Improved peptide detection with matrix‐assisted laser desorption/ionization mass spectrometry by trimethylation of amino groups

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Abstract
Trimethyl α‐amino derivatives of peptides (penta to deca) with a permanent positive charge on their α‐amino groups were prepared by in vacuo reaction with iodomethane and subjected to matrix‐assisted laser desorption/ionization mass spectrometry (MALDI‐MS). Compared to the unmodified peptide, the signal intensity of the trimethyl α‐amino derivative in MALDI‐MS is increased by at least an order of magnitude. Similarly, an octapeptide with a trimethylated ϵ‐amino group derived from the solitary lysine residue of the B‐chain of insulin also shows the same relative increase in signal intensity. Another advantage of the in vacuo methylation procedure is that trimethylation of a peptide amino group can be carried out readily with a combination of isotopes 13CH3I and 12CH3I or CD3I and CH3I, yielding a doublet signal either 3 or 9 units apart, respectively. The presence or absence of such a doublet signal can be used as a criterion to discriminate between peptide and non‐peptide signals in the mass spectrum. Copyright © 2002 John Wiley & Sons, Ltd.