Auger electron and photoabsorption spectra of glycine in the vicinity of the oxygen K-edge measured with an X-FEL

Jansen

et al. 2018

Chemistry A European J

Preservation of protein conformation upon transfer into the gas phase is key for structure determination of free single molecules, for example using X‐ray free‐electron lasers. In the gas phase, the helicity of melittin decreases strongly as the protein's protonation state increases. We demonstrate the sensitivity of soft X‐ray spectroscopy to the gas‐phase structure of melittin cations ([melittin+qH]q+, q=2–4) in a cryogenic linear radiofrequency ion trap. With increasing helicity, we observe a decrease of the dominating carbon 1 s–π* transition in the amide C=O bonds for non‐dissociative single ionization and an increase for non‐dissociative double ionization. As the underlying mechanism we identify inelastic electron scattering. Using an independent atom model, we show that the more compact nature of the helical protein conformation substantially increases the probability for off‐site intramolecular ionization by inelastic Auger electron scattering.

Section: Figurementioning

confidence: 99%

Soft X‐ray Spectroscopy as a Probe for Gas‐Phase Protein Structure: Electron Impact Ionization from Within

Bari¹,

Jansen

et al. 2018

Chemistry A European J

“…Depending on the final electronic state of the inner-shell excitation process and even more so, on the molecular orbitals involved in the subsequent Auger de-excitation, the electronic excitation of the melittin cations can cover a range of 50 eV or more, with an average of almost 20 eV [18,34].…”

Section: Small Neutral Lossesmentioning

confidence: 99%

Near-Edge Soft X-ray Absorption Mass Spectrometry of Protonated Melittin

J. Am. Soc. Mass Spectrom.

Bari

Boll

et al. 2018

We have investigated the photoionization and photofragmentation yields of gas-phase multiply protonated melittin cations for photon energies at the K-shell absorption edges of carbon, nitrogen, and oxygen. Two similar experimental approaches were employed. In both experiments, mass selected [melittin+qH] (q=2-4) ions were accumulated in radiofrequency ion traps. The trap content was exposed to intense beams of monochromatic soft X-ray photons from synchrotron beamlines and photoproducts were analyzed by means of time-of-flight mass spectrometry. Mass spectra were recorded for fixed photon energies, and partial ion yield spectra were recorded as a function of photon energy. The combination of mass spectrometry and soft X-ray spectroscopy allows for a direct correlation of protein electronic structure with various photoionization channels. Non-dissociative single and double ionization are used as a reference. The contribution of both channels to various backbone scission channels is quantified and related to activation energies and protonation sites. Soft X-ray absorption mass spectrometry combines fast energy deposition with single and double ionization and could complement established activation techniques. Graphical Abstract ᅟ.

“…However, as all proteins under study are composed from a wide range of amino acids, it is assumed that differences in the soft X-ray absorption induced excitation energy distributions average out. Sanchez-Gonzalez et al 22 have measured the kinetic energy distribution of Auger electrons emitted after C 1s ionization of gas phase glycine, the simplest amino acid, using 560 eV photons (Fig. 11).…”

Section: Electronic Excitation and Internal Temperaturementioning

confidence: 99%

“…Assuming a complete conversion of electronic excitation into vibrational excitation by IVR, the absolute internal temperature T depends on the excitation energy E exc as follows 23 T ¼ E exc cðT; nÞsk where s is the number of oscillators or degrees of freedom, k is the Boltzman constant, and c(T,n) a function that varies from 0 11 Auger-electron spectrum for core ionization of gas-phase glycine molecules using hn = 560 eV photons. 22 The top axis gives the excitation energy, as determined from the cutoff energy (here estimated as 269 eV). The inset sketches Auger electron emission involving the two most loosely bound electrons.…”

Section: Electronic Excitation and Internal Temperaturementioning

confidence: 99%

Near edge X-ray absorption mass spectrometry of gas phase proteins: the influence of protein size

Phys. Chem. Chem. Phys.

Schwob

Lalande

et al. 2016

Multiply protonated peptides and proteins in the gas phase can respond to near edge X-ray absorption in three different ways: (i) non dissociative ionization and ionization accompanied by loss of small neutrals, both known to dominate for proteins with masses in the 10 kDa range. (ii) Formation of immonium ions, dominating for peptides in the 1 kDa range. (iii) Backbone scission leading to sequence ions which is typically weaker and has mainly been observed for peptides in the 1 kDa range. We have studied carbon 1s photoexcitation and photoionization for a series of peptides and proteins with masses covering the range from 0.5 kDa to more than 10 kDa. The gas phase protonated molecules were trapped in a radiofrequency ion trap and exposed to synchrotron radiation. Time of flight mass spectrometry was employed for investigation of the photoionization and photofragmentation processes. A smooth transition from the photofragmentation regime to the non-dissociative photoionization regime is observed. Mass spectra are most complex in the few kDa regime, where non-dissociative ionization, backbone scission and immonium ion formation coexist. The observed correlation between protein size and fragmentation, i.e. radiation damage, is of relevance for soft X-ray microscopy.