Samples of approximately 3.5 µL volume were placed in a 0.6 mm i.d. and 0.84 o.d. quartz capillary and analyzed by ODNP, as described previously21 (link). A “pass through” NMR probe design built to fit inside a 3mm i.d. 6 mm o.d. quartz tube was used. The quartz tube was inserted into a high sensitivity microwave cavity (ER 4119HS-LC, Bruker Biospin) along with the NMR probe and sample. ODNP experiments were performed using a Bruker EMX CW EPR spectrometer and a Bruker Avance III NMR console. The samples were sealed in a capillary with a protective layer of Critoseal on the top and hot beeswax on the bottom. All ODNP measurements were performed at room temperature. The sample was irradiated with up to 6 W of microwaves at the EPR resonant frequency of the spin labels at ~10 GHz using a home-built microwave amplifier22 (link). The magnetic field was set on resonance at the central electron hyperfine transition, here at 9.8 GHz. The spin label concentration of each sample was determined from the double integral of its cw EPR spectrum measured at 1 mW irradiation power, 0.4 G modulation width. The concentration-dependent ODNP relaxation rates, kσ, kρ, and kLow, are normalized to the sample concentration derived from spin counting per integration of the cw EPR spectrum.