Amino Acids Peptides And Proteins Page 17
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471169
24-7
Reactions of Amino Acids
O
H
N
CH
COOH
CH
C
NH
CH
COOH
2
3
CH
CH
2
2
(
)
O
9O
9C
CH
NH
NH
3
2
(acetic anhydride)
N
N
histidine
N-acetylhistidine
O
O
OC 9 Cl
PhCH
2
H
N
CH
COOH
PhCH
O
C
NH
CH
COOH
2
2
(benzyl chloroformate)
CH
CH(CH
)
CH
CH(CH
)
2
3
2
2
3
2
leucine
N-benzyloxycarbonyl leucine
(90%)
The amino group of the N-benzyloxycarbonyl derivative is protected as the amide half
of a carbamate ester (a urethane, Section 21-16), which is more easily hydrolyzed than
most other amides. In addition, the ester half of this urethane is a benzyl ester that
undergoes hydrogenolysis. Catalytic hydrogenolysis of the N-benzyloxycarbonyl
amino acid gives an unstable carbamic acid that quickly decarboxylates to give the
deprotected amino acid.
O
H
O
H
CO
H
, Pd
2
2
CH
O
C
N
CH
COOH
CH
HO
C
N
C C H
COOH
2
3
H
N
CH
COOH
2
CH
C
H
2
2
C
H
2
CH(CH
)
CH(CH
)
3
2
3
2
CH(CH
)
3
2
toluene
a carbamic acid
N-benzyloxycarbonyl
leucine
leucine
PROBLEM 24-18
Give equations for the formation and hydrogenolysis of N-benzyloxycarbonyl methionine.
24-7C
Reaction with Ninhydrin
Ninhydrin is a common reagent for visualizing spots or bands of amino acids that have
been separated by chromatography or electrophoresis. When ninhydrin reacts with an
amino acid, one of the products is a deep violet, resonance-stabilized anion called
Ruhemann’s purple. Ninhydrin produces this same purple dye regardless of the struc-
ture of the original amino acid. The side chain of the amino acid is lost as an aldehyde.
Reaction of an amino acid with ninhydrin
O
O
O
OH
pyridine
H
N
CH
COOH
2
N
CO
2
2
OH
R
CHO
R
O
O
O
amino acid
ninhydrin
Ruhemann’s purple
The reaction of amino acids with ninhydrin can detect amino acids on a wide
variety of substrates. For example, if a kidnapper touches a ransom note with his fin-
gers, the dermal ridges on his fingers leave traces of amino acids from skin secretions.
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