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Azoles and pyridines are commonly incorporated into small molecule inhibitor scaffolds that target cytochromes P450 (CYPs) as a strategy to increase drug binding affinity, impart isoform-dependent selectivity, and improve metabolic stability. Optical absorbance spectra of the CYP-inhibitor complex are widely used to infer whether these inhibitors are ligated directly to the heme iron as catalytically inert, low-spin (type II) complexes. Here, we show that the low-spin complex between a drug-metabolizing CYP2C9 variant and 4-(3-phenylpropyl)-1H-1,2,3-triazole (PPT) retains an axial water ligand despite exhibiting elements of "classic" type II optical behavior. Hydrogens of the axial water ligand are observed by pulsed electron paramagnetic resonance (EPR) spectroscopy for both inhibitor-free and inhibitor-bound species and show that inhibitor binding does not displace the axial water. A (15)N label incorporated into PPT is 0.444 nm from the heme iron, showing that PPT is also in the active site. The reverse type I inhibitor, LP10, of CYP125A1 from Mycobacterium tuberculosis, known from X-ray crystal structures to form a low-spin water-bridged complex, is found by EPR and by visible and near-infrared magnetic circular dichroism spectroscopy to retain the axial water ligand in the complex in solution.

Ester-containing nitrones, including 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide are reported to be robust spin traps for superoxide. Spin trapping of superoxide affords the identical EPR spectra and lifetimes in homogenous aqueous solution and exhibits the same ratio of cis and trans isomers.

Hyperfine sublevel correlation (HYSCORE) spectra of paramagnetic centers and nuclear quadrupolar interactions (NQIs) of arbitrary strength have been considered. This is applied to estimate the spin of Hamiltonian parameters of the oxovanadium-EDTA complex.

The colour (reduction state) of rutile crystals plays a significant role in the preparation of TiO(110) surfaces. Electron paramagnetic resonance has been used to characterize the bulk defects of five samples with different degrees of bulk reduction.

A framework is developed for designing narrow-line nitroxides in order to understand the narrow line width of 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxyl. Nitroxide (4-carboxymethylidene-2,2,6,6-tetramethyl-1-piperidinyloxy), as a prototype, has allowed a variety of structural diversity such a nitroxide that can target tissue compartments for in vivo EPR imaging.

The pulsed electron nuclear double resonance spectroscopic studies are conducted to investigate the carotenoid (Car) oxidation taking place in the Cu(II)-substituted MCM-41 molecular sieves. Chemical oxidation is shown to lead to the formation of carotenoid radical cations.

Pulsed electron nuclear double resonance (ENDOR) and two-dimensional (2D)-hyperfine sublevel correlation spectroscopy (HYSCORE) are used for characterizing carotenoid radicals of zeaxanthin and violaxanthin photogenerated on silica-alumina. The formation of neutral radical of zeaxanthin has provided an effective nonphotochemical quencher of the excited state of chlorophyll for photoreaction in the presence of excess light.

The effect of noise, nonuniform line width and aliasing on the cepstrum is examined and the EPR spectra taken by both Fourier-transform EPR and CW-EPR methods are analyzed. The conditions for recording EPR spectra to be used in the ceptral analysis are less complicated than in ENDOR where the solvent, the temperature and the microwave power should be optimized to record a useful ENDOR spectrum.

The pulsed electron paramagnetic resonance (EPR) technique of hyperfine sublevel correlation spectroscopy(HYSCORE) is used to obtain structural information about vanadium(VO(super 2+))-exchanged ZSM-5. HYSCORE spectra are obtained for vanadium-exchanged ZSM-5 before and after dehydration and after the adsorption of ammonia.

Canthaxanthin and 8' -apo-beta-caroten-8' -al radical cations chemically prepared on activated silica-alumina and in CH2Cl2 solution containing AlCl3 are studied by pulsed EPR and electron nuclear double resonance (ENDOR).

The deprotonation of naturally occurring zeaxanthin (Zea)radicalcations (Zea•+) to form neutral radicals (#Zea•) and their involvement in the qE portion of nonphotochemicalquenching (NPQ) was examined. The radical cations are weak acids,and readily deprotonate to a long-lived neutral radical (#Zea•) that could serve as long-lived quenching sites. When#Zea•is eventually neutralized and Zea is reformedin the presence of D2O, the Zea has an opportunity to undergoH/D exchange. This paper examines evidence for H/D exchange specificto qE activity in Arabidopsis thaliana. We demonstrate that Zea•+formed chemically viaoxidation of Zea by Fe(III) in the presence of D2O undergoesH/D exchange with a significant intensity increase of the M+1 (d1Zea) and M+2 (d2Zea) mass peaks in the mass spectrum.Then leaves from wild-type A. thalianawere infiltrated with either D2O or H2O andexposed to light. The carotenoids were extracted and analyzed viaelectrospray ionization liquid chromatography/mass spectrometry (LC/MS)to examine the mass peak distribution of Zea. Only leaves exposedto light intensity that triggers qE in A. thaliana(>300 μE m–2s–1) showedH/D exchange. This result suggests that #Zea•canform by the deprotonation of the weak acid Zea•+during qE, and its possible impact on qE must be considered. [ABSTRACT FROM AUTHOR]

