Search

Herein we report the synthesis and electrochemical and photophysical behaviors of a nanocomposite TiO2/P-[Ru-pyr]2+, where [Ru(bpy)3]2+(bpy = 2,2′-bipyridine) cores bearing two pyrrole substituents on distinct bipyridines and a phosphonic anchoring group on the third bipyridine are immobilized on TiO2colloidal nanoparticles. Under visible light and in the presence of O2as a sacrificial electron scavenger, TiO2/P-[Ru-pyr]2+was converted into a suprastructure TiO2/P-oligo(Ru-pyr), where the [Ru(bpy)3]2+cores were linked together through a pyrrole coupling reaction. The photoinduced charge separated state (e–)TiO2/RuII-pyr+was revealed by electron paramagnetic resonance (EPR) spectroscopy ((e–)TiO2signal). The oxidative coupling of pyrroles was ascertained by identifying the additional organic radical EPR signal with DFT calculations of pyrrole-derived oligomers, combined with X-ray photoelectron spectroscopy (XPS) results. TiO2/P-oligo(Ru-pyr) exhibited a higher rate of electron injection from [Ru(bpy)3]2+* to TiO2than for colloidal nanoparticules without pyrrole denoted as TiO2/P-[Ru]2+. The composites were then electrophoretically deposited on a fluorine-doped tin oxide surface and tested as photoanodes. At a bias of 0.12 V vs Ag/Ag+0.01 M, FTO/TiO2/P-oligo(Ru-pyr) in the presence of tertiary amine as a sacrificial electron donor, displayed a current density double that of FTO/TiO2/P-[Ru]2+under similar conditions. XPS confirmed the stability of [Ru(bpy)3]2+cores after the photopolymerization process, but a partial desorption was observed after the electrophoretic deposition.

The [Fe2S2]-RsrR gene transcription regulator senses the redox status in bacteria by modulating DNA binding, while its cluster cycles between +1 and +2 states—only the latter binds DNA. We have previously shown that RsrR can undergo remarkable conformational changes involving a 100° rotation of tryptophan 9 between exposed (Out) and buried (In) states. Here, we have used the chemical modification of Trp9, site-directed mutagenesis, and crystallographic and computational chemical studies to show that (i) the Outand Instates correspond to oxidized and reduced RsrR, respectively, (ii) His33 is protonated in the Instate due to a change in its pKacaused by cluster reduction, and (iii) Trp9 rotation is conditioned by the response of its dipole moment to environmental electrostatic changes. Our findings illustrate a novel function of protonation resulting from electron transfer.

PylB is a radical S-adenosyl-l-methionine (SAM) enzyme predicted to convert l-lysine into (3R)-3-methyl-d-ornithine, a precursor in the biosynthesis of the 22nd proteogenic amino acid pyrrolysine. This protein highly resembles that of the radical SAM tyrosine and tryptophan lyases, which activate their substrate by abstracting a H atom from the amino-nitrogen position. Here, combining in vitroassays, analytical methods, electron paramagnetic resonance spectroscopy, and theoretical methods, we demonstrated that instead, PylB activates its substrate by abstracting a H atom from the Cγ position of l-lysine to afford the radical-based β-scission. Strikingly, we also showed that PylB catalyzes the reverse reaction, converting (3R)-3-methyl-d-ornithine into l-lysine and using catalytic amounts of the 5′-deoxyadenosyl radical. Finally, we identified significant in vitroproduction of 5′-thioadenosine, an unexpected shunt product that we propose to result from the quenching of the 5′-deoxyadenosyl radical species by the nearby [Fe4S4] cluster.

The radical S-adenosyl-l-methionine tryptophan lyase uses radical-based chemistry to convert l-tryptophan into 3-methyl-2-indolic acid, a fragment in the biosynthesis of the thiopeptide antibiotic nosiheptide. This complex reaction involves several successive steps corresponding to (i) the activation by a specific hydrogen-atom abstraction, (ii) an unprecedented •CO2–radical migration, (iii) a cyanide fragment release, and (iv) the termination of the radical-based reaction. In vitrostudy of this reaction is made more difficult because the enzyme produces a significant amount of a shunt product instead of the natural product. Here, using a combination of X-ray crystallography, electron paramagnetic resonance spectroscopy, and quantum and hybrid quantum mechanical/molecular mechanical calculations, we have deciphered the fine mechanism of the key •CO2–radical migration, highlighting how the preorganized active site of the protein tightly controls this reaction.

