble in
nonpolar solvent
more consistent with this
than this
•• O ••
+
H3NCH2C
•• O ••
•• •–
O•
••
••
H2NCH2C
called a zwitterion or dipolar ion
••
OH
••
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).
•• O ••
+
H3NCH2C
••
OH
••
Acid-Base Properties of Glycine
Therefore, the more stable neutral form of
glycine is the zwitterion.
•• O ••
+
H3NCH2C
•• •–
O•
••
•• O ••
+
H3NCH2C
••
OH
••
typical
carboxylic
acid: pKa ~5
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  carbon.
•• O ••
+
H3NCH2C
••
OH
••
typical
carboxylic
acid: pKa ~5
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.
•• O ••
+
H3NCH2C
•• •–
O•
••
HO
–
•• O ••
••
H2NCH2C
•• •–
O•
••
The pKa for removal of this proton is 9.60.
This value is about the same as that for NH4+
(9.3).
Isoelectric Point pI
•• O ••
+
H3NCH2C
••
OH
••
pKa = 2.34
•• O ••
+
H3NCH2C
•• •–
O•
••
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 = 9.60
•• O ••
••
H2NCH2C
•• •–
O•
••
Can be used for
identification or purification
Titration Curve Alanine
Table 27.3
Amino Acids with Acidic Side Chains
H
Aspartic acid
+
H3N
–
C
OCCH2
O
C
O
–
pKa1 =
pKa2 =
pKa3 =
pI =
1.88
3.65
9.60
2.77
O
For amino acids with acidic side chains, pI is the
average of pKa1 and pKa2.
Table 27.3
Amino Acids with Basic Side Chains
H
+
H3N
C
O
C
O
–
+
CH2CH2CH2CH2NH3
pKa1 =
pKa2 =
pKa3 =
pI =
2.18
8.95
10.53
9.74
Lysine
For amino acids with basic side chains, pI is the
average of pKa2 and pKa3.
Synthesis of Amino Acids
From -Halo Carboxylic Acids
O
CH3CHCOH + 2NH3
Br
H2O
O
–
CH3CHCO + NH4Br
+NH3
(65-70%)
Strecker Synthesis
O
CH3CH
NH4Cl
NaCN
CH3CHC
N
NH2
1. H2O, HCl, heat
2. HO–
O
–
CH3CHCO
+NH3
(52-60%)
Using Diethyl Acetamidomalonate
O
O
C
C
C
CH3CH2O
CH3CNH
H
OCH2CH3
O
Can be used like as diethyl malonate.
Example
O O
CH3CH2OCCCOCH2CH3
H
CH3CNH
O
1. NaOCH2CH3
2. C6H5CH2Cl
O O
CH3CH2OCCCOCH2CH3
CH3CNH
O
CH2C6H5
(90%)
O O
Example
HOCCCOH
–CO2
CH2C6H5
H3 N
+
O
HBr, H2O, heat
HCCOH
H3N
+
CH2C6H5
O O
(65%)
CH3CH2OCCCOCH2CH3
CH3CNH
O
CH2C6H5
Acylation of Amino Group
The amino nitrogen of an amino acid can be
converted to an amide with the customary
acylating agents.
O
O O
+
– +
H3NCH2CO
CH3COCCH3
O
O
CH3CNHCH2COH
(89-92%)
Esterification of Carboxyl Group
The carboxyl group of an amino acid can be
converted to an ester. The following illustrates
Fischer esterification of alanine.
O
+
– +
H3NCHCO
CH3CH2OH
CH3
HCl
O
Cl
–
+
H3NCHCOCH2CH3
CH3
(90-95%)
Ninhydrin Test
Amino acids are detected by the formation of a purple
color on treatment with ninhydrin.
O
O
OH
+
+ H3NCHCO–
OH
R
O
O
O–
O
RCH + CO2 + H2O +
N
O
O
Decarboxylation
Decarboxylation is a common reaction of amino acids. An example is the conversion of
L-histidine to histamine. Antihistamines act by
blocking the action of histamine.
N
–
CH2CHCO2
N
H
+ NH3
N
CH2CH2 NH2
enzymes
N
H
+ CO2,
Neurotransmitters
–
–
+
H3N
H
CO2
CO2
H
H
+
H3N
H
H
H
H2N
H
H
H
HO
HO
OH
L-Tyrosine
OH
L-3,4-Dihydroxyphenylalanine
L-DOPA for Parkinsons Disease
OH
Dopamine
Neurotransmitters
H
H2N
H
H
H
OH
HO
CH3NH
H
H
OH
HO
OH
Norepinephrine
OH
Epinephrine
Peptides
Peptides are compounds in which an amide
bond links the amino group of one -amino acid
and the carboxyl group of another.
An amide bond of this type is often referred to
as a peptide bond.
H
+
H3 N
N-terminus
Ala—Gly
H
O
C
C
CH3
N
H
C
O
C
C-terminus
H
Alanylglycine
–
O
AG