Synthesis and Characterization of PEGylated Trityl Radicals: Effect of PEGylation on Physicochemical Properties

Tetrathiatriarylmethyl (TAM, trityl) radicals have attracted considerable attention as spin probes for biological electron paramagnetic resonance (EPR) spectroscopy and imaging owing to their sharp EPR singlet signals and high biostability. However, their in vivo applications were limited by the short blood circulation lifetimes and strong binding with albumins. Our previous results showed that PEGylation is a feasible method to overcome the issues facing in vivo applications of TAM radicals. In the present study, we synthesized a series of new PEGylated TAM radicals (TTP1, TPP2, TNP1, TNP2, d-TNP1, and d-TNP3) containing various lengths and numbers of mPEG chains. Our results found that the pattern of PEGylation exerts an important effect on physicochemical properties of the resulting TAM radicals. Dendritic PEGylated TAM radicals, TNP1 and TNP2, have higher water solubility and lower susceptibility for self-aggregation than their linear analogues TPP1 and TPP2. Furthermore, dendritic PEGylated TAM radicals exhibit extremely high stability toward various biological oxidoreductants as well as in rat whole blood, liver homogenate, and following in vivo intravenous administration in mice. Importantly, the deuterated derivatives, especially d-TNP3, exhibit excellent properties including the sharp and O₂-sensitive EPR singlet signal, good biocompatibility, and prolonged kinetics with half-life time of ≥10 h in mice. These PEGylated TAM radicals should be suitable for a wide range of applications in in vivo EPR spectroscopy and imaging.