Environmental Engineering Reference
In-Depth Information
react with other components present in the cells, the most efficient of which is O
2
that
can form
O
2
•−
and then H
2
O
2
. The latter species are often detected in cells as discussed
in the earlier sections. It is also established that H
2
O
2
formation is the primary step of
many photoinduced processes in aqueous solution that finally lead to the formation of
the HO
•
radical (see chapter
“
Photoinduced and Microbial Generation of Hydrogen
Upon excitation, an electron is transferred from the Chls to the Pheo HA,
producing the charge-separated state P680
+
H
A
−
as assumed by earlier studies
(Germano et al.
2004
; Rockley et al.
1975
; Thurnauer et al.
1975
; Shuvalov and
Klevanik
1983
; Kirmaier and Holten
1987
; Holzapfel et al.
1990
). Similarly, in
PSI a primary charge separation occurs in the P700 reaction center that can lead
to the reduction of A
0
(two chlorophylloid primary electron acceptors), creating
the radical ion pair P700
+
A
0
−
(Krauß
2003
; Brettel
1997
; Müller et al.
2010
;
Webber and Lubitz
2001
; Fromme et al.
2001
). However, no concrete evidence
has been found for the formation of these types of radicals in PSI or PSII. Rather,
experimental studies support the idea that primary electron transfer reactions are
accompanied by molecular readjustments or reorganizations involving pigments
and proteins, or the interaction of pigment-protein complexes in the reaction
center (Dashdorj et al.
2004
; Kleinfeld et al.
1984
; Woodbury and Parson
1984
;
Kirmaier et al.
1985a
,
b
; Holten et al.
1986
; Kirmaier et al.
1986
; Tiede et al.
1987
; Mullineaux et al.
1993
; Savikhin et al.
2001
; Karapetyan
2004
).
It is also observed that chlorophyll-binding PsbS protein (22-kD protein of
PSII), which belongs to the family of light-harvesting proteins, can contribute only
to quenching but not to light harvesting (Li et al.
2000
,
2002
; Aspinall-O'Dea et
al.
2002
; Bergantino et al.
2003
). Indeed, the degree of fluorescence quenching in
vivo can correlate with the content of PsbS (Li et al.
2004
). Dissipation of energy
in PSI trimers of cyanobacteria takes place with a contribution of the long-wave-
length chlorophyll, and the excited state of which is quenched by the cation radical
of P700 or by P700 in its triplet state (Karapetyan
2004
). The low fluorescence
yield of Chls in light-harvesting antenna complexes is indicative of an additional
pathway of energy dissipation in oligomers, which would protect the PSII complex
of cyanobacteria against photodestruction (Karapetyan
2004
).
It can thus be hypothesized that excitation followed by charge transfer could
produce P680
+
O
2
•
−
instead of
P680
+
H
A
−
.O
2
is the primary acceptor for excited
electrons in aquatic media and is involved in the production of H
2
O
2
as dis-
cussed earlier. This result is supported by Laser flash photolysis studies, in which
a charge-transfer excited state has not been detected from the spectra. Recovery
kinetics, including observation of both triplet decay and ground-state folding reac-
tions, show that the flash transient obtained from the pinned form consists of a
triplet and of a ground state moiety in the unpinned configuration (Periasamy et al.
1978
). Experimental optical data and structure-based simulations showed nanosec-
ond absorption dynamics at ~685 nm, after excitation of PS I from
Synechocystis
sp. PCC 6803. It is suggested that the electrochromic shift of absorption bands
of the Chl
a
pigments may occur around the secondary electron acceptor, through
considerable protein relaxation (Dashdorj et al.
2004
; Savikhin et al.
2001
).
