Synthesis and characterization of a biocompatible 13 C 1 isotopologue of trityl radical OX071 for in vivo EPR viscometry

We describe the synthesis, characterization, and application of an isotopologue of the trityl radical OX071, labeled with C at the central carbon ( C ). This spin probe features large anisotropy of the hyperfine coupling with the C ( = 1/2), leading to an EPR spectrum highly sensitive to molecular tumbling. The high biocompatibility and lack of interaction with blood albumin allow for systemic delivery and measurement of tissue microviscosity by EPR.