Environmental Engineering Reference
In-Depth Information
formation of H
2
O
2
in the alkaline medium with pH 12.4 (Lobanov et al.
2008
;
Bruskov and Masalimov
2002
). Formation of H
2
O
2
from Chl can generally be
expressed as follows (Eq.
3.40
) (Lobanov et al.
2008
): at pH < 7,
1
/
2O
2
+
H
+
+
Chl
+
h
υ →
1
/
2H
2
O
2
+
Chl
+
(3.40)
where redox potentials (
Δϕ
°) and Gibbs energy changes (
Δ
G
0
) for the reduc-
tion of O
2
to H
2
O
2
with simultaneous oxidation of Chl to the radical cation
(
T
=
298 K) are
−
0.03 V and 5.8 kJ for H
2
O
2
generation, 1.83 V and
−
353 kJ
for the singlet excited state of Chl, as well as 1.23 V and
−
237 kJ for the triplet
excited state of Chl, respectively.Similarly at pH > 7 (Eq.
3.41
),
1
/
2O
2
+
Chl
+
h
υ →
1
/
2HO
2
−
+
Chl
+
(3.41)
where
Δϕ
° and
Δ
G
0
for the reduction of O
2
to HO
2
−
with simultaneous oxidation
of Chl to the cation radical (
T
=
298 K) are
−
0.80 V and 154 kJ for HO
2
−
gen-
eration, 1.06 V and
−
204 kJ for the singlet excited state of Chl, and 0.46 V and
−
89 kJ for the triplet excited state of Chl, respectively (Lobanov et al.
2008
).
In addition, H
2
O
2
is significantly formed photolytically in aqueous mixtures of
Chl and either micelles of cetyltrimethylammonium bromide (CTAB) or macro-
molecules of bovine serum albumin (BSA) in a noncovalent complex. Insuch a
case, Chl acts as a photocatalyst (Lobanov et al.
2008
). The Chl may affect the
donors of electron density, polarize chemical bonds, and stabilize reaction inter-
mediates (similar to enzyme-substrate complexes) by the occurrence of N-, O-,
and S-containing functional groups bound in proteins and lipids (Lobanov et al.
2008
).
Under certain physiological conditions such as exposure to high light inten-
sity or drought, reduction of O
2
in photosynthetic organisms can produce reac-
tive oxygen species (ROS), such as
O
2
•−
,H
2
O
2
or
1
O
2
.
These species can lead
to the closure of the stomata and cause low CO
2
concentrations in the chloro-
plasts (Krieger-Liszkay et al.
2008
; Asada
1992
,
2006
Halliwell and Gutteridge
1990
; Hideg et al.
2001
,
2002
; Trebst et al.
2002
). It is shown that a key ROS in
UV-irradiated leaves is
O
2
•−
,
whilst
1
O
2
is minor (Hideg et al.
2002
). Therefore,
H
2
O
2
may be produced in the plant cells via
O
2
•−
.
Under such conditions, the
plastoquinone pool can be in a very highly reduced state that would allow pho-
toinhibition, i.e. the light induced loss of PSII activity (Adir et al.
2003
). The
HO
•
produced photolytically from H
2
O
2
or
1
O
2
and ROS itself can react with
proteins, pigments, nucleic acids and lipids, and could also be connected to the
light-induced loss of PSII activity, to the degradation of the D1 polypeptide (PSII
reaction centre polypeptide) and to pigment bleaching (Krieger-Liszkay et al.
2008
; Aro et al.
1993
; Nishiyama et al.
2001
,
2004
; Vass et al.
1992
; Hideg et al.
1994
; Keren et al.
1997
; Halliwell and Gutteridge
1990
; Sopory et al.
1990
; Prasil
et al.
1992
; Hideg et al.
1998
; Okada et al.
1996
;
2006
; Allakhverdiev and Murata
2004
; Nixon et al.
2005
; Hideg et al.
2007
; Aro
2007
; Tyystjärvi
2008
). Such reac-
tions are often observed in water, where photoinduced generation of HO
•
either
from H
2
O
2
(both upon direct photolysis by sunlight and photo-Fenton reaction)
or other sources (e.g.
NO
2
−
and NO
3
−
) can decompose the DOM components
