Modulation of Ligand-Field Parameters by Heme Ruffling in Cytochromes <i>c</i> Revealed by EPR Spectroscopy

Electron paramagnetic resonance (EPR) spectra of variants of <i>Hydrogenobacter thermophilus</i> cytochrome <i>c</i>₅₅₂ (<i>Ht c</i>-552) and <i>Pseudomonas aeruginosa</i> cytochrome <i>c</i>₅₅₁ (<i>Pa c</i>-551) are analyzed to determine the effect of heme ruffling on ligand-field parameters. Mutations introduced at positions 13 and 22 in <i>Ht c</i>-552 were previously demonstrated to influence hydrogen bonding in the proximal heme pocket and to tune reduction potential (<i>E</i>_m) over a range of 80 mV [Michel, L. V.; Ye, T.; Bowman, S. E. J.; Levin, B. D.; Hahn, M. A.; Russell, B. S.; Elliott, S. J.; Bren, K. L. <i>Biochemistry</i> <b>2007</b>, <i>46</i>, 11753–11760]. These mutations are shown here to also increase heme ruffling as <i>E</i>_m decreases. The primary effect on electronic structure of increasing heme ruffling is found to be a decrease in the axial ligand-field term Δ/λ, which is proposed to arise from an increase in the energy of the d_<i>xy</i> orbital. Mutations at position 7, previously demonstrated to influence heme ruffling in <i>Pa c</i>-551 and <i>Ht c</i>-552, are utilized to test this correlation between molecular and electronic structure. In conclusion, the structure of the proximal heme pocket of cytochromes <i>c</i> is shown to play a role in determining heme conformation and electronic structure

Fe₃O₄ Nanocrystals Tune the Magnetic Regime of the Fe/Ni Molecular Magnet: A New Class of Magnetic Superstructures

A new class of organometallic–inorganic magnetic material was engineered by a sonochemically assisted self-assembly process between magnetite nanoparticles (biogenic Fe₃O₄, hard constituent) functionalized with isonicotinic acid and a metamagnetic organometallic complex ([Ni­(en)₂]₃[Fe­(CN)₆]₂·3H₂O, soft constituent). In such bottom-up methodology, hard and soft counterparts form well-organized microdimensional clusters that showed morphological fingerprints and magnetic behavior clearly distinct from those of the initial building units. In the engineered soft–hard material, the magnetite nanocrystals induced ferromagnetic ordering at room temperature of closer contact layers of [Ni­(en)₂]₃[Fe­(CN)₆]₂·3H₂O, thus demonstrating the ability to sensibly modify the [Ni­(en)₂]₃[Fe­(CN)₆]₂·3H₂O paramagnetic regime. The magnetic ordering of [Ni­(en)₂]₃[Fe­(CN)₆]₂·3H₂O was triggered by the intrinsic local field of the hard magnetic nanocrystals, which resembled, to some extent, the effects promoted by large, external magnetic fields

Solvent Controlled Generation of Spin Active Polarons in Two-Dimensional Material under UV Light Irradiation

Polarons belong to a class of extensively studied quasiparticles that have found applications spanning diverse fields, including charge transport, colossal magnetoresistance, thermoelectricity, (multi)ferroism, optoelectronics, and photovoltaics. It is notable, though, that their interaction with the local environment has been overlooked so far. We report an unexpected phenomenon of the solvent-induced generation of polaronic spin active states in a two-dimensional (2D) material fluorographene under UV light. Furthermore, we present compelling evidence of the solvent-specific nature of this phenomenon. The generation of spin-active states is robust in acetone, moderate in benzene, and absent in cyclohexane. Continuous wave X-band electron paramagnetic resonance (EPR) spectroscopy experiments revealed a massive increase in the EPR signal for fluorographene dispersed in acetone under UV-light irradiation, while the system did not show any significant signal under dark conditions and without the solvent. The patterns appeared due to the generation of transient magnetic photoexcited states of polaronic character, which encompassed the net 1/2 spin moment detectable by EPR. Advanced ab initio calculations disclosed that polarons are plausibly formed at radical sites in fluorographene which interact strongly with acetone molecules in their vicinity. Additionally, we present a comprehensive scenario for multiplication of polaronic spin active species, highlighting the pivotal role of the photoinduced charge transfer from the solvent to the electrophilic radical centers in fluorographene. We believe that the solvent-tunable polaron formation with the use of UV light and an easily accessible 2D nanomaterial opens up a wide range of future applications, ranging from molecular sensing to magneto-optical devices

Solvent Controlled Generation of Spin Active Polarons in Two-Dimensional Material under UV Light Irradiation

