Electron paramagnetic resonance (EPR) spectroscopy using paramagnetic probes has been employed asan important tool for the accurate determination of oxygen (O
2) concentrations in biological systems.However, paramagnetic probes are still limited by their intracellular penetrability. Various esterified tritylderivatives were synthesized and characterized, and an X-ray structure of one of the triyl radicals wasdetermined. The ester-derivatized trityls exhibited higher sensitivity to O
2 concentration compared to thetrityl tricarboxylate CT-03. Cyclic voltammetry was also carried out to assess the susceptibility of thetrityl radicals to oxidation and reduction. Among all of the ester-derivatized trityls studied, facile hydrolysisof the acetoxymethoxy esters to the respective carboxylate was observed using porcine liver esterase.This study demonstrates that cellular permeability of the trityl radicals can be achieved by varying thetype and number of ester groups. Therefore, ester-derivatized trityl radicals show great potential asintracellular EPR oximetry probes and imaging agents.
