COULOMBIC EFFECTS IN THE DISSOCIATION OF LARGE HIGHLY CHARGED IONS

Electrospray ionization has been shown to produce highly charged ions for classes of very large molecules. These ions (at relatively low m/z) can be generally activated and dissociated with good efficiency. The effect of coulombic interactions on the stability of large highly charged ions is discussed for a model based on evenly distributed charges for a linear polymer, i.e. a "charges on a string" model. Both activation energy for dissociation and net thermodynamic stability are considered. As a result of the long-range nature of the coulomb potential and the short-range nature of chemical bonding interactions, calculations predict a large coulombic effect on thermodynamic stability, and a somewhat smaller effect on activation energy for ion fragmentation. The most important consequence of this analysis is that as the charge of an ion increases, it should become easier to dissociate. Experimental support for the reduced stability of the higher charged ions is cited. Another consequence of coulombic interactions is that we expect ions with high charge to give fragmentation patterns different from those of ions with low charge. As charge increases, increased rates of dissociation are expected for sites towards the center of large molecules where the coulombic forces are greatest. Limitations of the models and future directions for more rigorous calculations are discussed. The present results are discussed within the context of preliminary experimental results.