charge separation ensures highly efficient use of the photon energy, while the subsequent thermal
steps move the charges ever-further apart to eliminate energy-wasting back reactions. The
resulting separated charges live long enough to provide the energy necessary to drive the
metabolic processes of the bacteria (Blankenship 2002; Woodbury and Allen 1995).
Photosystem I and II in Green Plants and
Cyanobacteria. Photosystem I (PSI)
functions to provide the chemical reducing
agents that fix CO 2 in the form of
carbohydrates. An x-ray structure of PSI,
obtained at 2.5-Å resolution, shows that the
PSI core is a large pigment-protein complex
(Grotjohann et al. 2004). The largest two
subunits bind the majority of the RC and core
antenna pigments. Once again, these proteins
display a symmetric structure analogous to
that found in the RC from purple bacteria.
While the active components of PSI are
chemically different from those of purple
bacteria, the central paradigm — light-
harvesting energy transfer from the antenna
to a chlorophyll-based primary electron
donor in the RC, photoinduced charge
separation, and several subsequent thermal
electron transfer steps — is maintained. The
overall complexity of PSI, as indicated by the
number and nature of the molecular species
participating in the overall process, is much
higher than that exhibited by the purple
bacteria.
Photosystem II (PSII) catalyzes one of the
most energetically demanding reactions in
biology: using the energy of light to drive a
catalyst capable of oxidizing water. The
crystal structure of PSII, at a resolution of
3.5 Å, reveals that the PSII core complex
consists of 19 proteins,^ while the central
protein subunits show striking similarities to
the protein structure of the bacterial RC
(Ferreira et al. 2004). Photoexcitation of the
primary chlorophyll electron donor in the
PSII RC once again results in electron
transfer, followed by a cascade of thermal
electron transfer steps. The important
PHOTOSYSTEM II: USING LIGHT TO SPLIT WATER2H 2 O O 2 + 4H+ + 4e4 hν
2H 2 O O 2 + 4H+ + 4e4 hν
2H 2 O O 2 + 4H+ + 4e4 hνPhoto-induced charge separation within PSII creates a
chlorophyll species called P680+, one of the most powerful
oxidants known in biology. P680+ provides the oxidizing
power to split water into O 2 and H+ in the Mn-containing,
oxygen-evolving complex (OEC) within the protein. The
structure of the OEC is shown in detail above. The OEC
contains one Ca and four Mn atoms, along with bridging
oxygen atoms between them. The OEC splits water by losing
four electrons, one at a time. These oxidative equivalents are
accumulated following sequential absorption of single
photons by the primary electron donor (P680) and
subsequent charge separation: P680 + Chl → P680+ + Chl-,
where Chl is a nearby chlorophyll electron acceptor.
Understanding the overall coupling between photo-induced
electron transfer within PSII and the functioning of the OEC is
essential to developing bio-inspired systems for fuels
production, because efficient water splitting, coupled to
catalytic reduction of H+ to H 2 , is one of the most important
routes to clean solar fuels.