Application of W-band 19F electron nuclear double resonance (ENDOR) spectroscopy to distance measurement using a trityl spin probe and a fluorine label.

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).

[A stochastic model for the NMR analysis of the heteroassociation of biologically active aromatic molecules]. / Stokhasticheskaia model' dlia IaMR-analiza geteroassotsiatsii biologicheski aktivnykh aromaticheskikh molekul.

A stochastic model for the NMR analysis of the heteroassociation of two aromatic compounds was developed, which takes into account all physically possible reactions of association of molecules in solution. Expressions for calculating the experimentally observed proton chemical shift were obtained in the general form, and an algorithm for calculating the parameters of heteroassociation using the stochastic model was proposed. The effects of limitations of the basic and general models, as compared with the stochastic model, on the model parameters of the heteroassociation of various biologically active aromatic molecules was analyzed. It was shown that the basic model can be used with a sufficient degree of accuracy for systems with a relatively small contribution of heteroassociation reactions to the total dynamic equilibrium in solution, whereas the general model describes satisfactorily the parameters of heteroassociation practically for all systems studied.