Jose A. Gascon

The mechanism of photosynthetic water splitting

Oxygenic photosynthesis, which provides the biosphere with most of its chemical energy, uses water as its source of electrons. Water is photochemically oxidized by the protein complex photosystem II (PSII), which is found, along with other proteins of the photosynthetic light reactions, in the thylakoid membranes of cyanobacteria and of green plant chloroplasts. Water splitting is catalyzed by the oxygen-evolving complex (OEC) of PSII, producing dioxygen gas, protons and electrons. O(2) is released into the atmosphere, sustaining all aerobic life on earth; product protons are released into the thylakoid lumen, augmenting a proton concentration gradient across the membrane; and photo-energized electrons pass to the rest of the electron-transfer pathway. The OEC contains four manganese ions, one calcium ion and (almost certainly) a chloride ion, but its precise structure and catalytic mechanism remain unclear. In this paper, we develop a chemically complete structure of the OEC and its environment by using molecular mechanics calculations to extend and slightly adjust the recently-obtained X-ray crystallographic model with reference to this structure and to some important recent experimental results.

Computational insights into the O2-evolving complex of photosystem II.

Mechanistic investigations of the water-splitting reaction of the oxygen-evolving complex (OEC) of photosystem II (PSII) are fundamentally informed by structural studies. Many physical techniques have provided important insights into the OEC structure and function, including X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy as well as mass spectrometry (MS), electron paramagnetic resonance (EPR) spectroscopy, and Fourier transform infrared spectroscopy applied in conjunction with mutagenesis studies. However, experimental studies have yet to yield consensus as to the exact configuration of the catalytic metal cluster and its ligation scheme. Computational modeling studies, including density functional (DFT) theory combined with quantum mechanics/molecular mechanics (QM/MM) hybrid methods for explicitly including the influence of the surrounding protein, have proposed chemically satisfactory models of the fully ligated OEC within PSII that are maximally consistent with experimental results. The inorganic core of these models is similar to the crystallographic model upon which they were based, but comprises important modifications due to structural refinement, hydration, and proteinaceous ligation which improve agreement with a wide range of experimental data. The computational models are useful for rationalizing spectroscopic and crystallographic results and for building a complete structure-based mechanism of water-splitting in PSII as described by the intermediate oxidation states of the OEC. This review summarizes these recent advances in QM/MM modeling of PSII within the context of recent experimental studies.

The MoD-QM/MM methodology for structural refinement of photosystem II and other biological macromolecules

Quantum mechanics/molecular mechanics (QM/MM) hybrid methods are currently the most powerful computational tools for studies of structure/function relations and structural refinement of macrobiomolecules (e.g., proteins and nucleic acids). These methods are highly efficient, since they implement quantum chemistry techniques for modeling only the small part of the system (QM layer) that undergoes chemical modifications, charge transfer, etc., under the influence of the surrounding environment. The rest of the system (MM layer) is described in terms of molecular mechanics force fields, assuming that its influence on the QM layer can be roughly decomposed in terms of electrostatic interactions and steric hindrance. Common limitations of QM/MM methods include inaccuracies in the MM force fields, when polarization effects are not explicitly considered, and the approximate treatment of electrostatic interactions at the boundaries between QM and MM layers. This article reviews recent advances in the development of computational protocols that allow for rigorous modeling of electrostatic interactions in extended systems beyond the common limitations of QM/MM hybrid methods. We focus on the moving-domain QM/MM (MoD-QM/MM) methodology that partitions the system into many molecular domains and obtains the electrostatic and structural properties of the whole system from an iterative self-consistent treatment of the constituent molecular fragments. We illustrate the MoD-QM/MM method as applied to the description of photosystem II as well as in conjunction with the application of spectroscopically constrained QM/MM optimization methods, based on high-resolution spectroscopic data (extended X-ray absorption fine structure spectra, and exchange coupling constants).

Force field parameters for large-scale computational modeling of sensitized TiO2 surfaces

Force field parameters for large scale computational modeling of sensitized TiO2-anatase surfaces are developed from ab initio molecular dynamics simulations and geometry optimization based on Density Functional Theory (DFT). The resulting force field, composed of Coulomb, van der Waals and harmonic interactions, reproduces the ab initio structures and the phonon spectra density profiles of TiO2-anatase nanostructures functionalized with catechol, a prototype of an aromatic linker commonly used to sensitize TiO2 nanoparticles with Ru(II)-polypyridyl dyes. In addition, simulations of interfacial electron injection and electron-hole relaxation dynamics demonstrate the capabilities of the resulting molecular mechanics force-field, as applied in conjunction with mixed quantum-classical methods, for modeling quantum processes that are critical for the overall efficiency of sensitized-TiO2 solar cells.

SURFACE EFFECTS ON THE STATISTICS OF THE LOCAL DENSITY OF STATES IN METALLIC NANOPARTICLES: MANIFESTATION ON THE NMR SPECTRA

In metallic nanoparticles, shifts in the ionization energy of surface atoms with respect to bulk atoms can lead to surface bands. Within a simple Tight Binding model we find that the projection of the electronic density of states on these sites presents two overlapping structures. One of them is characterized by the level spacing coming from bulk states and the other arises from the surface states. In very small particles, this effect contributes to an over-broadening of the NMR absorption spectra, determined by the Knight shift distribution of magnetic nuclei. We compare our calculated Knight shifts with experiments on aluminum nanoparticles, and show that the deviation of the scaling law as a function of temperature and particle size can be explained in terms of surface states.