Impact of disorder on formation of free radicals by gamma-irradiation: Multi-frequency EPR studies of trehalose polymorphs

Electron paramagnetic resonance (EPR) studies of the radiation-induced radicals in two anhydrous trehalose polymorphs, beta- crystalline (TREc) and glassy (TREg), were conducted with the aim to resolve whether different types of free radicals are induced in a differently disordered environment. A multifrequency approach (9.5 GHz, 94 GHz, and 244 GHz) was applied to improve the resolution of the complex EPR spectra. In addition, the thermal stability of the EPR spectra and the respective decay kinetics were analyzed in a series of thermal annealing studies in the temperature interval from 333 K to 363 K. It was found that in the crystalline matrix the transformation process of the induced radicals is more complex than in the glassy host matrix. Qualitative decomposition of the experimental spectra, assuming four contributing species, reproduced characteristic EPR spectral features in both matrices. These were interpreted as carbon-centered radicals while the possibility of the formation of alkoxy radicals due to the abstraction of a hydrogen atom could be ruled out. Only in one case, the tentative assignment of the EPR spectral components revealed the formation of the same radical species in both TREc and TREg. Furthermore, by thermal annealing TREg lost one of the radical species, whereas in TREc all 4 radical species pertained irrespective of the treatment. The results presented here, therefore, provide experimental evidence that the extent of disorder present in the material strongly affects the type and stability of radicals induced by ionizing radiation

Time-Resolved Infrared Spectroscopy Reveals the pH-Independence of the First Electron Transfer Step in the [FeFe] Hydrogenase Catalytic Cycle.

[FeFe] hydrogenases are highly active catalysts for hydrogen conversion. Their active site has two components: a [4Fe-4S] electron relay covalently attached to the H2 binding site and a diiron cluster ligated by CO, CN-, and 2-azapropane-1,3-dithiolate (ADT) ligands. Reduction of the [4Fe-4S] site was proposed to be coupled with protonation of one of its cysteine ligands. Here, we used time-resolved infrared (TRIR) spectroscopy on the [FeFe] hydrogenase from Chlamydomonas reinhardtii (CrHydA1) containing a propane-1,3-dithiolate (PDT) ligand instead of the native ADT ligand. The PDT modification does not affect the electron transfer step to [4Fe-4S]H but prevents the enzyme from proceeding further through the catalytic cycle. We show that the rate of the first electron transfer step is independent of the pH, supporting a simple electron transfer rather than a proton-coupled event. These results have important implications for our understanding of the catalytic mechanism of [FeFe] hydrogenases and highlight the utility of TRIR

Redox tuning of the H-cluster by second coordination sphere amino acids in the sensory [FeFe] hydrogenase from Thermotoga maritima

[FeFe] hydrogenases are exceptionally active catalysts for the interconversion of molecular hydrogen with protons and electrons. Their active site, the H-cluster, is composed of a [4Fe-4S] cluster covalently linked to a unique [2Fe] subcluster. These enzymes have been extensively studied to understand how the protein environment tunes the properties of the Fe ions for efficient catalysis. The sensory [FeFe] hydrogenase (HydS) from Thermotoga maritima has low activity and displays a very positive redox potential for the [2Fe] subcluster compared to that of the highly active prototypical enzymes. Using site directed mutagenesis, we investigate how second coordination sphere interactions of the protein environment with the H-cluster in HydS influence the catalytic, spectroscopic and redox properties of the H-cluster. In particular, mutation of the non-conserved serine 267, situated between the [4Fe-4S] and [2Fe] subclusters, to methionine (conserved in prototypical catalytic enzymes) gave a dramatic decrease in activity. Infra-red (IR) spectroelectrochemistry revealed a 50 mV lower redox potential for the [4Fe-4S] subcluster in the S267M variant. We speculate that this serine forms a hydrogen bond to the [4Fe-4S] subcluster, increasing its redox potential. These results demonstrate the importance of the secondary coordination sphere in tuning the catalytic properties of the H-cluster in [FeFe] hydrogenases and reveal a particularly important role for amino acids interacting with the [4Fe-4S] subcluster

L'Auto-vélo : automobilisme, cyclisme, athlétisme, yachting, aérostation, escrime, hippisme / dir. Henri Desgranges

07 avril 19271927/04/07 (A28,N9609)

Bismuth radical catalysis in the activation and coupling of redox-active electrophiles

Radical cross-coupling reactions represent a revolutionary tool to forge C(sp3)–C and C(sp3)–heteroatom bonds, by means of transition metals, photoredox or electrochemical approaches. This study demonstrates how a low-valent bismuth complex is able to undergo one-electron oxidative addition with redox-active alkyl radical precursors in an autonomous manner, mimicking the behavior of first-row transition metals. This reactivity paradigm for bismuth gives rise to unique radical-equilibrium complexes, which could be fully characterized in solution and solid state. The resulting Bi(III)–C(sp3) intermediates display divergent reactivity patterns depending on the α-substituents of the alkyl fragment. Mechanistic investigations on this reactivity led to the development of a bismuth-catalyzed C(sp3)–N cross-coupling reaction that operates under mild conditions and accommodates synthetically relevant N-heterocycles as coupling partners

Synthesis, Isolation and Characterization of Two Cationic Organo-bismuth(II) Pincer Complexes Relevant in Radical Redox Chemistry

Herein, we report the synthesis, isolation and characterization of two cationic organobismuth(II) compounds bearing N,C,N pincer frameworks, which are crucial intermediates in bismuth radical processes. X-ray crystallography uncovered a monomeric Bi(II) structure, while SQUID magnetometry in combination with NMR and EPR spectroscopy provide evidence for a para-magnetic S = 1/2 state. High resolution multifrequency EPR at X, Q, and W-band enable the precise assignment of the full g- and 209Bi A-tensors. Experimental data and DFT calculations reveal both complexes are metal-centered radicals with little delocalization onto the ligands

Following [FeFe] Hydrogenase Active Site Intermediates by Time-Resolved Mid-IR Spectroscopy

Time-resolved nanosecond mid-infrared spectroscopy is for the first time employed to study the [FeFe] hydrogenase from Chlamydomonas reinhardtii and to investigate relevant intermediates of the enzyme active site. An actinic 355 nm, 10 ns laser flash triggered photodissociation of a carbonyl group from the CO-inhibited state H-ox-CO to form the state H-ox, which is an intermediate of the catalytic proton reduction cycle. Time-resolved infrared spectroscopy allowed us to directly follow the subsequent rebinding of the carbonyl, re-forming H-ox-CO, and determine the reaction half-life to be t(1/2) approximate to 13 +/- 5 ms at room temperature. This gives direct information GO on the dynamics of CO inhibition of the enzyme