Conformation-Specific Spectroscopy of Peptide Fragment Ions in a Low-Temperature Ion Trap

Abstract

We have applied conformer-selective infrared-ultraviolet (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 collisioninduced 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 Nterminus, while a rearrangement reaction is confirmed for the a 4 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.

Rearrangement chemistry of a ions probed by IR spectroscopy

“…2), which is predicted at 3546 cm À1 . This infrared signature of the rearranged imine-amide structure has also been observed in the case of another a 4 ion (of YGGFL) [17], as well as in the case of the YG a 2 ion [22]. This $3540 cm À1 band is observed at higher frequency than typical amide NH stretching bands which are observed near 3450 cm À1 or even at lower frequency if they are involved in hydrogen bond.…”

“…For example, gas-phase IR studies were of outmost importance for characterizing the structure of b 2 ions. It was shown in particular that most of the b 2 ions have an oxazolone structure [9][10][11][12][13][14][15][16][17], as initially proposed by Yalcin et al [18]. This could be clearly demonstrated because the oxazolone CO stretching mode has a specific IR signature in the 1800-2000 cm À1 range where no other IR band can be expected for these systems.…”

“…The NH stretching region has been shown highly diagnostic for probing the structure for protonated peptide b n fragments [15][16][17]. The IR spectra of the a 2 , a 3 , and a 4 CID fragments of protonated polyglycine in the 3200-3800 cm À1 spectral range are given in Fig.…”

“…Gas phase IR spectra of a n fragment ions have been recorded by several groups [17,21,22,[23][24][25] under different experimental conditions. The ion storage devices in these experimental set-ups range from Penning and quadrupole Paul ion traps to a cryogenically cooled linear 22-pole ion trap [17].…”

“…The ion storage devices in these experimental set-ups range from Penning and quadrupole Paul ion traps to a cryogenically cooled linear 22-pole ion trap [17]. In addition, different collisional activation conditions have been used which may lead to substantially different internal energy distributions and isomer populations.…”