Perchlorinated Triarylmethyl Radical 99% Enriched 13C at the Central Carbon as EPR Spin Probe Highly Sensitive to Molecular Tumbling.

Soluble stable radicals are used as spin probes and spin labels for in vitro and in vivo electron paramagnetic resonance (EPR) spectroscopy and imaging applications. We report the synthesis and characterization of a perchlorinated triarylmethyl radical enriched 99% at the central carbon, 13C1-PTMTC. The anisotropy of the hyperfine splitting with the 13C1 (Ax = 26, Ay = 25, Az = 199.5 MHz) and the g (gx = 2.0015, gy = 2.0015, gz = 2.0040) are responsible for a strong effect of the radical tumbling rate on the EPR spectrum. The rotational correlation time can be determined by spectral simulation or via the line width or the apparent Az after calibration, so the spin probe 13C1-PTMTC can be used to measure media microviscosity with high sensitivity.