Arylamine moieties oxidized to radical cations are considered promising spin bearing units in high-spin-type compounds. Here, we report the first use of carbazole-3,6-diamine units as efficient, rigid spin containing units. The use of rigid spin bearing units enhances significantly spin exchange interactions. The design using density functional theory calculations shows the progressive increase of the exchange coupling constant dependent on the considered model molecules. Two of the most representative molecules containing flexible (dimer 1) and rigid spin coupling unit (dimer 2) were synthesized. Electrochemical and pulsed-electron paramagnetic resonance nutation studies showed that both dimers can be oxidized to yield a majority of dicationic diradicals exhibiting S= 1 ground states. The high values of the dimer 2exchange coupling constant obtained both computationally (J/kB= 145 K; HHeis.= −JS1S2) and experimentally (J/kB= 90–100 K) indicate the beneficial role of the carbazole moiety incorporated into spin bearing units.

Barium disilicide (BaSi2), composed of Earth-abundant and nontoxic elements, is a promising material for thin-film solar cells. The control of carrier type and carrier concentration by impurity doping is particularly important for the application of BaSi2to solar cells. However, the presence of defects in semiconductors such as BaSi2may have a significant impact on the electrical and optical properties of solar cells. In this study, we chose boron to act as a p-type impurity in BaSi2and studied boron-related defects using the Quantum Espresso method with density functional theory. The formation energy of interstitial boron defects was found to be lower than that of boron in Si vacancy sites. The hyperfine coupling constants of 137Ba with antisite boron defects (BSi) are very localized and differ significantly from those of 137Ba with interstitial boron defects (Bi). This suggests that neutral BSiand Bican be identified by electron paramagnetic resonance.

A new modular, easy-to-synthesize photocatalyst was prepared by assembling colloidal CdSe/ZnS quantum dots (QD) and gold nanoparticles (AuNP) via their ligands thanks to copper-catalyzed azide to alkyne cycloaddition (CuAAC) click chemistry. The resulting composite (QD–AuNP) photocatalyst was tested with a benchmark photoredox system previously reported by our group, for which QD alone acted as a photocatalyst but with a modest quantum yield (QY = 0.06%) and turnover number (TON = 350 in 3 h) due to poor charge separation. After optimization, the QD–AuNP composites exhibited much improved photocatalytic performances: up to five times higher TON (2600 in 3 h) and up to 24 times faster reaction in the first 10 min of visible irradiation. Such an improvement is attributed to an efficient electron transfer from QD to AuNP in the photoexcited QD–AuNP composites, which ensures a much better charge separation than that in QD alone. This was confirmed by studying both (i) the quenching of the QD photoluminescence during the synthesis of the QD–AuNP composites and (ii) the blue shift of the AuNP plasmon absorption band due to the accumulation of up to 7400 electrons per AuNP in QD–AuNP composites under visible light irradiation in the presence of electron donors.

We present a 7Li MAS NMR study carried out before (pristine material) and during the first cycle of charge/discharge of Li[Li0.2Mn0.61Ni0.18Mg0.01]O2layered oxide, a promising active material for positive electrode in Li-ion batteries. For the pristine material, at least five NMR signals were observed. To analyze these results, we developed an 18 cation local model (first and second spheres) aiming at identifying very precise cationic (Li+, Mn4+/Ni2+) configurations compatible with all our NMR data while satisfying local electroneutrality constraints (the key ingredient of our approach). Our results strongly suggest that the material presents two types of coexisting nanoscale domains. The first type is highly ordered and consists of pure Li2MnO3cores (volume ∼58%), while the second more disordered type concentrates most of the Ni and is labeled LiMO2-like (volume ∼20%) where M = Mn1/2Ni1/2. Finally, at the interphase of these two Ni-free and Ni-rich domains, there are slightly Ni-contaminated Li2MnO3-like regions, most probably surrounding the Li2MnO3domains and thus labeled “Ni-poor boundaries” (volume ∼21%). This partition is confirmed by the behavior of the NMR signals during the first electrochemical cycle. At the initial state of charge (≤4.3 V), Li-ion extraction occurs mainly from the (Ni-rich) Li1–xMO2-like domains via Ni2+oxidation. At higher states of charge (≥4.5 V), the Li2MnO3-like domains become highly involved via oxygen-based (ir)reversible oxidation processes, leading to significant structural transformations. During discharge, only ∼60% of the initial lithium is reinserted into the structure. The (Ni-rich) LiMO2-like domains are fully refilled (via reversible Ni4+reduction into Ni2+), while the ordered Li2MnO3-like domains experience a significant size decrease after the first cycle of charge/discharge. The originality of the present approach consists of analyzing NMR data with a new model that includes at its heart local electroneutrality constraints. This model allowed us to shed light on the processes occurring in the Li-rich Mn/Ni layered oxide compound during the first electrochemical cycle on the microscopic level.

