UV Photodissociation of Protonated Gly-Trp and Trp-Gly Dipeptides and Their Complexes with Crown Ether in an Electrostatic Ion Storage Ring

Abstract

Photodissociation of protonated GW, WG (G = glycine, W = tryptophan), and their complexes with 18-crown-6-ether (CE) was performed in an electrostatic ion storage ring using a tunable laser system. On the basis of lifetime measurements, action spectra were obtained from 210 to 360 nm. These reveal that whereas [GW + H](+) absorbs maximally at approximately 220 nm, the absorption maximum is <210 nm for [WG + H](+), which is in good accordance with density functional theory calculations that predict band maxima at 221 and 212 nm, respectively. This difference in absorption is ascribed to the ammonium group interacting with the indole ring in the case of GW, thereby lowering the energy of the excited state more than that of the ground state. A broad band at higher wavelengths is observed for WG but not for GW, which again may be linked to differences in conformational structures between the two ions. Absorption spectra for the two CE tagged ions are very similar to each other: The high-energy band is now <210 nm for both peptide ions, and they display an absorption band with a maximum at 270 nm. The crown ether targets the ammonium protons, preventing an interaction between ammonium and indole, and the photophysics of the two complexes is therefore similar. The complexes have significantly longer lifetimes with respect to dissociation than the bare ions. Finally, we report product ion mass spectra at two different excitation wavelengths, 210 and 270 nm. There are significant differences between the two peptides, and the crown ether enhances certain channels, such as the loss of CO + H(2)O and the tryptophan side chain.