Environmental Engineering Reference
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N
OH
(a)
(b)
N
O
A
+
A
- C
6
H
5
N
2
+
D
radical coupling
N
OH
N
NN
O
N
OH
N
OH
N
OH
OH
N
N
N
O
O
HO
(
2
)
HO
A
(
3
)
(
1
)
H
m/z
= 321
N
OH
m/z
= 319
N
n = 3;
m/z
= 637
HO
SOG
N
OH
N
- C
6
H
5
N
2
+
E
O
+
C
+
D,
+
E
B
+ H
2
O
N
OH
N
OH
N
OH
N
OH
N
N
N
N
O
OH
+
C
O
NH
O
O
O
N
O
OH
- e
-
,
- H
+
N
HO
N
n = 3;
m/z
= 737
(
5
)
n = 2;
m/z
= 533
(
4
)
HN
N
N
N
- N
2
n = 3;
m/z
= 765
(
6
)
D
E
n = 3;
m/z
= 841
(
7
)
Fig. 4 Proposed mechanism for the biotransformation of SOG by CotA-laccase (a) and proposed
structures (1)
-
(7) for the oxidation products (b). The oxidation of azo dyes occurs without the
cleavage of the azo bond, through a highly non-speci
c free radical mechanism resulting in the
formation of phenolic type compounds. Following this mechanism, CotA-laccase oxidizes one
hydroxyl group of SOG generating the phenoxyl radical A, sequentially oxidized to a carbonium ion
(B). The water nucleophilic attack on the phenolic carbonium, followed by N
C bond cleavage,
produces diazenylbenzene (C) and the 4-hydroxy-1,2-benzoquinone. The diazenylbenzene (C) can
lead to the radical (D) and then, to a benzene radical (E) upon loss of a nitrogen molecule. All these
radicals were involved in further coupling reactions (adapted from Pereira et al.
2009b
)
-
(
a)
(b)
1.0
150
125
0.8
100
0.6
75
2h
1h30
0.4
50
1h
30min
10min
0.2
25
2.5min
t0
0.0
0
300
400
500
600
700
800
0102030405060
Wavelength [nm]
Retention time [min]
Fig. 5 Time course for acid blue 62 (AB62) biotransformation as monitored by absorbance
(a) and by HPLC (b). [(b) white square AB62 and products: black circle R
t
13 min and, black
square R
t
40 min] (adapted from Pereira et al.
2009a
)
(R
t
= 50 min) (Fig.
5
b). The proposed mechanism of biotransformation of AB62 by
laccases is illustrated in Fig.
6
, showing the pathway for formation of an azo bound
in (4) which is responsible for the color observed in the reaction mixtures.
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