Thermal Vaporization of Biological Nanoparticles:  Fragment-Free Vacuum Ultraviolet Photoionization Mass Spectra of Tryptophan, Phenylalanine−Glycine−Glycine, and β-Carotene

Abstract

A simple, new way to introduce fragile biomolecules into the gas phase via thermal vaporization of nanoparticles is described. The general utility of this technique for the study of biomolecules is demonstrated by coupling this source to tunable synchrotron vacuum ultraviolet radiation. Fragment-free photoionization mass spectra of tryptophan, phenylalanine-glycine-glycine, and beta-carotene are detected with signal-to-noise ratios exceeding 100. The 8.0 eV photoionization mass spectrum of tryptophan nanoparticles vaporized at 373 K is dominated by a single parent ion peak that exhibits a 20-fold enhancement over the methylene indole fragment ion. The degree of dissociative photoionization of tryptophan can be precisely controlled either by the thermal energy imparted into the neutral tryptophan molecule or by the energy of the ionizing photon. The results reveal how approximately 0.5 eV changes in internal energy affect both the photoionization mass spectrum of tryptophan and the appearance energy of the daughter ion fragments. This method allows the ionization energies of glycine (9.3 +/- 0.1 eV), tryptophan (7.3 +/- 0.2 eV), phenylalanine (8.6 +/- 0.1 eV), phenylalanine-glycine-glycine (9.1 +/- 0.1 eV), and beta-carotene (<7.0 eV) molecules to be determined directly from the photoionization efficiency spectra.

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

“…In the same way, further increase in energy would allow the ionization of the indole residue and of the peptide bond accounting for the tryptophan side chain and the formation of a-type product ion series. Indeed, ionization energies for tryptophan and aromatic amino-acid containing peptides were measured at 7.3 and 9.1 eV, respectively [37]. Fragmentation of the peptide would then occur close to the initial position of the radical owing to excess energy let in the peptide.…”

Chemical dynamics, molecular energetics, and kinetics at the synchrotron

“…80 Despite its importance, the details of the biosynthesis of lignin remain unknown, and the specific chemical changes that occur after genetically modifying plants are not completely clear. 81 It is therefore important to be able to image the spatial localization of lignin and its key thermally desorbed biomolecules, 64 However, fragments formed by dissociative photoionization do show remarkable shifts in appearance energy upon an increase of the internal energy of the neutral molecule. 82 Understanding the dissociative photoionization pathways of these molecules as a function of internal energy that is deposited in the sputtering step, coupled with analysis of fragmentation pathways as a function of photon energy is a necessary first step towards successful application of imaging mass spectroscopy to biological and organic molecular systems.…”

7.87 eV laser desorption postionization mass spectrometry of adsorbed and covalently bound bisphenol a diglycidyl methacrylate

“…The ϳ7.8 eV IP of the Bis-GMA group rendered threshold SPI via 7.87 eV radiation possible for all the compounds and attaching the higher IP Azo or Cyanato tags did not lower the overall IP of the tagged Bis-GMA complex. However, the presence of the Benz tag with an IP of 7.4 eV moved SPI away from threshold for Benz-Bis-GMA, dramatically increasing itsÅphotoionizationÅcrossÅsectionÅ [21]ÅcomparedÅwith untagged Bis-GMA. Prior experiments and ab initio calculations supported the argument that the IP of a compound is equal to or less than that of its lowest IP componentÅ [31,Å32].ÅThus,ÅtaggingÅaÅhighÅIPÅanalyte with a low IP species can be used to allow SPI with relatively low photon energies.…”

“…However, 10.5 eV photons are ϳ2.7 eV higher energy than the ϳ7.8 eV IP of Bis-GMA-methacryloyl overlayers (see below). This ϳ2.7 eV excess energy contributed to the extensive fragmentation observed during 10.5 eV SPI [21]. Furthermore, the 118 nm pulsed radiation was generated by frequency tripling of the third harmonic of a Nd:YAG laser in pure Xe gas [6,7], but the VUV intensity was only in the nW range and generated spectra with relatively low signal to noise.…”

“…The mW power range of the fluorine laser makes it perhaps the only laboratory VUV source of sufficient intensity to saturate single photon ionization (SPI) in molecular species [25]. Its 7.87 eV radiation can be used for threshold ionization of various classes of species including aromatic sulfides, aromatic amines, polyaromatic hydrocarbons, some pharmaceutical compounds, and tryptophan-containing peptides [20,21,25,26]. Threshold ionization has the added advantage of imparting relatively little excess energy into the cation, limiting the extent of fragmentation.…”