Electron capture dissociation (ECD) of proteins in Fourier transform ion cyclotron resonance mass spectrometry usually leads to charge reduction and backbone-bond cleavage, thereby mostly retaining labile, intramolecular noncovalent interactions. In this report, we evaluate ECD of the 84-kDa noncovalent heptameric gp31 complex and compare this with sustained off-resonance irradiation collisionally activated dissociation (SORI-CAD) of the same protein. Unexpectedly, the 21+ charge state of the gp31 oligomer exhibits a main ECD pathway resulting in a hexamer and monomer, disrupting labile, intermolecular noncovalent bonds and leaving the backbone intact. Unexpectedly, the charge separation over the two products is highly proportional to molecular weight. This indicates that a major charge redistribution over the subunits of the complex does not take place during ECD, in contrast to the behavior observed when using SORI-CAD. We speculate that the ejected monomer retains more of its original structure in ECD, when compared to SORI-CAD. ECD of lower charge states of gp31 does not lead to dissociation of noncovalent bonds. We hypothesize that the initial gas-phase structure of the 21+ charge state is significantly different from the lower charge states. These structural differences result in the different reaction pathways when using ECD.

Anal. Chem.

Geels, R. B. J., van der Vies, S. M., Heck, A., & Heeren, R. (2006). Electron capture dissociation as structural probe for noncovalent gas-phase protein assemblies. Anal. Chem., 78, 7191–7196. doi:10.1021/ac060960p