Ionisation bias undermines the use of matrix-assisted laser desorption/ionisation for estimating peptide deamidation: Synthetic peptide studies demonstrate electrospray ionisation gives more reliable response ratios

Joanna P. Simpson*, Martin Fascione, Ed Bergström, Julie Wilson, Matthew J. Collins, Kirsty E.H. Penkman, Jane Thomas-Oates

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


RATIONALE: Although mass spectrometry is routinely used to determine deamination in peptide mixtures, the effects of ionisation source choice have not yet been investigated. In particular, matrix-assisted laser desorption/ionisation (MALDI) has become a popular tool with which to measure levels of glutamine deamidation in ancient proteins. Here we use model synthetic peptides to rigorously compare MALDI and electrospray ionisation.

METHODS: We use two synthetic peptides, with glutamine (Q) in one substituted for glutamic acid (E) in the other, to investigate the suitability of MALDI and ESI sources for the assessment of deamidation in peptides using mass spectrometry. We also compare measurements of the same Q and E-containing peptide mixtures using two different mass analysers (time of flight and Fourier-transform ion cyclotron resonance).

RESULTS: When standard mixtures of the Q- and E-containing peptides were analysed using MALDI, under-representation of the E-containing peptide was observed. This observation was consistent between analyses carried out using either TOF or FT-ICR-MS. When the same mixtures were analysed using ESI FT-ICR-MS, no ionisation bias was observed.

CONCLUSIONS: MALDI may not be a suitable ionisation method for the determination of deamidation in peptide mixtures. However, ESI was successfully used to determine the ratio in known mixtures of Q and E containing peptides. These preliminary observations warrant further investigation into ionisation bias when measuring deamidation in other peptide sequences.
Original languageEnglish
Pages (from-to)1049-1057
Number of pages9
JournalRapid Communications in Mass Spectrometry
Issue number12
Early online date25 Mar 2019
Publication statusPublished - 30 Jun 2019

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