12 t apex qe

All MS data were acquired on a Bruker 12 T Apex-Qe hybrid FT-ICR mass spectrometer (Bruker Daltonics, Billerica, MA, USA). The parameters for the ion sampling interface and the ion transfer were kept the same as described previously [31 (link)] to ensure that no collisional activation-induced H/D scrambling occurs. An ion accumulation time of 0.2 s in the collision cell was used for the acquisition of survey-scan mass spectra, while 0.3 s was used for obtaining ECD data. The ECD experiments were performed on the entire ion population of XIAP within the ICR cell without precursor selection. Top-down ECD experiments on unlabeled XIAP were done by infusing the protein (2.5 μM) in an aqueous solution containing 0.1% formic acid. The ECD parameters are as follows: electron pulse length, 11 ms; electron beam bias, 1.4 V; grid potential, 12 V; and cathode filament heater current, 1.2 A. Up to 600 scans were accumulated for each ECD spectrum over the m/z range 250–2600; this corresponds to an accumulation time of 10 min. Mass calibration was performed using the ECD fragments of ubiquitin.