Degree of ionization in MALDI of peptides: Thermal explanation for the gas-phase ion formation

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Title
Degree of ionization in MALDI of peptides: Thermal explanation for the gas-phase ion formation
Author(s)
Y J Bae; Y S Shin; Jeong Hee Moon; M S Kim
Bibliographic Citation
Journal of American Society for Mass Spectrometry, vol. 23, no. 8, pp. 1326-1335
Publication Year
2012
Abstract
Degree of ionization (DI) in matrix-assisted laser desorption ionization (MALDI) was measured for five peptides using a-cyano-4-hydroxycinnanmic acid (CHCA) as the matrix. DIs were low 10-4 for peptides and 10-7 for CHCA. Total number of ions (i.e., peptide plus matrix) was the same regardless of peptides and their concentration, setting the number of gas-phase ions generated from a pure matrix as the upper limit to that of peptide ions. Positively charged cluster ions were too weak to support the ion formation via such ions. The total number of gas-phase ions generated by MALDI, and that from pure CHCA, was unaffected by the laser pulse energy, invalidating laser-induced ionization ofmatrixmolecules as themechanismfor the primary ion formation. Instead, the excitation of matrix by laser is simply a way of supplying thermal energy to the sample. Accepting strong Coulomb attraction felt by cations in a solid sample, we propose three hypotheses for gas-phase peptide ion formation. In Hypothesis 1, they originate from the dielectrically screened peptide ions in the sample. In Hypothesis 2, the preformed peptide ions are released as part of neutral ion pairs, which generate gas-phase peptide ions via reaction with matrix-derived cations. In Hypothesis 3, neutral peptides released by ablation get protonated via reaction with matrix-derived cations.
Keyword
Degree of ionizationGas phase proton transferIon formation mechanismLaser fluence dependenceMALDIPeptidePreformed ion
ISSN
1044-0305
Publisher
Amer Chem Soc
DOI
http://dx.doi.org/10.1007/s13361-012-0406-y
Type
Article
Appears in Collections:
Division of Biomedical Research > Disease Target Structure Research Center > 1. Journal Articles
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