Action spectroscopy of a protonated peptide in the ultraviolet range

Abstract

Action spectroscopy of protonated substance P, a model undecapeptide, has been probed from 5.2 eV to 20 eV. For photon energy above the ionization threshold measured at 10.3 ± 0.1 eV, the radical dication is observed along with side chain losses and abundant formation of all kinds of sequence ions. Below the photoionization threshold, the photoproducts involve side chain cleavages and backbone cleavages into a-, b-/y-, and c-sequence ions. Different electronic excited states appear to produce different fragment ions. Norrish type I and II reactions are proposed to account for some photoproducts. This study bridges the gap between laser activation and electron-induced dissociation of peptides. Moreover, our results report for the first time a comprehensive picture of the photochemical fragmentation of a gas phase peptide in a wide photon energy range.

Charge Transfer Dissociation (CTD) Mass Spectrometry of Peptide Cations: Study of Charge State Effects and Side-Chain Losses

“…charge transfer. Helium cations have an electron affinity of~24.6 eV, and given that they are generated from a 6 kV saddle field ion source, there is more than enough energy to overcome the Coulombic repulsion barrier to enable charge transfer to occur [8,21,24,26,27]. In addition to the charge-increased species, charge-reduced product ions were observed in He-CTD spectra of 2+ and 3+ substance P cations.…”

“…Amino acid side-chain losses have been well-noted and referred to as (M • -X) regions in variety of tandem MS approaches, including UVPD [18,19], action spectroscopy [21], fs-LID [20], EID [17], EED [16], ECD [31][32][33][34][35][36][37][38], ETD [39], CID [40], and MAD [9]. Figure 2 to show more clearly the side-chain losses from the ionized product ions.…”

“…Gas-phase oxidation, or increasing the charge state of gas-phase ions has been observed in a variety of fragmentation methods, including He-MAD [9,26], EID and EED [16,17], and photon-based dissociation methods [20,21].…”

“…For example, bradykinin in Figure 6a shows four different small losses: 30 Da (HCHO) [17], 44 Da ( • C(NH 2 ) = NH 2 + from Arg) [21], 62 Da ( • C(NH 2 ) = NH 2 + + H 2 O) [21], and 91 Da ( • CH 2 (C 6 H 5 ) from Phe) [21], two of which are of significantly higher intensity compared with that in CTD experiment of 1+ substance P. The appearance of fragments corresponding to side-chain losses from phenylalanine and arginine in the (M • -X) region of [M + H] 2+• is consistent with the fact that bradykinin possesses twice the amount of phenylalanine and arginine residues, and that these residues are at or adjacent to the Cterminus in bradykinin.…”

“…To combat these issues, significant interest has been placed in the development of new ion activation methods, such as electronic excitation dissociation (EED) [16], electron ionization dissociation (EID) [17], ultraviolet photodissociation (UVPD) [18,19], femtosecond laser-induced ionization/ dissociation (fs-LID) [20], action spectroscopy (synchrotron radiation) [21], and metastable atom-activated dissociation (MAD) [22,23]. These fragmentation methods all possess a common feature-the capability of dissociating low charge state (1+ & 2+) precursor ions, thus providing complementary structural information to ETD/ECD.…”