1. 1
1
A
A
(a) Draw general structure of α–amino acid
(b) Name a given amino acid according to the IUPAC nomenclature.
(c) Define the terms zwitterion and isoelectric point, pI
(d) Draw the structures of a given amino acid
(i) In acidic medium
(ii) In basic medium
(iii) At pI
Objective :
1
CHAPTER 12 : INTRODUCTION AMINO ACID
2. ❑ Amino acids are molecules with two functional
groups: Amino group (-NH2) and Carboxyl
group (- COOH)
❑ Amino acids & proteins = biopolymers
❑ Amino acid commonly found in proteins are α-
amino acid
❑ Acid amino on which both carboxyl and amino
groups are bonded to the SAME CARBON atom.
General structure of α-amino acid
H2
N
C C-
OH
H O
R
Carboxyl
group
Amino
group
IUPAC nomenclature
❑ The names of α-amino acids given are common names.
❑ In IUPAC nomenclature, α-amino acids are named as
carboxyl (-COOH)group as the parent name whereas
amino(NH2) group as substituent group.
❑ However, IUPAC names are not normally used for α-amino
acids
❑ Determine longest carbon chain -COOH always as
Carbon1(parent) -NH2 group as a substituent
H-CH-
C
NH
2
O
H
O
Parent🡪
ethanoic acid
aminoethanoic acid
(glycine)
(1)
2
3. ❑ Determine longest carbon chain
❑ -COOH always as Carbon1(parent)
❑ -NH2 group as a substituent
CH3-
CH-C
N
H2
O
H
O
2-aminopropanoic acid
(alanine)
Parent 🡪 propanoic acid
CH3CH2-
CH-C
NH
2
O
H
O
2-aminobutanoic acid
Parent 🡪 butanoic acid
(2)
(3)
3
6. Question
Give the IUPAC name
(i) CH2(NH2)COOH
(ii) CH3CH(NH2)COOH
(iii) CH2(OH)CH(NH2)COOH
(iv) NH2CH2CH2CH2CH(NH2)COOH
(v) HOOCCH2CH2CH2CH(NH2)COOH
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7. Dipolar ions = zwitterion
❑ Amino acids are white crystalline solids
❑ α-Amino acids are dipolar ions. The term used for dipolar ion is zwitterion or
internal salt.
❑ Zwitterion = is molecule of amino acid that has both positive charge and
negative charge in the same molecule but at different atom. The net charge of
zwitterion is zero.
❑ In neutral solution and in solid state, amino acids exist as zwitterion.
❑ A zwitterion formed when a proton from the –COOH group is donated to the –NH2
group of the same molecule.
Physical Properties
❑ Amino acids are amphoteric in solution where pH = pI.
❑ Which it can react as acid or base depending on the pH of the solution
7
8. A zwitterion formed when a proton from the –COOH
group is donated to the –NH2 group of the same molecule.
H-CH-
C
NH
2
O-
H
O
H-CH-C
N+H3
O-
O
Neutral
(uncharge)
Zwitterion (dipolar ion)
However, the
molecules has
no net charge
Zwitterion
Isoelectric point, pI
❑ Isoelectric point = The pH whereby the concentration of
zwitterion is at maximum
❑ At pI, zwitterion has no net charge.
❖ Each amino acids has it’s own specific pI.
❖ For example: ALANINE
❖ Isoelectric point for Alanine is at pH 6.02
❖ At this pH, alanine exist as zwitterion
H-CH-C
N+H3
O-
O
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9. H-
CH-C
N
H2
O
H
O
Amino acids
H-
CH-C
N+
H3
O
H
O
In Acidic Solution
H-CH-C
NH2
O-
O
In Basic Solution At isoelectronic, pI
H-CH-
C
N+H
3
O
-
O
In acidic
Cations are
predominates
In basic
Anions are
predominates
In NEUTRAL
Amino acids can exists in 3 forms depending on the pH of the solution.
Zwitterions are
Predominates
Exists at
isoelectronic points
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