Efficient Synthesis and Characterization of PEGylated/Deuterated Derivatives of Monophosphonated Tetrathiatriarylmethyl Radicals

Abstract

Electron paramagnetic resonance (EPR) with the use of exogenous spin probes is an indispensable tool to measure pH in vitro and in vivo. Monophosphonated tetrathiatriarylmethyl (p1TAM) radicals are so far the most promising EPR probes for pH measurement. However, their biological applications were limited by extremely low efficiency in synthesis and potential albumin binding, such that they could not be administered systematically. In the present work, an efficient procedure for p1TAM synthesis was reported, with a total yield of 25% from the corresponding triarylmethanol (compared to 1.6% with the commonly used procedure). Further PEGylation prevented the interaction between p1TAM and albumin. The EPR spectra of nondeuterated (POP) and deuterated (dPOP) PEGylated derivatives of p1TAM were also recorded at different pH values. The two probes had similar pH sensitivities with pKa values of 6.80 and 6.79, respectively. POP had a much more complex EPR spectrum than dPOP, with the latter always exhibiting simple EPR doublet signals regardless of the pH. These results suggested that dPOP is more advantageous for pH measurement, relative to POP and p1TAM. Moreover, dPOP was shown to have excellent biological stability and high O2 sensitivity, and thus might have the potential to be used as the probe for simultaneous pH and O2 measurements in biological system.

Keywords

• electron paramagnetic resonance (EPR)
• tetrathiatriarylmethyl (TAM) radicals
• pH
• oxygen
• probes