Polarons belong to a class of extensively studied quasiparticles that have found applications spanning diverse fields, including charge transport, colossal magnetoresistance, thermoelectricity, (multi)ferroism, optoelectronics, and photovoltaics. It is notable, though, that their interaction with the local environment has been overlooked so far. We report an unexpected phenomenon of the solvent-induced generation of polaronic spin active states in a two-dimensional (2D) material fluorographene under UV light. Furthermore, we present compelling evidence of the solvent-specific nature of this phenomenon. The generation of spin-active states is robust in acetone, moderate in benzene, and absent in cyclohexane. Continuous wave X-band electron paramagnetic resonance (EPR) spectroscopy experiments revealed a massive increase in the EPR signal for fluorographene dispersed in acetone under UV-light irradiation, while the system did not show any significant signal under dark conditions and without the solvent. The patterns appeared due to the generation of transient magnetic photoexcited states of polaronic character, which encompassed the net 1/2 spin moment detectable by EPR. Advanced ab initio calculations disclosed that polarons are plausibly formed at radical sites in fluorographene which interact strongly with acetone molecules in their vicinity. Additionally, we present a comprehensive scenario for multiplication of polaronic spin active species, highlighting the pivotal role of the photoinduced charge transfer from the solvent to the electrophilic radical centers in fluorographene. We believe that the solvent-tunable polaron formation with the use of UV light and an easily accessible 2D nanomaterial opens up a wide range of future applications, ranging from molecular sensing to magneto-optical devices

Triggering Two-Step Spin Bistability and Large Hysteresis in Spin Crossover Nanoparticles via Molecular Nanoengineering

The local entrapment of the spin crossover complex Fe­(II)-tris­[2-(2′-pyridyl)­benzimidazole] into the pluronic polymeric matrix (P123, PEG20–PPG70–PEG20, MW ∼ 5800) yielded the formation of magnetic nanoparticles of ∼26 nm (SCO-Np). Formation of SCO-Np was driven by the emergence of noncovalent interactions between the aromatic −NH group of the benzimidazole moieties present in Fe­(II)-tris­[2-(2′-pyridyl)­benzimidazole] with the aliphatic ether (−O−) groups of the pluronic polymeric matrix. The nanoparticles show spin crossover behavior, two-step spin bistability, and wide magnetic hysteresis, expressed in the temperature range of 170–280 K (Δ<i>T</i>_max = 38 K). The neat SCO molecules, Fe­(II)-tris­[2-(2′-pyridyl)­benzimidazole], on the contrary show only first-order spin transition and negligible hysteresis. The developed matrix-confinement approach of SCO molecules shown in this work yielded an unprecedented and significant improvement of the magnetic cooperativity compared to the neat spin crossover system, despite the decreased dimension of the magnetic domain in the nanosized architecture

UV-Vis spectra of reconstituted R2F.

<p>(A) UV-Vis spectrum of 75 µM R2F prior to reconstitution (dark-yellow line), R2F reconstituted with ferrous iron (blue line), and after incubation with 4 mM HU for 5 min (red line) at room temperature. (B) UV-Vis spectrum of 50 µM Mn^II₂-R2F reconstituted with 25 µM of NrdI_hq and O₂ (g) (blue line), 50 µM Mn^II₂-R2F with 25 µM of NrdI_ox (dark-yellow line), and manganese reconstituted R2F treated with 4 mM HU for 5 min at room temperature (red line).</p

Zero-Valent Iron Nanoparticles with Unique Spherical 3D Architectures Encode Superior Efficiency in Copper Entrapment

The large-scale preparation of spherical condensed-type superstructures of zero-valent iron (nZVI), obtained by controlled solid-state reaction through a morphologically conserved transformation of a magnetite precursor, is herein reported. The formed 3D nanoarchitectures (S-nZVI) exhibit enhanced entrapment efficiency of heavy metal pollutants, such as copper, compared to all previously tested materials reported in the literature, thus unveiling the relevance in the material’s design of the morphological variable. The superior removal efficiency of these mesoporous S-nZVI superstructures is linked to their extraordinary ability to couple effectively processes such as reduction and sorption of the metal pollutant

Resonance Raman parameters for the Fe(III)-O-Fe(III) protein sites and phenoxyl ν_7a bands of different RNR proteins.

<p>Resonance Raman parameters for the Fe(III)-O-Fe(III) protein sites and phenoxyl ν_7a bands of different RNR proteins.</p

The Mn-substituted R2F-protein from <i>C. ammoniagenes</i> (PDB code 3MJO) [<b>26</b>] (A). and sketch of themolecular structure of the tyrosyl radical (B).

<p>In panel (A) the numbers in bold represent atomic distances (Å). The structure in (B) describes the dihedral angle θ (C6-C1-Cβ-Cα) used to illustrate the orientation of the α-carbon: θ = 0° corresponds to the α-carbon in the plane of the phenoxyl ring, while θ = 90° is perpendicular to the plane.</p

Spin-density distribution of an isolated tyrosyl radical obtained by density functional theory (DFT/UB3LYP/6-311++G(d,p) in gas phase, neutral form after geometry optimization (2> = 0.7507, the dihedral angle θ was constrained to 60°).

<p>In this constrained conformation, the calculated atomic spin densities (from Mulliken population analyses) reveal much larger positive values located on H(β1) with respect to the H(β2) proton.</p