The effect of ion size on rate of dissociation: RRKM calculations on model large polypeptide ions.

The larger an ion is, the less likely it is to decompose on mass spectrornetry time scales at given critical and internal energies. This is an obstacle to obtaining structural information on large molecules by mass spectrometry. We performed RRKM calculations on model ions with masses from 0.27 kDa to 102.4 kDa to explore what such calculations predict regarding this limitation. According to the calculations, it is impractical to add enough energy to fragment very large ions unless the decomposition has a low critical energy. It is suggested that ion-molecule reactions that are either very low in their critical energies or exothermic may be a feasible approach to fragmenting ionized macromolecules.