Synthesis and Characterization of Ester-Derivatized Tetrathiatriarylmethyl Radicals as Intracellular Oxygen Probes

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Abstract
Electron paramagnetic resonance (EPR) spectroscopy using paramagnetic probes has been employed as an important tool for the accurate determination of oxygen (O2) concentrations in biological systems. However, paramagnetic probes are still limited by their intracellular penetrability. Various esterified trityl derivatives were synthesized and characterized, and an X-ray structure of one of the triyl radicals was determined. The ester-derivatized trityls exhibited higher sensitivity to O2 concentration compared to the trityl tricarboxylate CT-03. Cyclic voltammetry was also carried out to assess the susceptibility of the trityl radicals to oxidation and reduction. Among all of the ester-derivatized trityls studied, facile hydrolysis of 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 the type and number of ester groups. Therefore, ester-derivatized trityl radicals show great potential as intracellular EPR oximetry probes and imaging agents.