Ion mobility experiments were also performed on the ECDmodified SELECT SERIES Cyclic IMS QToF (Waters Corp., UK) using the MS parameters detailed above. For these experiments the instrument was operated in mobility mode with parameters: Automatic ADC; 1 Push Per Bin; Racetrack Bias 70 V; Twave Height 28 V; Twave Velocity 375 ms -1 . The instrument was operated in single pass mode (5 ms separation time) for all separations detailed in this study, since multiple passes were not found to appreciably increase resolution between calmodulin's mobility peaks (Supplementary Figure ) in line with previous observations of cytochrome C. For classic native top-down experiments in which no conformer selection was required, the following cyclic control sequence was used: Inject -Separate for 5 ms -Eject Ions and Acquire. Complete cyclic sequences for all additional experiments are presented in Table within the Supplementary Methods of this manuscript. Further information regarding the geometry of the SELECT SERIES Cyclic IMS QToF (Waters Corp., UK) and the preparation of cyclic control sequences for protein analysis, including activation, are described in detail elsewhere. Electron Capture Dissociation Electron capture dissociation was performed at the postmobility ECD cell (e-MSion, US), with the voltages across the block (L1; L2; LM3; L4; FB; LM5; L6; L7) tuned using reported c and z ions for calmodulin in order to give the optimum dissociation whilst retaining ion transmission. The values used were L1 1.0; L2 -30.0; LM3 10.0; L4 12.0; FB 3.4; LM5 8.5; L6 -30.0; L7 1.0. For all experiments the filament current was maintained at 2.3 A. The Transfer Collision Energy was increased to 16 V, providing supplemental activation to reduce non-specific fragment binding. For calmodulin-peptide complexes the FB was dropped to 2.9 but all other parameters remained the same.