Factors Affecting the Production of Aromatic Immonium Ions in MALDI 157 nm Photodissociation Studies

Abstract

Abstract. Immonium ions are commonly observed in the high energy fragmentation of peptide ions. In a MALDI-TOF/TOF mass spectrometer, singly charged peptides photofragmented with 157 nm VUV light yield a copious abundance of immonium ions, especially those from aromatic residues. However, their intensities may vary from one peptide to another. In this work, the effect of varying amino acid position, peptide length, and peptide composition on immonium ion yield is investigated. Internal immonium ions are found to have the strongest intensity, whereas immonium ions arising from C-terminal residues are the weakest. Peptide length and competition among residues also strongly influence the immonium ion production. Quantum calculations provide insights about immonium ion structures and the fragment ion conformations that promote or inhibit immonium ion formation.

Use of Ultraviolet Photodissociation Coupled with Ion Mobility Mass Spectrometry To Determine Structure and Sequence from Drift Time Selected Peptides and Proteins

“…There are notable differences in the low m / z region, where UVPD produces the histidine immonium ion (H) at m / z 110 in comparatively high abundance. This is in accordance with previous studies, in which immonium ions from histidine, tryptophan, tyrosine, and phenylalanine were found to be produced abundantly in photodissociation but only to a lesser extent in low-energy CID. , The histidine immonium in LHRH as an internal fragment is most likely produced by a combination of y- and a-type cleavage, in this case y-type cleavage of the a 2 fragment also visible in the obtained spectrum . While the a 2 ion is produced by both CID and UVPD, the conversion from a 2 into H appears increased by UVPD, even though the overall intensity of the a 2 ion remains higher than H. Again, this has been seen previously and could be attributed to a high stability of the a 2 ion. , …”

“…30,31 The histidine immonium in LHRH as an internal fragment is most likely produced by a combination of y-and atype cleavage, in this case y-type cleavage of the a 2 fragment also visible in the obtained spectrum. 30 While the a 2 ion is produced by both CID and UVPD, the conversion from a 2 into H appears increased by UVPD, even though the overall intensity of the a 2 ion remains higher than H. Again, this has been seen previously and could be attributed to a high stability of the a 2 ion. 32,33 The histidine residue in LHRH has been shown to be involved in many interactions, with the C-terminus among others, giving the molecule a ring-like shape in which the histidine side chain sits inside.…”

“…Similar results are shown for GHRP-6 (Figure b), again the dominant fragments are b and y ions, but there is an increase in intensity of the histidine immonium ion and tryptophan side chain peak from the UVPD compared to CID (Figure S2a). The terminal histidine immonium ion in this case may have been produced by a rearrangement of the oxazolone ring of the corresponding b ion . Compared to LHRH, the immonium ion is more abundant in the spectrum of GHRP, and this may be attributed to two reasons: GHRP has only 6 residues (compared to 10) and therefore has less vibrational modes available to it which could dissipate energy.…”

“…The terminal histidine immonium ion in this case may have been produced by a rearrangement of the oxazolone ring of the corresponding b ion. 30 Compared to LHRH, the immonium ion is more abundant in the spectrum of GHRP, and this may be attributed to two reasons: GHRP has only 6 residues (compared to 10) and therefore has less vibrational modes available to it which could dissipate energy. Second, it has been shown that the closer the precursor residue to the N-terminus, the more abundant is the immonium ion.…”

A Novel Triethylphosphonium Charge Tag on Peptides: Synthesis, Derivatization, and Fragmentation

“…The modification of this mass spectrometer to allow 157 nm photodissociation with a Coherent F 2 Laser (Santa Clara, CA, USA) was described previously [32]. Peptide ions travelling through the instrument absorb a single 157 nm photon from an unfocused 2–4 mJ pulse with a 5×10 mm rectangular beam profile.…”