Electron capture and collisionally activated dissociation mass spectrometry of doubly charged hyperbranched polyesteramides

Electron capture dissociation (ECD) of doubly protonated hyperbranched polyesteramide oligomers (1100-1900 Da) was examined and compared with the structural information obtained by low energy collisionally activated dissociation (CAD). Both the ester and amide bonds of the protonated species were cleaved easily upon ECD with the formation of odd electron (OE.+) or even electron (EE+) fragment ions. Several mechanistic schemes are proposed that describe the complex ECD fragmentation behavior of the multiply charged oligomers. In contrast to studies of biomolecules, the present results indicate that consecutive cleavages induced by intramolecular H-shifts are significant for ECD and of less importance for low energy CAD. The capture of an electron by the ionized species results in fragmentation associated with a redistribution of the excess internal energy over the products and the subsequent bond cleavage. Low energy, multiple collision CAD is found to be a more selective dissociation method than ECD in view of the observation that only amide bonds are cleaved for most of the hyperbranched polymers examined with CAD in this study. ECD appears not to provide complementary structural information compared to CAD in the study of hyperbranched polymers, even though a significantly more complex ECD fragmentation behavior is observed. ECD is shown to be of use for the structural characterization of large oligomers that may not dissociate upon low energy CAD. This is a direct result of the fact that ECD produces ionized hyperbranched oligomers with a relatively high internal energy.