Triarylmethyl Radicals: An EPR Study of 13C Hyperfine Coupling Constants

Abstract

Abstract Triarylmethyl (TAM) radicals are widely used in electron paramagnetic resonance (EPR) spectroscopy as spin labels and in EPR imaging as spin probes for in vivo oxymetry. One of the key advantages of TAMs is the extremely narrow EPR line, especially in case of deuterated analogs (~2.5 μT). Another advantage is their slow spin relaxation even at physiological temperatures; in particular, this characteristic enables the use of pulsed dipolar EPR methods for distance measurements in biomolecules. In this study, a large series of TAM radicals and their deuterated analogs was synthesized, and the corresponding spectroscopic parameters including 13C hyperfine constants were determined for the first time. We observed negligible dependence of 13C hyperfine constants on the solvent or on the structure or number of substituents at para-C atoms of the aromatic rings. In addition, we demonstrated that 13C signals at natural abundance can be used for successful distance measurements at room temperature by pulsed electron double resonance (PELDOR or DEER).