Synthesis and Characterization of Pyridine-Containing Tetrathiatriarylmethyl (TAM) Stable Radicals for Biomedical Magnetic Resonance Applications.

Tetrathiatriarylmethyl (TAM, trityl) radicals are widely used as spin probes, spin labels, and dynamic nuclear polarization agents in magnetic resonance applications due to their exceptional biostability, long relaxation times, and narrow electron paramagnetic resonance (EPR) linewidths. However, existing tetrathiatriarylmethyl radicals rely exclusively on benzene-based aryl frameworks, limiting structural diversity and functional tunability. In this article, we report on the synthesis and characterization of pyridine-containing TAM radicals designed to expand the chemical space of trityl probes. Incorporation of a pyridine ring introduces a single 14N nucleus, yielding a well-resolved 1:1:1 hyperfine triplet with aN ≈ 1.2 G while preserving narrow intrinsic line widths. Highly hydrophilic derivatives exhibit improved aqueous solubility and substantially reduced albumin binding compared to less hydrophilic analogues. The lead probe PyTAM-COOH shows excellent stability in air saturated PBS at room temperature and oxygen-dependent line width broadening with a sensitivity of 0.56 mG per mmHg. These results demonstrate that pyridine incorporation is a viable strategy to tune TAM electronic properties while maintaining favorable EPR spectral features, thereby expanding the toolbox of trityl radicals for biomedical magnetic resonance applications.