Photosynthesis in Nature: A New Look - Photobiogeochemistry of Organic Matter - page 584

can absorb between 676 and 684 nm (Telfer et al. 1990 ; Durrant et al. 1995 ;
Renger and Marcus 2002 ). Red shifts are commonly observed in in vitro Chl a
systems, such as thin films, monolayers and colloidal dispersions, used as models
for the in vivo system (Katz et al. 1991 ). It is known that red shifts occur when the
release of electrons takes place in the functional groups that is bound to the com-
ponent system (see also chapter “ Colored and Chromophoric Dissolved Organic
Matter in Natural Waters ” , “ Fluorescent Dissolved Organic Matter in Natural
Waters ” ) (Mostofa et al. 2009 ; Senesi 1990 ). Note that Chl a has a broad absorp-
tion spectrum and can form dimers or aggregates through self assembly, which
typically leads to changes in its optical properties (Shipman et al. 1976 ; Hynninen
and Lötjönen 1993 ; Closs et al. 1963 ; Katz et al. 1963 ; Fong 1974 ; Shipman et
al. 1975 ; Katz 1990 , 1994 ; Frackowiak et al. 1994 ). Formation of the dimer often
occurs through H-bonding in the N-heterocyclic base pair (Catalan et al. 2004 ),
which can support the occurrence of H-bonding between N and H 2 O (Fig. 7 b).
Two possible hydrogen bonds were also discussed in earlier studies. First, for-
mation of H-bonds might occur between central Mg and H 2 O according to the
Mg…OH 2 interaction (Hynninen and Lötjönen 1993 ). Second, the keto carbonyl
group of Chl a may participate in the formation of Chl a dimers, either through
coordination with Mg or through H-bonding of the H-X type, where X = O,
N and S (Shipman et al. 1976 ; Closs et al. 1963 ; Katz et al. 1963 ; Fong 1974 ;
Shipman et al. 1975 ; Katz 1990 ). However, these two previous assumptions are
not possible electronically because the outer shells of Mg are entirely full, after
bonding with two covalent bonds and two unpaired π -electron systems with four
N-atoms of the Chl a . Therefore, Mg has less probability to accept further elec-
trons or H-bonding with other groups. Moreover, the formation of such proposed
bonding systems is not consistent with the easiest way of electron release via
absorption in the longer wavelength region.
Crystal structures of the reaction center have identified two chlorophyll mono-
mers forming a dimer with a partial structural overlap, which are thus stabilized
by van der Waals interactions (Nilsson Lill 2011 ). The structure of the chlorophyll
dimer has been optimized using dispersion-corrected density functional theory
(B3LYP-DCP) and it has been found that the dimerization energy is approximately
− 17 kcal mol − 1 (Nilsson Lill 2011 ). Electrons may be rapidly released from these
resonance configurations upon irradiation of the Chl a dimmer, according to the
proposed dimer formation (Fig. 6 ). This can be understood from the interaction
mechanism between the functional group [-CH 2 -(NH 3

+

)-CH-COO − ] in trypto-

+

)CHCOO − ] and metal ions, where the functional group

phan [C 8 H 5 (NH)-CH 2 (NH 3

)-CH-COO − ] can display resonance configuration that is responsible
for the longer wavelength fluorescence emission spectra (see chapter “ Complexation
of Dissolved Organic Matter With Trace Metal Ions in Natural Waters ” ).
PSII acts as one component and upon irradiation, the released electron may not
accept the same component of PSII that can be understood from aquatic ecosystem. For
example, in aqueous media fulvic acid or humic acid upon irradiation can donate the
electron to O 2 and form O 2 •− and then H 2 O 2 , which is a well-accepted mechanism by
all aquatic scientists. Therefore, it is hypothesized that the released electron in PSII may

+

[-CH 2 -(NH 3