Models of fragmentations induced by electron attachment to protonated peptides.

Invoking a number of theoretical levels ranging from HF/STO-3G to CCSD(T)/aug-cc-pVQZ, we have made a detailed survey of six potential energy surfaces (NH4+, NH4*, [CH3CONHCH3]H+, [CH3CONHCH3]H*, [HCONHCH2CONH2]H+, [HCONHCH2CONH2]H*). In conjunction with this, ab inito direct dynamics calculations have been conducted, tracing out several hundred reaction trajectories to reveal details of the electron-capture dissociation mechanism. The model calculations suggest the possibility of a bimodal pattern where some of the radicals, formed upon recombination, dissociate almost directly within one picosecond, the remaining radicals being subject to complete energy redistribution with a subsequent dissociation occurring at the microsecond timescale. Both processes give rise to c and z backbone fragments, resulting from cleavage of N-Calpha bonds of the peptide chain.