Mixed Valence {Ni²⁺Ni¹⁺} Clusters as Models of Acetyl Coenzyme ASynthase Intermediates

Acetyl coenzyme A synthase (ACS) catalyzes the formation and deconstruction of the key biological metabolite, acetyl coenzyme A (acetyl-CoA). The active site of ACS features a {NiNi} cluster bridged to a [Fe4S4]n+ cubane known as the A-cluster. The mechanism by which the A-cluster functions is debated, with few model complexes able to replicate the oxidation states, coordination features, or reactivity proposed in the catalytic cycle. In this work, we isolate the first bimetallic models of two hypothesized intermediates on the paramagnetic pathway of the ACS function. The heteroligated {Ni2+Ni1+} cluster, [K(12-crown-4)2][1], effectively replicates the coordination number and oxidation state of the proposed “Ared” state of the A-cluster. Addition of carbon monoxide to [1]− allows for isolation of a dinuclear {Ni2+Ni1+(CO)} complex, [K(12-crown-2)n][2] (n = 1–2), which bears similarity to the “ANiFeC” enzyme intermediate. Structural and electronic properties of each cluster are elucidated by X-ray diffraction, nuclear magnetic resonance, cyclic voltammetry, and UV/vis and electron paramagnetic resonance spectroscopies, which are supplemented by density functional theory (DFT) calculations. Calculations indicate that the pseudo-T-shaped geometry of the three-coordinate nickel in [1]– is more stable than the Y-conformation by 22 kcal mol–1, and that binding of CO to Ni1+ is barrierless and exergonic by 6 kcal mol–1. UV/vis absorption spectroscopy on [2]− in conjunction with time-dependent DFT calculations indicates that the square-planar nickel site is involved in electron transfer to the CO π*-orbital. Further, we demonstrate that [2]− promotes thioester synthesis in a reaction analogous to the production of acetyl coenzyme A by ACS

The [2Fe-2S] cluster of mitochondrial outer membrane protein mitoNEET has an O₂-regulated nitric oxide access tunnel

The mitochondrial outer membrane iron-sulphur ([Fe-S]) protein mitoNEET has been extensively studied as a target of the anti-inflammatory and type-2 diabetes drug pioglitazone and as a protein affecting mitochondrial respiratory rate. Despite these extensive past studies, its molecular function has yet to be discovered. Here, we applied an interdisciplinary approach and discovered an explicit nitric oxide (NO) access site to the mitoNEET [2Fe-2S] cluster. We found that O2 and pioglitazone block NO access to the cluster, suggesting a molecular function for the mitoNEET [2Fe-2S] cluster in mitochondrial signal transduction. Our discovery hints at a new pathway via which mitochondria can sense hypoxia through O2 protection of the mitoNEET [2Fe-2S] cluster, a new paradigm in understanding the importance of [Fe-S] clusters for gasotransmitter signal transduction in eukaryotes. </p

Optimizing the Au Particle Doping Size for Enhanced Photocatalytic Disinfection under Low-Intensity Visible Light

Here, we present the effect of 1.2-9.9 nm Au particles on crystal violet-treated polymer under a low intensity of visible light. The use of Au particles ≤ 6.3 nm promoted charge carrier transfer from crystal violet to Au particles. Photospectroscopy analyses and DFT computations revealed that a change in the electronic band structure caused by the size reduction of the particle altered the charge carrier transfer pathway in crystal violet. Especially for crystal violet─1.2 nm Au particles, charge carrier transfer predominantly occurs at the S1 of crystal violet because the T1 state lacks sufficient potential energy for transfer. 1.2 nm Au particles on crystal violet not only most significantly enhanced the generation of O2•-, H2O2, and •OH by minimizing unnecessary side reactions or energy loss but also showed the most potent disinfection activity against Staphylococcus aureus, even at low visible light flux levels (0.037-0.054 mW cm-2), which resulted in a 5.3 log reduction in viable bacteria after 6 h exposure to visible light. This finding provides fundamental insights into the Au effect as a cocatalyst in photocatalysts and the development of light-activated self-sterilizing surfaces that can be applied to various hospital surfaces to prevent the spread of pathogens, which remains a global challenge

