27 research outputs found
Electron Transfer from Cyt b559 and Tyrosine-D to the S2 and S3 states of the water oxidizing complex in Photosystem II at Cryogenic Temperatures
The Mn4CaO5 cluster of photosystem II (PSII) catalyzes the oxidation of water to molecular oxygen through the light-driven redox S-cycle. The water oxidizing complex (WOC) forms a triad with Tyrosine(Z) and P-680, which mediates electrons from water towards the acceptor side of PSII. Under certain conditions two other redox-active components, Tyrosine(D) (Y-D) and Cytochrome b (559) (Cyt b (559)) can also interact with the S-states. In the present work we investigate the electron transfer from Cyt b (559) and Y-D to the S-2 and S-3 states at 195 K. First, Y-D (aEuro cent) and Cyt b (559) were chemically reduced. The S-2 and S-3 states were then achieved by application of one or two laser flashes, respectively, on samples stabilized in the S-1 state. EPR signals of the WOC (the S-2-state multiline signal, ML-S-2), Y-D (aEuro cent) and oxidized Cyt b (559) were simultaneously detected during a prolonged dark incubation at 195 K. During 163 days of incubation a large fraction of the S-2 population decayed to S-1 in the S-2 samples by following a single exponential decay. Differently, S-3 samples showed an initial increase in the ML-S-2 intensity (due to S-3 to S-2 conversion) and a subsequent slow decay due to S-2 to S-1 conversion. In both cases, only a minor oxidation of Y-D was observed. In contrast, the signal intensity of the oxidized Cyt b (559) showed a two-fold increase in both the S-2 and S-3 samples. The electron donation from Cyt b (559) was much more efficient to the S-2 state than to the S-3 state
PHILOSOPHICAL TRANSACTIONS -OF THE ROYAL SOCIETY QM/MM computational studies of substrate water binding to the oxygen-evolving centre of photosystem II
This paper reports computational studies of substrate water binding to the oxygen-evolving centre (OEC) of photosystem II (PSII), completely ligated by amino acid residues, water, hydroxide and chloride. The calculations are based on quantum mechanics/molecular mechanics hybrid models of the OEC of PSII, recently developed in conjunction with the X-ray crystal structure of PSII from the cyanobacterium Thermosynechococcus elongatus. The model OEC involves a cuboidal Mn3Ca04Mn metal cluster with three closely associated manganese ions linked to a single |Li4-oxo-ligated Mn ion, often called the 'dangling manganese'. Two water molecules bound to calcium and the dangling manganese are postulated to be substrate molecules, responsible for dioxygen formation. It is found that the energy barriers for the Mn(4)-bound water agree nicely with those of model complexes. However, the barriers for Ca-bound waters are substantially larger. Water binding is not simply correlated to the formal oxidation states of the metal centres but rather to their corresponding electrostatic potential atomic charges as modulated by charge-transfer interactions. The calculations of structural rearrangements during water exchange provide support for the experimental finding that the exchange rates with bulk lsO-labelled water should be smaller for water molecules coordinated to calcium than for water molecules attached to the dangling manganese. The models also predict that the Sx->S2 transition should produce opposite effects on the two water exchange rates
QM/MM computational studies of substrate water binding to the oxygen-evolving centre of photosystem II
This paper reports computational studies of substrate water binding to the oxygen-evolving centre (OEC) of photosystem II (PSII), completely ligated by amino acid residues, water, hydroxide and chloride. The calculations are based on quantum mechanics/molecular mechanics hybrid models of the OEC of PSII, recently developed in conjunction with the X-ray crystal structure of PSII from the cyanobacterium Thermosynechococcus elongatus. The model OEC involves a cuboidal Mn3CaO4Mn metal cluster with three closely associated manganese ions linked to a single μ4-oxo-ligated Mn ion, often called the ‘dangling manganese’. Two water molecules bound to calcium and the dangling manganese are postulated to be substrate molecules, responsible for dioxygen formation. It is found that the energy barriers for the Mn(4)-bound water agree nicely with those of model complexes. However, the barriers for Ca-bound waters are substantially larger. Water binding is not simply correlated to the formal oxidation states of the metal centres but rather to their corresponding electrostatic potential atomic charges as modulated by charge-transfer interactions. The calculations of structural rearrangements during water exchange provide support for the experimental finding that the exchange rates with bulk 18O-labelled water should be smaller for water molecules coordinated to calcium than for water molecules attached to the dangling manganese. The models also predict that the S1→S2 transition should produce opposite effects on the two water-exchange rates
Electron Transfer Kinetics in CdS Nanorod–[FeFe]-Hydrogenase Complexes and Implications for Photochemical H<sub>2</sub> Generation
This Article describes the electron
transfer (ET) kinetics in complexes
of CdS nanorods (CdS NRs) and [FeFe]-hydrogenase I from Clostridium acetobutylicum (CaI). In the presence
of an electron donor, these complexes produce H<sub>2</sub> photochemically
with quantum yields of up to 20%. Kinetics of ET from CdS NRs to CaI
play a critical role in the overall photochemical reactivity, as the
quantum efficiency of ET defines the upper limit on the quantum yield
of H<sub>2</sub> generation. We investigated the competitiveness of
ET with the electron relaxation pathways in CdS NRs by directly measuring
the rate and quantum efficiency of ET from photoexcited CdS NRs to
CaI using transient absorption spectroscopy. This technique is uniquely
suited to decouple CdS→CaI ET from the processes occurring
in the enzyme during H<sub>2</sub> production. We found that the ET
rate constant (<i>k</i><sub>ET</sub>) and the electron relaxation
rate constant in CdS NRs (<i>k</i><sub>CdS</sub>) were comparable,
with values of 10<sup>7</sup> s<sup>–1</sup>, resulting in
a quantum efficiency of ET of 42% for complexes with the average CaI:CdS
NR molar ratio of 1:1. Given the direct competition between the two
processes that occur with similar rates, we propose that gains in
efficiencies of H<sub>2</sub> production could be achieved by increasing <i>k</i><sub>ET</sub> and/or decreasing <i>k</i><sub>CdS</sub> through structural modifications of the nanocrystals. When
catalytically inactive forms of CaI were used in CdS–CaI complexes,
ET behavior was akin to that observed with active CaI, demonstrating
that electron injection occurs at a distal iron–sulfur cluster
and is followed by transport through a series of accessory iron–sulfur
clusters to the active site of CaI. Using insights from this time-resolved
spectroscopic study, we discuss the intricate kinetic pathways involved
in photochemical H<sub>2</sub> generation in CdS–CaI complexes,
and we examine how the relationship between the electron injection
rate and the other kinetic processes relates to the overall H<sub>2</sub> production efficiency