Ubiquitin was also analyzed on a Thermo Scientific Orbitrap UHMR mass spectrometer equipped with Po charge reduction to reduce the charge states created during ESI, ultimately generating 1+ precursor ions. Ubiquitin was introduced using a syringe-pumped nanoelectrospray source with an applied voltage of 2800 V. The 210 Po source was positioned 1 centimeter below the electrospray emitter to allow significant charge reduction. A trapping gas setting of 3 was used along with an in-source trapping voltage of 150 V to effectively transmit low-charge ubiquitin ions. MS/MS spectra were acquired at a resolution setting of 400,000 and a fixed injection time of 500 ms. 193 nm UVPD was performed with a single laser pulse at 5.0 mJ. Additional details about each of the proteins used in this study are provided in Table .