ELECTRON TRANSFER DISSOCIATION OF A MELECTIN PEPTIDE: CORRELATING THE PRECURSOR ION STRUCTURE WITH PEPTIDE BACKBONE DISSOCIATIONS

Electron transfer dissociation (ETD) of doubly and triply charged ions from the amphipathic N-terminal decapeptide GFLSILKKVL-NH2 segment of melectin gave different distributions of fragment ions. The triply charged ions generated extensive series of fragment ions of c and z type that covered the entire sequence from both the N and C termini. In contrast, electron transfer to the doubly charged ions caused backbone cleavages that occurred at residues close to the N and C termini. Attachment of a free low-energy electron to the doubly charged ions caused primary dissociations close to the N and C termini that were followed by consecutive dissociations of z ions. The structure of gaseous doubly charged ions from the melectin peptide was elucidated by a combination of exhaustive conformational search by force-field molecular dynamics, large-scale gradient optimization using the semiempirical PM6 method, and density functional theory single-point energy and gradient optimization calculations. The most stable doubly charged ions were found to be protonated at the lysine e-amino groups and have globular conformations. The backbone cleavages in ETD correlated with the electronic structure of cation-radicals produced by electron attachment to the most stable conformers. The charged lysine ammonium groups direct the incoming electron to the pi* orbitals at the proximate amide groups at Phe, Leu, Lys and Val residues that show the highest spin densities. Electron attachment at these amide groups weakens the N-C-alpha bonds between the Phe-Leu, Leu-Ser, Lys-Lys and Lys-Val residues and causes backbone dissociations.