VUV state-selected photoionization of thermally-desorbed biomolecules by coupling an aerosol source to an imaging photoelectron/photoion coincidence spectrometer: case of the amino acids tryptophan and phenylalanine

Abstract

Gas phase studies of biological molecules provide structural and dynamical information on isolated systems. The lack of inter- or intra-molecular interactions facilitates the interpretation of the experimental results through theoretical calculations, and constitutes an informative complement to the condensed phase. However advances in the field are partially hindered by the difficulty of vaporising these systems, most of which are thermally unstable. In this work we present a newly developed aerosol mass thermodesorption setup, which has been coupled to a Velocity Map Imaging (VMI) analyzer operated in coincidence with a Wiley-McLaren Time of Flight spectrometer, using synchrotron radiation as a single photon ionization source. Although it has been previously demonstrated that thermolabile molecules such as amino acids can be produced intact by the aerosol vaporisation technique, we show how its non-trivial coupling to a VMI analyzer plus the use of electron/ion coincidences greatly improves the concept in terms of the amount of spectroscopic and dynamic information that can be extracted. In this manner, we report on the valence shell ionization of two amino acids, tryptophan and phenylalanine, for which threshold photoelectron spectra have been recorded within the first 3 eV above the first ionization energy using synchrotron radiation emitted from the DESIRS beamline located at SOLEIL in France. Their adiabatic ionization energies (IEs) have been measured at 7.40 ± 0.05 and 8.65 ± 0.02 eV, respectively, and their spectra analyzed using existing theoretical data from the literature. The IE values agree well with previously published ones, but are given here with a considerably reduced uncertainty by up to a factor of 5. The photostability of both amino acids is also described in detail, through the measurement of the state-selected fragmentation pathways via the use of threshold electron/ion coincidences (TPEPICO), with appearance energies for the different photofragments given for the vaporization temperatures studied, in correlation with the different molecular orbitals involved as identified from the Threshold Photoelectron Spectra (TPES).

Development and characterization of a single particle laser ablation mass spectrometer (SPLAM) for organic aerosol studies

“…In the literature, EI (70 eV) and PI (15 eV) reference mass spectra can be found only for benzoic acid, phthaldialdehyde and phthalic acid (NIST, 2008;Gaie-Levrel et al, 2008). Under these conditions, it is very difficult to proceed to molecular speciation from our single particle mass spectra.…”

Formation and Fragmentation of Radical Peptide Anions: Insights from Vacuum Ultra Violet Spectroscopy

“…They suggest that laser induced fragmentation occurs via a Rydberg excitation (and/or electron detachment mechanism) and subsequent C α -C bond cleavage [9]. The ionization potentials of tryptophan and phenylalanine in the gas phase are, respectively, 7.4 and 8.65 eV [42]. Ionization thresholds for neutral polypeptides are, however, slightly higher in energy (around 9.8 eV) [43].…”

Nanofocusing, shadowing, and electron mean free path in the photoemission from aerosol droplets

“…Wilson et al [10,11], the DESIRS beamline team at SOLEIL [12] and our group [13] used angle-resolved photoelectron spectroscopy and VUV radiation to visualize shadowing effects in the photoelectron angular distributions of small dielectric particles. The introduction of velocity map imaging for probing particle photoemission was crucial because it is currently the only method that allows one to obtain detailed information on the PEY, the eKE, and the PAD for small dielectric particles (~10 to 300 nm in radius).…”