1B-O1-1525 PDF
Difference of Electron Capture and Transfer Dissociation Mass Spectrometry on Zn2+-Polyhistidine Complexes in the Absence of Remote Protons
The Zn2+−polyhistidine complexes were used as a model to investigate the mechanism of ECD/ETD fragmentation. Since we were focusing on the involvement of remote protons in the ECD/ETD process of a metal−peptide complex, the C-terminal carboxyl group in polyhistidine was amidated to avoid the production of remote protons in the complex. Although the complex does not have any remote proton, ETD mass spectra of shows the fragment ions due to the N−Cα bond cleavage. The results strongly suggest that the ETD of the Zn2+-polyhistidine led to aminoketyl anion radical formation followed by N–Cα bond cleavage. In contrast, the ECD of Zn2+-polyhistidine complexes differs dramatically from the ETD, which originate from the different electronic states of the charge-reduced complexes formed by ECD and ETD. ECD produced charge-reduced product with excited states, which undergoes hydrogen radical transfer, giving a long-lived radical intermediate with a [1H,2H]-type imidazoline group. Subsequently, the [1H,2H]-type imidazoline group is reported to release a hydrogen radical. As a result, [bn–H+Zn]+ originated from the fragmentation of even-electron complexes, [M–H+Zn]+. [bn–H+Zn]+ then produced [an–H+Zn]+ by loss of carbon monoxide. The detail of the fragmentation processes are investigated by DFT calculation with M06-2X/6-31+G(d,p) level.