Fascinating N-terminal C-alpha-C bond cleavages in a series of nonbasic tyrosine-containing peptide radical cations have been observed under low-energy collision-induced dissociation (CID), leading to the generation of rarely observed x-type radical fragments, with significant abundances. CID experiments of the radical cations of the alanyltyrosylglycine tripeptide and its analogues suggested that the Nterminal C alpha-C bond cleavage, yielding its [x(2) + H](center dot+) radical cation, does not involve an N-terminal alpha-carbon-centered radical. Theoretical examination of a prototypical radical cation of the alanyltyrosine dipeptide, using density functional theory calculations, suggested that direct N-terminal C-alpha-C bond cleavage could produce an ion-molecule complex formed 'between the incipient a(1)(+) and x(1)(center dot) fragments. Subsequent proton transfer from the iminium nitrogen atom in a(1)(+) to the acyl carbon atom in x(1)(center dot) results in the observable [x(1) + H](center dot+). The barriers against this novel C-alpha-C bond cleavage and the competitive N-C-alpha bond cleavage, forming the complementary [c(1) + 2H](+)/[z(1) - H](center dot+) ion pair, are similar (ca. 16 kcal mol(-1)). Rice-Ramsperger-Kassel-Marcus modeling revealed that [x(1) + H](center dot+) and [c(1) + 2H](+) species are formed with comparable rates, in agreement with energy-resolved CID experiments for [AY](center dot+).