Oxidation potentials of the ZnCe₆ and ZnCe₆ without carboxylic acids in continuum solvents with different values for the dielectric constant.

<p>Oxidation potentials of the ZnCe₆ and ZnCe₆ without carboxylic acids in continuum solvents with different values for the dielectric constant.</p

Factors Controlling the Redox Potential of ZnCe₆ in an Engineered Bacterioferritin Photochemical ‘Reaction Centre’

<div><p>Photosystem II (PSII) of photosynthesis has the unique ability to photochemically oxidize water. Recently an engineered bacterioferritin photochemical ‘reaction centre’ (BFR-RC) using a zinc chlorin pigment (ZnCe₆) in place of its native heme has been shown to photo-oxidize bound manganese ions through a tyrosine residue, thus mimicking two of the key reactions on the electron donor side of PSII. To understand the mechanism of tyrosine oxidation in BFR-RCs, and explore the possibility of water oxidation in such a system we have built an atomic-level model of the BFR-RC using ONIOM methodology. We studied the influence of axial ligands and carboxyl groups on the oxidation potential of ZnCe₆ using DFT theory, and finally calculated the shift of the redox potential of ZnCe₆ in the BFR-RC protein using the multi-conformational molecular mechanics–Poisson-Boltzmann approach. According to our calculations, the redox potential for the first oxidation of ZnCe₆ in the BRF-RC protein is only 0.57 V, too low to oxidize tyrosine. We suggest that the observed tyrosine oxidation in BRF-RC could be driven by the ZnCe₆ di-cation. In order to increase the efficiency of tyrosine oxidation, and ultimately oxidize water, the first potential of ZnCe₆ would have to attain a value in excess of 0.8 V. We discuss the possibilities for modifying the BFR-RC to achieve this goal.</p></div

Calculated pH - dependence of the E_m of ZnCe₆ in BFR-RC (solid line, slope −46.4, R = 0.994) and in water (dashed line, slope −18.4, R = 0.950).

<p>Calculated pH - dependence of the E_m of ZnCe₆ in BFR-RC (solid line, slope −46.4, R = 0.994) and in water (dashed line, slope −18.4, R = 0.950).</p

A: Differential pulse voltammogram of ZnCe₆ in DMSO with 0.1 M TBAPF₆. [ZnCe₆] = 1 mM. Voltammogram of the solvent is shown by dotted line. B: Spectral changes during the oxidation of ZnCe₆ at 0.54 V. See Results for details.

<p>A: Differential pulse voltammogram of ZnCe₆ in DMSO with 0.1 M TBAPF₆. [ZnCe₆] = 1 mM. Voltammogram of the solvent is shown by dotted line. B: Spectral changes during the oxidation of ZnCe₆ at 0.54 V. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0068421#s3" target="_blank">Results</a> for details.</p

Phosphorus(V) Porphyrin-Manganese(II) Terpyridine Conjugates: Synthesis, Spectroscopy, and Photo-Oxidation Studies on a SnO₂ Surface

A major challenge in designing artificial photosynthetic systems is to find a suitable mimic of the highly oxidizing photoactive species P₆₈₀ in photosystem II. High-potential phosphorus­(V) porphyrins have many attractive properties for such a mimic but have not been widely studied. Here, we report the synthesis and photophysical characterization of a novel phosphorus­(V) octaethylporphyrin–oxyphenyl–terpyridine conjugate (PPor-OPh-tpy, <b>1</b>) and its corresponding manganese­(II) complex (PPor-OPh-Mn­(tpy)­Cl₂, <b>2</b>). The X-ray structure of <b>2</b> shows that the Mn­(II) and P­(V) centers are 11.783 Å apart and that the phenoxy linker is not fully conjugated with the terpyridine ligand. The porphyrin fluorescence in <b>1</b> and <b>2</b> is strongly quenched and has a shorter lifetime compared to a reference compound without the terpyridine ligand. This suggests that electron transfer from tpy or Mn­(tpy) to the excited singlet state of the PPor may be occurring. However, femtosecond transient absorbance data show that the rate of relaxation to the ground state in <b>1</b> and <b>2</b> is comparable to the fluorescence lifetimes. Thus, if charge separation is occurring, its lifetime is short. Because both <b>1</b> and <b>2</b> are positively charged, they can be electrostatically deposited onto the surface of negatively charged SnO₂ nanoparticles. Freeze-trapping EPR studies of <b>2</b> electrostatically bound to SnO₂ suggest that excitation of the porphyrin results in electron injection from ¹PPor* into the conduction band of SnO₂ and that the resulting PPor^•+ species acquires enough potential to photo-oxidize the axially bound Mn­(II) (tpy) moiety to Mn­(III) (tpy)

Atomic level model of the <i>E. coli</i> bacterioferritin.

<p>A - homodimer showing two identical subunits each hosting two manganese ions and ZnCe₆ bound at the interface of the subunits. B – manganese binding site. C - ZnCe₆ in its binding site. The ensemble of carboxylic acid conformations used to compute pKas is shown in yellow. Model is based on the X-ray diffraction structure PDB ID: 3E1M.</p

Contributions from aminoacid side chains to the shift of ZnCe₆ E_m.

<p>Contributions from aminoacid side chains to the shift of ZnCe₆ E_m.</p

Photoinduced Charge Separation in a Ferrocene−Aluminum(III) Porphyrin−Fullerene Supramolecular Triad

Light-induced electron transfer is investigated in a ferrocene−aluminum(III) porphyrin−fullerene supramolecular triad (FcAlPorC60) and the constituent dyads (AlPorC60 and FcAlPorPh). The fullerene unit (C60) is bound axially to the aluminum(III) porphyrin (AlPor) via a benzoate spacer, and ferrocene (Fc) is attached via an amide linkage to one of the four phenyl groups in the meso positions of the porphyrin ring. The absorption spectra and voltammetry data of the complexes suggest that the ground state electronic structures of the Fc, AlPor, and C60 entities are not significantly perturbed in the dyads and triad. Time-resolved optical and transient electron paramagnetic resonance (EPR) data show that photoexcitation of the AlPorC60 dyad results in efficient electron transfer from the excited singlet state of the porphyrin to fullerene, producing the charge-separated state AlPor•+−C60•−. The fluorescence and transient EPR data also suggest that some energy transfer from the porphyrin to fullerene may occur. The lifetime of the radical pair AlPor•+−C60•− measured by transient absorbance spectroscopy is found to be 39 ns in o-dichlorobenzene at room temperature. At 200 K, transient EPR experiments place a lower limit of 5 μs on the radical pair lifetime. In the triad, the data suggest that excitation of the porphyrin gives rise to the charge-separated state Fc•+−AlPor−C60•− in two electron transfer steps. Photocurrent measurements demonstrate that both dyads and the triad have good photovoltaic performance. However, when Fc is appended to AlPorC60, the expected improvement of the radical pair lifetime and the photovoltaic characteristics is not observed

Details of the protein environment of Tyr25.

<p>Details of the protein environment of Tyr25.</p

Aminoacids contributing the most to the E_m shift of ZnCe₆.

<p>Grey spheres represent hydrophobic side chains.</p