TRAP isCA∆ -SIDC. In 2005 Ruotolo, Robinson, and coworkers published a convincing report that native-like protein quaternary structure could be maintained in the gas phase after nESI from ammonium acetate. The report was founded on ion mobility measurements for TRAP from Bacillus subtilus. Careful CCS calibration and comparison with predicted CCS values for model structures indicated that the 21+ charge state likely featured a collapsed, globular, 11mer structure while the 19+ charge state was likely an intact cyclic ring. Since then, the Wysocki lab has published SID data comparing the 21+, 15+, and 16-charge states. The 21+ and 15+ had distinctly different SID fingerprints, with the latter showing native-like fragmentation for a ring-like quaternary structure. The 16-had similar features to the 15+. This distinct SID behavior persuaded us to include TRAP here as well, to illustrate the effect of overly energetic isCA on an undecameric protein complex .