Vacuum Ultraviolet Postionization of Aromatic Groups Covalently Bound to Peptides

Abstract

Experiments demonstrate that peptides with ionization potentials (IPs) above 7.87 eV can be single-photon-ionized in the gas phase with a molecular fluorine laser following prior chemical derivatization with one of several aromatic tags acting as chromophores. 4-(Dimethylamino)benzoic acid, 1-naphthylacetic acid, and 9-anthracenecarboxylic acid (denoted Benz, Naph and Anth, respectively) behave as chromophores, allowing single-photon ionization for vacuum ultraviolet (VUV) laser light by lowering the IP of the tagged peptide. Anth-tagged peptides that are laser-desorbed from a substrate and subsequently postionized produce mass spectra dominated by the intact radical cation, although protonated ions and fragmented species are also observed. Electronic structure calculations on Anth-tagged peptides indicate that in addition to lowering the ionization potential, the presence of the aromatic tag increases charge localization on and delocalization across the ring structure, which presumably stabilizes the radical cation. Measurements on several tagged peptides confirm this calculation and show that the stabilizing effect of the tag increases with the size of the conjugated system in the order Benz < Naph < Anth. The tagged hexapeptide Anth-GAPKSC exhibits the parent ion, whereas the Benz- and Naph-tagged peptides do not. These results are supported by the experimental comparison of Anth-tagged vs untagged tryptophan, further suggesting that VUV postionization of tagged high-IP species is a promising method for expanding the capabilities of mass spectrometric analyses of molecular species.

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

“…These results were compared with previous work using 10.5 eV LDPI-MS to analyze the native Bis-GMAmethacryloyl overlayer [4]. This work provides further verification of the hypothesis that tagging of analytes with low IP compounds allows their detection by 7.87 eV SPI [26,31,32].…”

“…A VUV grade MgF 2 plano-convex lens (35.3 cm focal length at 180 nm) focused the 6 ϫ 3 mm 2 dimension 157 nm laser beam above the sample plate, and the actual fluorine laser power was ϳ1 MW/cm 2 (20% maximum power), a power range previously shown to be well within the singleÅ photonÅ ionizationÅ regimeÅ [32].Å HighÅ purityÅ premix gas of 0.075% fluorine in helium was used for the fluorine laser discharge media.…”

“…PreviousÅ workÅ [32]Å revealedÅ thatÅ protonationÅ (MÅ ϩ H ϩ ) following SPI was a significant channel in parent ion formation during 7.87 eV LDPI-MS of anthracene labeled hexapeptides. High-resolution mass spectra of Benz-Bis-GMA in the region of the parent ion were collected to determine whether secondary protonation of the parent ion following its formation via SPI may have occurred here.…”

“…Previous work demonstrated that aromatic tagging can be used to facilitate resonant multiphoton ionization of analytes whose underivatized counterparts cannot be similarly detected [27][28][29][30]. Several of the authors previously used a similar strategy to show that aromatic tagging of molecular analytes lowers the overall IP of the tagged molecular complex, allowing 7.87 eV SPI [26,[31][32][33]. Whether by single or multiphoton excitation, apparently aromatic tags can serve as chromophores to allow photoionization [10].…”

“…Whether by single or multiphoton excitation, apparently aromatic tags can serve as chromophores to allow photoionization [10]. Furthermore, aromatic chemical tags such as anthracene that lower the IP also delocalize charge across the radical cation and permit-in at least some cases [26,32]-the formation of intact parent ions.…”