Peptide quantitation with methyl iodide isotopic tags and mass spectrometry

• Published in: Analyst (London) Vol. 135; no. 6; pp. 1456 - 146
• Main Authors: Blagojevic, Voislav, Zhidkov, Nickholas, Tharmaratnam, Samuel, Pham, Van Thong, Kaplan, Harvey, Bohme, Diethard K
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 01.06.2010
• Subjects: Analytical chemistry; Chemical reactions; Chemistry; Deuterium Exchange Measurement; Exact sciences and technology; Hydrocarbons, Iodinated - chemistry; Iodides; Isotope Labeling; Labels; Mass spectrometers; Mass spectrometry; Methylation; Peptides; Peptides - analysis; Spectrometric and optical methods; Spectrometry, Mass, Electrospray Ionization - methods; Stability; Iodides; Peptides; Sample; Use; Standard; Concentration; Method; Mixture; Degradation; Sensitivity; Storage; Chemical reaction; Dynamics; Methylation; Mass spectrometry; Quantitative analysis
• ISSN: 0003-2654; 1364-5528
• DOI: 10.1039/c0an00003e

A novel method is presented for the quantitation of peptides based on their methylation by in vacuo chemical reaction with methyl iodide. Samples of two small peptides, hexaglycine and pentaalanine, were labeled with CH 3 I and CD 3 I, representing the "unknown" and "standard" respectively, and then subjected to a series of tests using mass spectrometry to ascertain the suitability of the isotopic labels for peptide quantitation. The experiments show methyl iodide to be a very quantitative label, exhibiting a linear relationship in concentration over the dynamic range of the mass spectrometer used in the analysis (up to 4 orders of magnitude) both as pure samples and in a complex mixture of peptides. The tendency of trimethylated peptides to preferentially form a 2 fragment ions in MS 2 produces a significant increase in sensitivity, especially when the mass spectrometer is used in the MRM mode. Tests were also performed to verify the stability of the label against H/D exchange and its suitability for long-term storage, showing little degradation while in solution and during subsequent chemical processing. Isotopic labeling with methyl iodide offers an inexpensive, efficient and reliable method for peptide quantitation. Features such as directed dissociation in tandem mass spectrometry and the presence of permanently charged quaternary amine provide opportunities for increased sensitivity compared to other methods.