Search

Being a low‐toxic and hydrophilic representative of TAM, OX063 has shown its suitability for in‐vivo and in‐cell EPR experiments and design of spin labels. Using 13C labeling, we investigated the course of oxidative degradation of OX063 into quinone‐methide (QM) under the influence of superoxide as well as further thiol‐promoted reduction of QM into TAM radical, which formally corresponds to substitution of a carboxyl function by a hydroxyl group. We found these transformations being quantitative in model reactions mimicking specific features of biological media and confirmed the presence of these reactions in the blood and liver homogenate of mice in vitro. The emergence of the trityl with the hydroxyl group can be masked by an initial TAM in EPR spectra and may introduce distortions into EPR‐derived oximetry data if they have been obtained for objects under hypoxia. 13C labeling allows one to detect its presence, considering its different hyperfine splitting constant on 13C1 (2.04 mT) as compared to OX063 (2.30 mT). The potential involvement of these reactions should be considered when using TAM in spin‐labeling of biopolymers intended for subsequent EPR experiments, as well as in the successful application of TAM in experiments in vivo and in cell. [ABSTRACT FROM AUTHOR]

The combination of fluorine labeling and pulsed electron-nuclear double resonance (ENDOR) is emerging as a powerful technique for obtaining structural information about proteins and nucleic acids. In this work, we explored the capability of Mims 19F ENDOR experiments on reporting intermolecular distances in trityl- and 19F-labeled DNA duplexes at three electron paramagnetic resonance (EPR) frequencies (34, 94, and 263 GHz). For spin labeling, we used the hydrophobic Finland trityl radical and hydrophilic OX063 trityl radical. Fluorine labels were introduced into two positions of a DNA oligonucleotide. The results indicated that hyperfine splittings visible in the ENDOR spectra are consistent with the most populated interspin distances between 19F and the trityl radical predicted from molecular dynamic (MD) simulations. Moreover, for some cases, ENDOR spectral simulations based on MD results were able to reproduce the conformational distribution reflected in the experimental ENDOR line broadening. Additionally, MD simulations provided more detailed information about the melting of terminal base pairs of the oligonucleotides and about the configuration of the trityls relative to a DNA end. [ABSTRACT FROM AUTHOR]

Recently, Marina Bennati and coworkers (M. Bennati et al., Angew. Chem., Int. Ed., 2020, 59, 373–379., M. Bennati et al., J. Magn. Reson., 2021, 333, 107091) proposed to use electron nuclear double resonance (ENDOR) spectroscopy in the W-band for a pair of labels, nitroxide and 19F, for measurements of short (0.5–1.0 nm) distances in biomolecules. In our paper, we investigated the suitability of high-field ENDOR spectroscopy in the W-band for pairs of triarylmethyl and fluorine labels using five newly synthesized model compounds. It is shown that the application of strong magnetic fields allows distinguishing nuclear frequencies of 19F and protons with sufficient resolution. On the one hand, in contrast to nitroxides, for triarylmethyl radicals, it is not necessary to obtain spectra in different orientations owing to low g-factor anisotropic and long electron spin relaxation times of triarylmethyls. On the other hand, the size of the triarylmethyl radical is substantially larger than that of nitroxide and comparable with measured distances. We theoretically analyzed the suitability of the dipole–dipole approach for triarylmethyl to be used in a 19F ENDOR experiment and determined limitations of this approach. Finally, for comparison, we performed paramagnetic relaxation enhancement (PRE) NMR on the same compounds. In addition, we applied this approach to study the process of a thiol exchange between molecules of triarylmethyl-labeled and 19F-labeled human serum albumin (HSA). [ABSTRACT FROM AUTHOR]

The noncatalytic bromination of benzene is shown experimentally to require high 5-14 M concentrations of bromine to proceed at ambient temperatures to form predominantly bromobenzene, along with detectable (<2%) amounts of addition products such as tetra and hexabromocyclohexanes. The kinetic order in bromine at these high concentrations is 4.8 ± 0.06 at 298 K and 5.6 ± 0.11 at 273 K with a small measured inverse deuterium isotope effect using D6-benzene of 0.97 ± 0.03 at 298 K. These results are rationalized using computed transition states models at the B3LYP+D3/6-311++G(2d,2p) level with an essential continuum solvent field for benzene applied. The model with the lowest predicted activation free energies agrees with the high experimental kinetic order in bromine and involves formation of an ionic, concerted, and asynchronous transition state with a Br8 cluster resembling the structure of the known Br9−. This cluster plays three roles; as a Br+ donor, as a proton base, and as a stabilizing arm forming weak interactions with two adjacent benzene CH hydrogens, these aspects together combining to overcome the lack of reactivity of benzene induced by its aromaticity. The computed inverse kinetic isotope effect of 0.95 agrees with experiment, and arises because CBr bond formation is essentially complete, whereas CH cleavage has not yet commenced. The computed free energy barriers for the reaction with 4Br2 and 5Br2 for a standard state of 14.3 M in bromine are reasonable for an ambient temperature reaction, unlike previously reported theoretical models involving only one or two bromines. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Although 1,1′-bi-2-naphthol (BINOL) is known to racemize in aqueous mineral acids and much more readily in superacids, it retains its initial configuration in one of the strongest superacid system, HSO3F-SbF5-SO2ClF due to protective C8,C8′-diprotonation. [ABSTRACT FROM AUTHOR]

