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27.1
Classification of Amino Acids
Classification of Amino Acids
amino acids are classified as , , , etc. to
indicate where the nitrogen atom is relative to the
carboxylic acid:
O
H
H
O
N 
H
N
H
O
H

O
N
-Amino Acid
-Amino Acid
H
O

N
H


H

-Amino Acid
O
Selected Amino Acids
1-Aminocyclopropanecarboxylic Acid
NH2
an -amino acid that is an
intermediate in the biosynthesis
of ethylene
OH

O

H
3-Aminopropanoic Acid
O
H
N
O

N
H

N
H
O

H

O
a -amino acid that is one
of
the structural units present
in
coenzyme A
4-Aminobutanoic
Acid
a -amino acid involved in
the transmission of nerve
impulses
-Amino Acids are Most
Ubiquitous in Nature
H
H
O
N 
N
O
H

O
N
-Amino Acid
-Amino Acid
H
O

N
H

H
H
O

H

-Amino Acid
O
Revision: Amines are Brønsted Bases
N
H
X
N
X
Amines are electron-rich and have a reactive
lone pair, which can form a covalent bond to
a proton to form an ammonium ion.
Amino Acids Exist as Zwitterions
While their name implies that amino acids
are compounds that contain an amine (—
NH2) and a carboxylic acid (-CO2H), these
groups are actually present as their
conjugate acid (—NH3+) and conjugate base
(—CO2–), respectively.
Amino Acids Exist as Zwitterions
Basic
Amine
H
H
O
N 
H
H
H
H
O
H
O
N 
Acidic
Carboxylic
Acid
H
O
H
N 
Resonance-Stabilized Zwitterion
O
O
Zwitterions Defined
Zwitterionic Compounds/Zwitterions
neutral compounds having formal unit electrical
charges of opposite sign. Some chemists restrict
the term to compounds with the charges on nonadjacent atoms. Sometimes referred to as inner
salts, dipolar ions (a misnomer), e.g. +H3N-CH2CO2ammonioacetate (glycine).
IUPAC Compendium of Chemical Terminology
Amino Acids Exist as Zwitterions
H
H
O
H
H
N
N
H
O
O
H
O
What evidence do we have for this
behavior………
Properties of Glycine Reflect
its Zwitterionic Structure
High Water Solubility
glycine is soluble in water but not in nonpolar solvents.
High Melting Point
when heated to 233°C it decomposes
before it melts.
Properties of Glycine Reflect
its Zwitterionic Structure
The physical properties
of glycine are
consistent with this
structure
H
O
H
O
H
N
H
H
O
N
H
O
Polar Zwitterion
27.2
Stereochemistry of -Amino
Acids
Non-Polar Side Chains: Alanine
O
Alanine
(Ala, A)
H3N
Me
CO2
O
H
H3N
H
Me
Alanine, valine, leucine, and isoleucine have alkyl
groups as side chains, which are non-polar and
hydrophobic
Non-Polar Side Chains: Valine
O
Valine
(Val, V)
H3N
Me
CO2
O
H3N
H
H
Me
Me
Me
Non-Polar Side Chains: Phenylalanine
O
Phenylalanine
(Phe, F)
H3N
CO2
O
H3N
H
The side chain in phenylalanine (a nonpolar amino acid) is a benzyl group.
H