Wassermann, Tobias N.Boyarkine, OlegPaizs, BelaRizzo, Thomas R.2011-12-022011-12-022011-12-02201210.1007/s13361-012-0368-0https://infoscience.epfl.ch/handle/20.500.14299/72951WOS:000303685800006We have applied conformer-selective IR-UV double-resonance photofragment spectroscopy at low temperatures in an ion trap mass spectrometer for the spectroscopic characterization of peptide fragment ions. We investigate b- and a-type ions formed by collision-induced dissociation from protonated leucine-enkephalin. The vibrational analysis and assignment are supported by nitrogen-15 isotopic substitution of individual amino acid residues and assisted by density functional theory calculations. Under such conditions, b-type ions of different size are found to appear exclusively as linear oxazolone structures with protonation on the N-terminus, while a rearrangement reaction is confirmed for the a4 ion in which the side chain of the C-terminal phenylalanine residue is transferred to the N-terminal side of the molecule. The vibrational spectra that we present here provide a particularly stringent test for theoretical approaches.enpeptide fragmentationcold ion spectroscopyb- and a-type fragment ionstandem mass spectrometrytext::journal::journal article::research article