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

Abstract

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).