Ionization and Photofragmentation of Isolated Metalloporphyrin Cations Investigated by VUV Action Spectroscopy*.

Kaja Schubert,Luke Macaleese,Cornelius L Pieterse,Sadia Bari,Marion Girod,Simone Techert,Thomas Schlathölter,Simon Dörner,Lucas Schwob

doi:10.1002/chem.202101515

Abstract

We investigated the photoionization and fragmentation of isolated metal protoporphyrin IX cations (MPPIX+ with M=Fe, Co, Zn) by means of vacuum‐ultraviolet (VUV) action spectroscopy in the energy range of 8.5–35 eV. Experiments were carried out in the gas phase by interfacing an electrospray ionization tandem mass spectrometer with a synchrotron beamline. The mass spectra and partial ion yields show that photoexcitation of the precursor ions predominantly leads to .CH2COOH radical side‐chain losses of the macrocycle with additional methyl radical (.CH3) side‐chain losses. Ionization, in contrast, leads to the formation of the intact ionized precursor and various doubly charged fragments which are mostly due to side‐chain cleavages. Although statistical fragmentation dominates, we found evidence for non‐statistical processes such as new fragments involving for example single and double H2O losses, indicating that different relaxation mechanisms are at play upon photoionization compared to photoexcitation. The measured ionization energies were 9.6±0.2 eV, 9.4±0.2 eV and 9.6±0.2 eV for FePPIX+, CoPPIX+ and ZnPPIX+, respectively.

Highlights

VUV action spectroscopy has shown its value in previous photofragmentation studies on isolated peptides, by coupling electrospray ionization tandem mass spectrometers (ESI-MS) to synchrotron beamlines
As metalloporphyrins cations can be transferred into the gas phase with electrospray ionization sources, in this study, we could apply for the first time VUV action spectroscopy to isolated metal protoporphyrin IX cations (FePPIX+, CoPPIX+, ZnPPIX+) by interfacing an ESI-MS instrument to the U125-2_NIM VUV beamline of the BESSY II synchrotron (Helmholtz-Zentrum Berlin)
The isotopic distribution for FePPIX+ and CoPPIX+ are similar, whereas the zinc isotopes lead to a broader m/z distribution

Summary

Introduction

VUV action spectroscopy has shown its value in previous photofragmentation studies on isolated peptides, by coupling electrospray ionization tandem mass spectrometers (ESI-MS) to synchrotron beamlines Such experiments have been pioneered a decade ago, providing information on deexcitation mechanisms after photoexcitation and ionization.[34,35,36,37,38,39] As metalloporphyrins cations can be transferred into the gas phase with electrospray ionization sources, in this study, we could apply for the first time VUV action spectroscopy to isolated metal protoporphyrin IX cations (FePPIX+, CoPPIX+, ZnPPIX+) by interfacing an ESI-MS instrument to the U125-2_NIM VUV beamline of the BESSY II synchrotron (Helmholtz-Zentrum Berlin). In order to help assigning isobaric fragments, collision-induced dissociation (CID) data have been recorded for FePPIX+ on a high-resolution tandem mass-spectrometer

Results and Discussion

Conclusion

Conflict of Interest