Metodi sperimentali e teorici per la spettroscopia EPR di nitronilnitrossidi

Lo scopo del presente lavoro consiste nell’analizzare gli spettri CW-EPR di un radicale nitronilnitrossido (MTPNN) in cristalli liquidi (ZLI-1083) al variare della temperatura tramite un approccio computazionale integrato quantomeccanico – idrodinamico; tale metodologia di calcolo si contraddistingue rispetto al più classico procedimento di simulazione per l’esiguo numero dei parametri richiesti al fine di ottenere lo spettro. ================================================================================= The aim of the present work is to analyze the temperature-dependent CW-EPR spectra of a nitronylnitroxide monoradical (MTPNN) in a nematic phase (ZLI-1083) by a quantomechanical-hydrodynamic integrated computational approach; as just a few free phænomenological parameters are needed in order to calculate the spectrum, this method differs significantly from the “classical” simulation approach to the interpretation of EPR spectra

Study of the spectral and relaxation properties of nitronylnitroxide monoradicals: an experimental and computational approach

The present study deals with a thorough characterization of the ESR-related properties of nitronylnitroxide radicals; more in detail we determined the mutual orientation between the eigenframes of the magnetic interaction tensors and the spin relaxation properties in fluid solution. These latter measurements show that the transverse spin relaxation rates of nitronylnitroxides are slower with respect to nitroxide radicals; the analysis of the relaxation rate in terms of contributions, which relies on the application of the Redfield spin relaxation theory, allows to understand the different behaviour. All these considerations promote the routine use of nitronylnitroxide radicals in spin labeling studies, even though the analysis of the spectra is more involved due to the delocalization of the unpaired electron spin density over two nitrogen nuclei rather than one. We showed, however, that the introduction of new computational tools for the calculation of the spectral profiles relying on the a priori determination of the parameters affecting the lineshape allowed the reproduction of a series of experimental spectra recorded in a partially oriented fluid with minimal resort to best fitting procedures; this result further confirms the feasibility of a routine use of nitronylnitroxides as paramagnetic probes in ESR studies.Il presente studio affronta una valutazione dettagliata delle proprietà dei radicali nitronilnitrossido inerenti alla spettroscopia ESR; nello specifico vengono determinate le mutue orientazioni tra i sistemi di riferimento principali dei tensori magnetici e le proprietà di rilassamento di spin in fluido. Queste ultime misure, in particolare, mettono in luce un più lento rilassamento trasversale da parte dei radicali nitronilnitrossido, e la separazione dei contributi secondo la teoria di rilassamento di Redfield porta a determinare la causa della differenza di comportamento. Tutte queste valutazioni vanno nella direzione di favorire l’utilizzo di radicali nitronilnitrossido a fianco dei radicali nitrossido negli studi di spin labeling, sebbene i primi siano più complessi da un punto di vista dell’analisi degli spettri a causa della delocalizzazione della densità di spin elettronico su due nuclei d’azoto anziché su uno. Nondimeno l’introduzione di nuove metodologie di calcolo dei profili spettrali, le quali fanno affidamento su una valutazione a priori dei parametri che influenzano lo spettro, ha reso possibile la riproduzione di una serie di tracciati sperimentali acquisiti in un fluido parzialmente orientato con una introduzione solo marginale di parametri da sottoporre ad ottimizzazione, confermando dunque la possibilità di un uso comune dei radicali nitronilnitrossido nella spettroscopia ESR in qualità di sonde paramagnetiche

First determination of the spin relaxation properties of a nitronyl nitroxide in solution by Electron Spin Echoes at X-band. A comparison with Tempone

We studied by electron spin echo pulse methods the spin relaxation properties of a phenyl nitronyl nitroxide radical (2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide, PTIO) at X-band in fluid toluene solution in a wide temperature range, and in a water/glycerol 1:1 mixture near room temperature. The relaxation properties of PTIO have been compared with that of Tempone, as a widely used nitroxide. By a new procedure, based on experimental results on the temperature dependences of the relaxation times T1 and T2, and on the approximation of an isotropic Brownian rotational diffusion, we separated non-secular, spin rotational and residual terms from the transverse relaxation rate to isolate secular and pseudosecular contributions. By comparing the results for the two radicals we found the differences in the magnetic properties that give rise to slower transverse (T2) and longitudinal (T1) electron spin relaxation for PTIO in the whole temperature range explored in this work