35 research outputs found
Magnetic interactions in the catalyst used by nature to split water: a DFT plus U multiscale study on the Mn4CaO5 core in photosystem II
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Erratum to: The Italian Registry of Endovascular Treatment in Acute Stroke: rationale, design and baseline features of patients
The S-2 State of the Oxygen-Evolving Complex of Photosystem II Explored by QM/MM Dynamics: Spin Surfaces and Metastable States Suggest a Reaction Path Towards the S-3 State
Split and polish: Quantum mechanics/molecular mechanics simulations reveal the role of spin surfaces, kinetics, and thermodynamics on the interconversion between two structural models of the {Mn4CaO5} cluster (see picture) in the S2 state of the water-splitting Kok's cycle in photosystem-II. The results account for the temperature, illumination, and procedure dependence of historical EPR experiments and suggest a detailed pathway for the S2 to S3 transition. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Pathway for Mn-cluster oxidation by tyrosine-Z in the S2 state of photosystem II
Water oxidation in photosynthetic organisms occurs through the five intermediate steps S0-S4 of the Kok cycle in the oxygen evolving complex of photosystem II (PSII). Along the catalytic cycle, four electrons are subsequently removed from the Mn4CaO5 core by the nearby tyrosine Tyr-Z, which is in turn oxidized by the chlorophyll special pair P680, the photo-induced primary donor in PSII. Recently, two Mn4CaO5 conformations, consistent with the S2 state (namely, S2 A and S2 B models) were suggested to exist, perhaps playing a different role within the S 2-to-S3 transition. Here we report multiscale ab initio density functional theory plus U simulations revealing that upon such oxidation the relative thermodynamic stability of the two previously proposed geometries is reversed, the S2 B state becoming the leading conformation. In this latter state a proton coupled electron transfer is spontaneously observed at ∼100 fs at room temperature dynamics. Upon oxidation, the Mn cluster, which is tightly electronically coupled along dynamics to the Tyr-Z tyrosyl group, releases a proton from the nearby W1 water molecule to the close Asp-61 on the femtosecond timescale, thus undergoing a conformational transition increasing the available space for the subsequent coordination of an additional water molecule. The results can help to rationalize previous spectroscopic experiments and confirm, for the first time to our knowledge, that the water-splitting reaction has to proceed through the S2 B conformation, providing the basis for a structural model of the S3 state
Magnetic coupling constants and vibrational frequencies by extended broken symmetry approach with hybrid functionals
The description of the electronic structure and magnetic properties of multi-centers transition metal complexes, especially of mixed-valence compounds, still represents a challenge for density functional theory (DFT) methods. The energies and the geometries of the correctly symmetrized low-spin ground state are estimated using the Heisenberg-Dirac-van Vleck spin Hamiltonian within the extended broken symmetry method introduced by Marx and co-workers [Nair et al., J. Chem. Theory Comput. 4, 1174-1188 (2008)]. In the present work we extend the application of this technique, originally implemented using the DFT+U scheme, to the use of hybrid functionals, investigating the ground-state properties of di-iron and di-manganese compounds. The calculated magnetic coupling and vibrational properties of ferredoxin molecular models are in good agreements with experimental results and DFT+U calculations. Six different mixed-valence Mn(III)-Mn(IV) compounds have been extensively studied optimizing the geometry in low-spin, high-spin, and broken-symmetry states and with different functionals. The magnetic coupling constants calculated by the extended broken symmetry approach using B3LYP functional presents a remarkable agreement with the experimental results, revealing that the proposed methodology provides a consistent and accurate DFT approach to the electronic structure of multi-centers transition metal complexes. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4752398
A Spotlight on the Compatibility between XFEL and Ab Initio Structures of the Oxygen Evolving Complex in Photosystem II
The Mn4CaO5 cluster of photosystem II promotes a crucial step in the oxygenic photosynthesis, namely, the water-splitting reaction. The structure of such cluster in the S-1 state of the Kok-Joliot's cycle has been recently resolved by femtosecond X-ray free-electron laser (XFEL) measurements. However, the XFEL results are characterized by appreciable discrepancies with previous X-ray diffraction (XRD), as well as with S-1 models based on ab initio calculations. We provide here a unifying picture based on a combined set of DFT-based structures and molecular dynamics simulations of the S-0 and S-1 states. Our findings indicate that the XFEL results cannot be interpreted on the grounds of a single structure. A combination of two S-1 stable isomers together with a minority contribution of the S-0 state is necessary to reproduce XFEL results within 0.16 angstrom
Vibrational fingerprints of the Mn4CaO5 cluster in Photosystem II by mixed quantum-classical molecular dynamics
A detailed knowledge of the structures of the catalytic steps along the Kok-Joliot cycle of Photosystem II may help to understand the strategies adopted by this unique enzyme to achieve water oxidation. Vibrational spectroscopy has probed in the last decades the intermediate states of the catalytic cycle, although the interpretation of the data turned out to be often problematic. In the present work we use QM/MM molecular dynamics on the S-2 state to obtain the vibrational density of states at room temperature. To help the interpretation of the computational and experimental data we propose a decomposition of the Mn4CaO5 moiety into five separate parts, composed by "diamond" motifs involving four atoms. The spectral signatures arising from this analysis can be easily interpreted to assign experimentally known bands to specific molecular motions. In particular, we focused in the low frequency region of the vibrational spectrum of the S-2 state. We can therefore identify the observed bands around 600-620 cm(-1) as characteristic for the stretching vibrations involving Mn1-O1-Mn2 or Mn3-O5 moieties. (C) 2016 Elsevier B.V. All rights reserved
Photocatalytic and Photovoltaic Properties of TiO2 Nanoparticles Investigated by Ab Initio Simulations
4noTitanium dioxide and TiO2 -based materials are widely used in environmental- and energy-related applications like photocatalysis and photovoltaics, where they are usually employed as nanocrystals or nanostructures. The present contribution is aimed at filling the gap between the vast literature devoted to the simulation of electronic and photochemical properties of TiO2 crystals and surfaces, and the few theoretical studies of photoactivated processes involving instead TiO2 nanostructures.
