Ab-initio electronic structure methods are used to explore potential energy profiles pertinent to the fragmentations of gas-phase radicals thought to be formed in the new negative-ion mode EDD mass spectroscopic studies of peptides. Barriers to fragmentation as well as the associated overall energy differences are computed for the observed Calpha-C backbone bond cleavage as well as for side-chain loss for a variety of side chains (valine, arginine, glutamic acid, and tyrosine). It is found that Calpha-C bond cleavage is favored over side-chain loss, although loss of a tyrosine side chain may compete with Calpha-C cleavage because the tyrosine radical formed can delocalize its unpaired electron over its aromatic ring. In addition, it is found that fragmentation of the nitrogen-centered radicals formed in EDD results in cleavage to produce so-called a*/x fragments rather than a/x* fragments both because producing the former involves a significantly smaller barrier and is nearly thermoneutral, while cleavage to yield a/x* is significantly endothermic.
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