Environmental Engineering Reference
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hydroxyl radical can then degrade the proteins or amino acid residues. Such an
effect can reduce the contents of proteins or amino acid residues, which are often
observed in PSI and PSII (Neufeld et al. 2004 ; Shutova et al. 2005 ). The decom-
position of proteins or amino acids (e.g. tryptophan) is also generally observed in
sunlit water environments because of the effects of HO
and other ROS (Mostofa
et al. 2007 , 2010 , 2011 ; Moran et al. 2000 ).
Fourth, are there any O 2 or H 2 O 2 molecules that may remain undetected among
the 1,300 water molecules found in PSII? It is consistent to detect O 2 and H 2 O 2
molecules in the PSII structure, which have often been observed in earlier studies.
The occurrence of a large number of H 2 O molecules suggests that O 2 may remain
and be dissolved in those water molecules. Furthermore, H 2 O 2 may be produced
photolytically from O 2 as discussed before. Two facts may be responsible for not
detecting O 2 or H 2 O 2 : (i) O 2 and H 2 O 2 may disappear during the primary process-
ing of the photosynthetic cells before examination; and (ii) former studies did not
focus on the occurrence of H 2 O 2 in PSII. In a recent study, it has been assumed
that H 2 O 2 may be “lost” amongst 1,300 H 2 O molecules (Umena et al. 2011 ). The
most likely reasons would be the structural similarity and the fact that H 2 O 2 occur-
rence in the PSII structure was not expected. It should be noted that H 2 O 2 may be
decomposed to H 2 O during the processing of photosynthetic cells for the determi-
nation of PS crystal structure.
It has been shown that two H 2 O molecules in four reaction-center Chls are
linked through H-bonding between water ligand and Chl D1 (Umena et al. 2011 ),
and it may well be H 2 O 2 that can make H-bonding in the proposed structure. It is
also shown that two balls labeled I and II represent a single water molecule, dis-
ordered at two different positions separated by 1.8 Å. Position-I is able to H-bond
to YD (redox-active tyrosine residue located at D2-tyr 160), whereas position-II is
not able to H-bond to YD (Supplementary part) (Umena et al. 2011 ). On this basis,
it can be assumed that H 2 O 2 may occur in that structure instead of H 2 O. Note that
the bond length of O-O in H 2 O 2 is 1.49 Å, which is larger than in the ground
(triplet) state of molecular oxygen ( 3 O 2 , 1.21 Å) (Abrahams et al. 1951 ). Among
the 1300 H 2 O molecules in each PSII monomer, a few of them are detected as dis-
ordered (Umena et al. 2011 ), a case in which the probability to mistakenly detect
H 2 O instead of H 2 O 2 is relatively high. Future studies will be important to find out
any presence of H 2 O 2 instead of H 2 O in the crystal structure of PSII.
The first two questions will be discussed comprehensively in the next section.
4.2 Mechanism for Electron Transfer and O 2 -Release
in Photosystem II Reaction Centers
Upon excitation by light, the electron release takes place at the central part of the
reaction center (RC), at the primary donor P700 in PSI or P680 in PSII (Figs. 4 , 5 )
(Müller et al. 2010 ; Nilsson Lill 2011 ; Umena et al. 2011 ). It is suggested that the
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