Triarylmethyl radicals, TAMs, have been the popular choice for electron paramagnetic resonance (EPR) spectroscopy and imaging applications because of their extraordinary stability in cells and tissues, narrow linewidths resulting in high analytical resolution at μM concentrations, and enhanced sensitivity to O2. In this Communication we propose a general approach and provide experimental data allowing for improvement of the functionality of TAM radicals by developing pH sensitive derivatives. The magnetic resonance parameters, both g-factor and hydrogen hyperfine splitting, of several TAM radicals containing different number of carboxyl groups and hydrogen atoms attached to the aryl substituents were found to be pH dependent. This pH effect was measured using X-band and Q-band EPR spectroscopy and is related to reversible protonation of carboxyl groups. Taking into account that both oxygen and pH play key roles in cellular metabolism and homeostasis, dual function pH and oxygen probes based on TAM radicals may become useful tools in biomedical research.
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