Carbon–sulfur bond formation at aliphatic positions is a challenging reaction that is performed efficiently by radical S-adenosyl-L-methionine (SAM) enzymes. Here we report that 1,3-thiazolidines can act as ligands and substrates for the radical SAM enzyme HydE, which is involved in the assembly of the active site of [FeFe]-hydrogenase. Using X-ray crystallography, in vitro assays and NMR spectroscopy we identified a radical-based reaction mechanism that is best described as the formation of a C-centred radical that concomitantly attacks the sulfur atom of a thioether. To the best of our knowledge, this is the first example of a radical SAM enzyme that reacts directly on a sulfur atom instead of abstracting a hydrogen atom. Using theoretical calculations based on our high-resolution structures we followed the evolution of the electronic structure from SAM through to the formation of S-adenosyl-L-cysteine. Our results suggest that, at least in this case, the widely proposed and highly reactive 5′-deoxyadenosyl radical species that triggers the reaction in radical SAM enzymes is not an isolable intermediate.

The use of CdSe-ZnS type I quantum dots as redox photocatalysts with visible light in water is investigated by monitoring the oxidation reaction of 8-oxo-2′-deoxyguanosine and the reduction of nitrophenylalanine derivatives. The detection of reaction intermediates and the identification and quantitation of reaction products establish that both reactions are photocatalyzed simultaneously by CdSe-ZnS type I quantum dots. The photocatalyzed reactions are observed without the need for sacrificial reactants and with high turnover numbers (>400). These results demonstrate that CdSe-ZnS type I quantum dots can work efficiently as redox photocatalysts in water solution.

57Fe Q-band ENDOR has been used to study the [4Fe–4S]1+state created by γ irradiation of single crystals of the synthetic model compound [N(C2H5)4]2[Fe4S4(SCH2C6H5)4] enriched in 57Fe. This compound is an excellent biomimetic model of the active sites of many 4 iron–4 sulfur proteins, enabling detailed and systematic studies of its oxidized [4Fe–4S]3+and reduced [4Fe–4S]1+paramagnetic states. Taking advantage of the fact that Q-band ENDOR, in contrast with X-Band ENDOR, allows for a very good separation of the 57Fe transitions from those of the protons, the complete hyperfine tensors of the four iron atoms for the [4Fe–4S]1+species has been measured with precision. For each iron atom, the electron orbital and electron spin isotropic contributions have been determined separately. Moreover, it is remarkable that two 57Fe hyperfine tensors attributed to the ferrous pair of iron atoms are very different. In effect, one tensor presents a much larger anisotropic part and a much smaller isotropic part than those of the other. This difference has been interpreted in terms of a differential electron orbital hyperfine interaction among the two ferrous ions.

Abstract.: Within the general context of homometallic spin-coupled Cu(II) dimers, we propose to relate the antiferromagnetic part of the exchange coupling constant, J AF, to the quantity ΔP 2(Cu), the difference of copper squared spin populations as calculated for the high-spin (i.e. triplet) and broken-symmetry spin states, through J AF≈−UΔP 2(Cu), where U is interpreted as the covalent–ionic term. This proportionality is illustrated for three “bare” Cu(II) dimers (i.e. without peripheral ligation, so as to enhance the antiferromagnetic contribution) bridged by azido, hydroxo, or oxo groups. This provides an alternative quantifier of the exchange phenomenon to that usually used, i.e. Δ2, the square of the singly occupied molecular orbital splitting in the triplet state. Moreover, and quite interestingly, the quantity ΔP 2(Cu) can become negative (i.e. induce ferromagnetism) without apparently affecting the proportionality relation.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.