Tandem electrospray mass spectrometric studies of proton and sodium ion adducts of neutral peptides with modified N- and C-termini: Synthetic model peptides and microheterogeneous peptaibol antibiotics

Abstract

The fragmentations of [M+H]⁺ and [M+Na]⁺ adducts of neutral peptides with blocked N- and C-termini have been investigated using electrospray ion trap mass spectrometry. The N-termini of these synthetically designed peptides are blocked with a tertiarybutyloxycarbonyl (Boc) group, and the C-termini are esterified. These peptides do not possess side chains that are capable of complexation and hence the backbone amide units are the sole sites of protonation and metallation. The cleavage patterns of the protonated peptides are strikingly different from those of sodium ion adducts. While the loss of the N-terminal blocking group occurs quite readily in the case of MS/MS of [M+Na]⁺, the cleavage of the C-terminal methoxy group seems to be a facile process in the case of MS/MS of [M+H]⁺. Fragmentation of the protonated adducts yields only b_n ions, while y_n and an ions are predominantly formed from the fragmentation of sodium ion adducts. The a_n ions arising from the fragmentation of [M+Na]⁺ lack the N-terminal Boc group (and are here termed a_n^∗ ions). MS/MS of [M+Na]+ species also yields bn ions of substantially lower intensities that lack the N-terminal Boc group (b_n^∗). A similar distinction between the fragmentation patterns of proton and sodium ion adducts is observed in the case of peptides possessing an N-terminal acetyl group. An example of the fragmentation of the H⁺ and Na⁺ adducts of a naturally occurring peptaibol from a Trichoderma species confirms that fragmentation of these two ionized species yields complementary information, useful in sequencing natural peptides. Inspection of the isotopic pattern of b_n ions derived from [M+H]⁺ adducts of peptaibols provided insights into the sequences of microheterogeneous samples. This study reveals that the combined use of protonated and sodium ion adducts should prove useful in de novo sequencing of peptides, particularly of naturally occurring neutral peptides with modified N- and C-termini, for example, peptaibols.