The object of this study is the interaction of the cyclic anhydride 2 of (18 α,19 β)-19-hydroxy-2,3-secooleanane-2,3,28-trioic acid 28,19-lactone ( 1) with primary and secondary amines. It was shown that the products of steric control (the corresponding 2-amino-2-oxo-3-oic acids=2-amides) were formed solely upon the opening of the anhydride cycle by secondary amines ( Scheme 2), whereas the interaction with primary amines yielded a mixture of isomeric amides ( Scheme 10). In the latter case, the solvent provided a noticeable effect on the reaction selectivity, which was demonstrated in the case of 4-methoxybenzylamine. The interaction between the resulting 3-amides and oxalyl chloride yielded the corresponding cyclic imides, whereas under these conditions, 2-amides formed spiropyrrolidinetriones ( Scheme 4). [ABSTRACT FROM AUTHOR]

Reduced derivatives of 2,3-seco-28-oxo-19 β,28-epoxy-18 α-olean-2,3-dicarboxylic acid and its cyclic anhydride were prepared. Reduction of the starting 2,3-secodicarboxylic acid by NaBH-I produced the 2,3-seco-2,3-dihydroxy derivative. Reaction of the starting anhydride with LiAlH gave the 2,3-seco2,3,19 β,28-tetrahydroxy derivative. Cyclization using acidic reagents of the 2,3-seco-2,3-hydroxy- and 2,3seco-2,3,19 β,28-tetrahydroxy derivatives gave the corresponding cyclic ethers containing an oxepane ring. The anhydride ring was reduced by NaBH to the corresponding ε-lactone, the structure of which was confirmed by an x-ray crystal structure. [ABSTRACT FROM AUTHOR]

The combination of fluorine labeling and pulsed electron-nuclear double resonance (ENDOR) is emerging as a powerful technique for obtaining structural information about proteins and nucleic acids. In this work, we explored the capability of Mims 19 F ENDOR experiments on reporting intermolecular distances in trityl- and 19 F-labeled DNA duplexes at three electron paramagnetic resonance (EPR) frequencies (34, 94, and 263 GHz). For spin labeling, we used the hydrophobic Finland trityl radical and hydrophilic OX063 trityl radical. Fluorine labels were introduced into two positions of a DNA oligonucleotide. The results indicated that hyperfine splittings visible in the ENDOR spectra are consistent with the most populated interspin distances between 19 F and the trityl radical predicted from molecular dynamic (MD) simulations. Moreover, for some cases, ENDOR spectral simulations based on MD results were able to reproduce the conformational distribution reflected in the experimental ENDOR line broadening. Additionally, MD simulations provided more detailed information about the melting of terminal base pairs of the oligonucleotides and about the configuration of the trityls relative to a DNA end.

Recently, Marina Bennati and coworkers (M. Bennati et al. , Angew. Chem., Int. Ed. , 2020, 59 , 373-379., M. Bennati et al. , J. Magn. Reson. , 2021, 333 , 107091) proposed to use electron nuclear double resonance (ENDOR) spectroscopy in the W-band for a pair of labels, nitroxide and 19 F, for measurements of short (0.5-1.0 nm) distances in biomolecules. In our paper, we investigated the suitability of high-field ENDOR spectroscopy in the W-band for pairs of triarylmethyl and fluorine labels using five newly synthesized model compounds. It is shown that the application of strong magnetic fields allows distinguishing nuclear frequencies of 19 F and protons with sufficient resolution. On the one hand, in contrast to nitroxides, for triarylmethyl radicals, it is not necessary to obtain spectra in different orientations owing to low g -factor anisotropic and long electron spin relaxation times of triarylmethyls. On the other hand, the size of the triarylmethyl radical is substantially larger than that of nitroxide and comparable with measured distances. We theoretically analyzed the suitability of the dipole-dipole approach for triarylmethyl to be used in a 19 F ENDOR experiment and determined limitations of this approach. Finally, for comparison, we performed paramagnetic relaxation enhancement (PRE) NMR on the same compounds. In addition, we applied this approach to study the process of a thiol exchange between molecules of triarylmethyl-labeled and 19 F-labeled human serum albumin (HSA).