Interpretation of the characteristic fragmentation mechanisms through determining the initial ionization site by natural spin density: A study on the derivatives of tryptophan and tryptamine

The characteristic fragmentation mechanisms of the derivatives of tryptophan and tryptamine are investigated by the density functional theory (DFT) at the B3LYP/6–31+G (d, p) level. The main primary α-cleavage fragmentation has been predicted through determining the initial ionization site by calculated spin density of molecular radical cation and the variation of molecular structures from neutral to cationic form. The results show that the more intensive base peak produced by main primary α-cleavage fragmentation predicted in the present agrees well with those of experimental mass spectra, which is useful for characterizing the fragmentation mechanisms of such indole alkaloids. More importantly, the novel method for determining the most likely initial ionization site proposed in this study is prior to that based on the ionization energy (IE). It will play an important role in unraveling the mechanisms of radical-driven fragmentation not only for small molecules, but also for gas-phase peptides occurring in electron capture dissociation (ECD), electron transfer dissociation (ETD), or low-energy CID of peptide radical cations. However, this study is inappropriate for negative systems.

Graphical abstractElucidation of the characteristic fragmentation patterns for the derivatives of tryptophan and tryptamine through determining the initial ionization site by atomic spin density.Figure optionsDownload full-size imageDownload as PowerPoint slide