Protonated leucine enkephalin (YGGFL) was studied by ultraviolet photodissociation (UVPD) from 225 to 300 nm utilizing an optical parametric oscillator tunable wavelength laser system (OPO). Fragments were identified by absolute mass measurement in a 9.4 T Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS). Bond cleavage was preferred in the vicinity of the two aromatic residues, resulting in high ion abundances for a 4 , a 1 , b 3 , y 2 and y 1 fragments. a, b and y ions dominated the mass spectrum, and full sequence coverage was achieved for those types. Photodissociation was most effective at the short wavelength end of the studied range, which is assigned to the onset of the L a π-π∗ transition of the tyrosine chromophore, but worked well also at the L b π-π∗ chromophore absorption maxima in the 35 000-39 000 cm -1 region. Several side-chain and internal fragments were observed. H atom loss is observed only above 41 000 cm -1 , consistent with the requirement of a curve crossing to a repulsive 1 πσ∗ state. It is suggested that the photochemically generated mobile H atom plays a role in further backbone cleavages, similar to the mechanism for electron capture dissociation. The b 4 fragment is most intense at the L b chromophore absorptions, undergoing additional fragmentation at higher photon energies. The high resolution of the FT-ICR MS revealed that out of all x and z-type fragments only x 3 and x 4 were formed, with low intensity. Other previously reported x- and z-fragments were re-assigned to internal fragments, based on exact mass measurement.
Herburger, A., Van Der Linde, C., & Beyer, M. K. (2017). Photodissociation spectroscopy of protonated leucine enkephalin. Physical Chemistry Chemical Physics, 19(17), 10786–10795. https://doi.org/10.1039/c6cp08436b