The cytochrome bC1 and related complexes are essential energy-conserving components of mitochondrial and bacterial electron transport chains. They orchestrate a complex sequence of electron and proton transfer reactions resulting in the oxidation of quinol, the reduction of a mobile electron carrier, and the translocation of protons across the membrane to store energy in an electrochemical proton gradient. The enzyme can also catalyze substantial rates of superoxide production, with deleterious physiological consequences. Progress on understanding these processes has been hindered by the lack of observable enzymatic intermediates. We report the first direct detection of a semiquinone radical generated by the Qo site using continuous wave and pulsed EPR spectroscopy. The radical is a ubisemiquinone anion and is sensitive to both specific inhibitors and mutations within the Qo site as well as O2, suggesting that it is the elusive intermediate responsible for superoxide production. Paramagnetic interactions show that the new semiquinone species is buried in the protein, probably in or near the Qo site but not strongly interacting with the 2Fe2S cluster. The semiquinone is substoichiometric, even with conditions optimized for its accumulation, consistent with recently proposed models where the semiquinone is destabilized to limit superoxide production. The discovery of this intermediate provides a critical tool to directly probe the elusive chemistry that takes place within the Qo site. [ABSTRACT FROM AUTHOR]

Pulsed Dipolar Spectroscopy (PDS) methods of Electron Paramagnetic Resonance (EPR) were used to detect and characterize reversible non-covalent dimers of Human Serum Albumin (HSA), the most abundant protein in human plasma. The spin labels, MTSL and OX063, were attached to Cys-34 and these chemical modifications of Cys-34 did affect the dimerization of HSA, indicating that other post-translational modifications can modulate dimer formation. At physiologically relevant concentrations, HSA does form weak, non-covalent dimers with a well-defined structure. Dimer formation is readily reversible into monomers. Dimerization is very relevant to the role of HSA in the transport, binding, and other physiological processes. [ABSTRACT FROM AUTHOR]

The d-group Kramers ions, having strong zero field splitting (ZFS) with axial symmetry and a negative D value for the ZFS Hamiltonian, are widely considered as candidates for use as single molecular magnets (SMMs). An important need is the means to switch the SMM between its states in a reasonably short and predictable period of time, which is generally not available. We propose an approach, Zeeman–far infrared (ZeFIR) double resonance, in which circularly polarized alternating magnetic fields in the far infrared (FIR) range induce selective magnetic dipole transitions between different Kramers doublets of the SMM and polarized microwave (mw) pulses transfer excitation inside the upper Kramers doublet. A combination of FIR and mw pulses allows unidirectional switching between +S and −S states of the ion. The proposed approach is considered for a model quartet system with total spin S = 3/2, which seems to be the most promising object for selective resonance manipulations of its states by circularly polarized radiation. [ABSTRACT FROM AUTHOR]

The cytochrome bc complexes found in mitochondria, chloroplasts and many bacteria play critical roles in their respective electron transport chains. The quinol oxidase (Qo) site in this complex oxidizes a hydroquinone (quinol), reducing two one-electron carriers, a low potential cytochrome b heme and the ‘Rieske’ iron-sulfur cluster. The overall electron transfer reactions are coupled to transmembrane translocation of protons via a ‘Q-cycle’ mechanism, which generates proton motive force for ATP synthesis. Since semiquinone intermediates of quinol oxidation are generally highly reactive, one of the key questions in this field is: how does the Qo site oxidize quinol without the production of deleterious side reactions including superoxide production? We attempt to test three possible general models to account for this behavior: 1) The Qo site semiquinone (or quinol-imidazolate complex) is unstable and thus occurs at a very low steady-state concentration, limiting O2 reduction; 2) the Qo site semiquinone is highly stabilized making it unreactive toward oxygen; and 3) the Qo, site catalyzes a quantum mechanically coupled two-electron/two-proton transfer without a semiquinone intermediate. Enthalpies of activation were found to be almost identical between the uninhibited Q-cycle and superoxide production in the presence of antimycin A in wild type. This behavior was also preserved in a series of mutants with altered driving forces for quinol oxidation. Overall, the data support models where the rate-limiting step for both Q-cycle and superoxide production is essentially identical, consistent with model 1 but requiring modifications to models 2 and 3. [ABSTRACT FROM AUTHOR]

The mitochondrial cytochrome bc1 complex catalyzes the transfer of electrons from ubiquinol to cyt c while generating a proton motive force for ATP synthesis via the ‘Q-cycle’ mechanism. Under certain conditions electron flow through the Q-cycle is blocked at the level of a reactive intermediate in the quinol oxidase site of the enzyme, resulting in ‘bypass reactions,’ some of which lead to superoxide production. Using analogs of the respiratory substrates ubiquinol-3 and rhodoquinol-3, we show that the relative rates of Q-cycle bypass reactions in the Saccharomyces cerevisiae cyt bc1 complex are highly dependent by a factor of up to 100-fold on the properties of the substrate quinol. Our results suggest that the rate of Q-cycle bypass reactions is dependent on the steady state concentration of reactive intermediates produced at the quinol oxidase site of the enzyme. We conclude that normal operation of the Q-cycle requires a fairly narrow window of redox potentials with respect to the quinol substrate to allow normal turnover of the complex while preventing potentially damaging bypass reactions. [ABSTRACT FROM AUTHOR]

The unexpected formation of a highly strained polycyclic amine was observed in a one-pot synthesis from cyclopentanone, dimethyl fumarate and ammonium acetate. This multistep reaction includes 1,3-dipolar cycloaddition of dimethyl fumarate to the cyclic azomethine ylide formed in situ from cyclopentanone and ammonia. The polycyclic amine product was easily converted into a sterically shielded polycyclic nitroxide. The EPR spectra and spin relaxation behavior of the nitroxide were studied in solution. The spin relaxation seems well suited for the use as a biological spin label and are comparable with those of cyclic nitroxides with two spirocyclic moieties adjacent to the N-O · group., (Copyright © 2019, Dobrynin et al.; licensee Beilstein-Institut.)