More specifically, photocatalytic and photovoltaic processes promoted by model TiO2 nanoparticles (NPs) have been investigated by using ab
initio simulations based on the U-corrected density functional theory, and on the time-dependent density functional perturbation theory. We focus on well-investigated processes like the photogeneration of charge carriers in UV-irradiated NPs, the photoreduction of dioxygen and photooxidation of methanol catalyzed by NPs, and the splitting of photogenerated charge carriers occurring at a model NP-dye interface. Our results provide indications on some crucial points of such processes, showing that:
(i) excited charge carriers photogenerated within bare NPs are preferentially trapped as small polarons at surface undercoordinated Ti3+ and O− sites;
(ii) dioxygen and methanol are efficient scavengers of such electrons and holes, respectively, and trigger surface redox processes likely involving proton coupled electron transfer (PCET) steps;
(iii) dye-sensitized NPs are instead characterized by low-energy excited states in which electrons and holes photogenerated within the dye are efficiently split by the TiO2 /dye junction, thus confirming the expected spontaneous formation of charge-separated states in TiO2 -based photovoltaic devices;
(iv) cost-effective theoretical tools can be fruitfully employed to obtain reliable predictions of the photocatalytic properties of nanostructured metal oxides and of the photovoltaic properties of hybrid organic photovoltaic devices.nonenoneMattioli, G.; Amore Bonapasta, A.; Bovi, D.; Giannozzi, P.Mattioli, G.; Amore Bonapasta, A.; Bovi, D.; Giannozzi, Paol
Mechanism of Water Delivery to the Active Site of Photosystem II along the S<sub>2</sub> to S<sub>3</sub> Transition
The two water molecules serving as
substrate for the oxygen evolution
in Photosystem II are already bound in the S<sub>2</sub> state of
the Kok–Joliot’s cycle. Nevertheless, an additional
water molecule is supposed to bind the cluster during the transition
between the S<sub>2</sub> and S<sub>3</sub> states, which has been
recently revealed to have the Mn<sub>4</sub>CaO<sub>5</sub> catalytic
cluster arranged in an open cubane fashion. In this Letter, by means
of ab initio calculations, we investigated the possible pathways for
the binding of the upcoming water molecule. Upon the four different
possibilities checked in our calculations, the binding of the crystallographic
water molecule, originally located nearby the Cl<sup>–</sup> binding site, showed the lowest activation energy barrier. Our findings
therefore support the view in which the W2 hydroxyl group and the
O5 oxygen act as substrates for the oxygen evolution. Within this
framework the role of the open and closed Mn<sub>4</sub>CaO<sub>5</sub> conformers is clarified as well as the exact mechanistic events
occurring along the S<sub>2</sub> to S<sub>3</sub> transition
Mechanism of Water Delivery to the Active Site of Photosystem II along the S<sub>2</sub> to S<sub>3</sub> Transition
The two water molecules serving as
substrate for the oxygen evolution
in Photosystem II are already bound in the S<sub>2</sub> state of
the Kok–Joliot’s cycle. Nevertheless, an additional
water molecule is supposed to bind the cluster during the transition
between the S<sub>2</sub> and S<sub>3</sub> states, which has been
recently revealed to have the Mn<sub>4</sub>CaO<sub>5</sub> catalytic
cluster arranged in an open cubane fashion. In this Letter, by means
of ab initio calculations, we investigated the possible pathways for
the binding of the upcoming water molecule. Upon the four different
possibilities checked in our calculations, the binding of the crystallographic
water molecule, originally located nearby the Cl<sup>–</sup> binding site, showed the lowest activation energy barrier. Our findings
therefore support the view in which the W2 hydroxyl group and the
O5 oxygen act as substrates for the oxygen evolution. Within this
framework the role of the open and closed Mn<sub>4</sub>CaO<sub>5</sub> conformers is clarified as well as the exact mechanistic events
occurring along the S<sub>2</sub> to S<sub>3</sub> transition