Sequence specific stabilization of a linear analog of the antifungal lipopeptide iturin A(2) by sodium during low energy electrospray ionization massspectrometry conditions

The structures and stability of sodiated species of 8-Beta, a linear lipope ptide analog (beta -aminotetradecanoyl-NYNQPNS) of the antifungal peptide i turin A(2), were evaluated by electrospray ionization mass spectrometry (ES I-MS). Association of the lipopeptide, 8-Beta, with sodium afforded protect ion from fragmentation at high cone voltages and increasing collision energ y conditions in the ESI-MS. The order of decreasing stability was found as 8-Beta . 1Na > 8-Beta . 2Na > 8-Beta . 3Na > 8-Beta. Substantial difference s were found between fragmentation patterns of the free and sodiated molecu lar species. Breakage of the N-terminal peptide bond of L-Pro generated the major product ions of the free 8-Beta parent ion. Impaired fragmentation o f the sodium adducts of 8-Beta, indicated that this bond is protected by so dium complexation. Fragmentation patterns of the sodiated lipopeptide furth er revealed two specific binding sites for a nonsolvated sodium ion within the two type II beta -turn sequences (beta -aminotetradecanoyl-NYN and QPNS ) of the natural iturin A(2). It is proposed that specific interaction with sodium takes place with most of the peptide bond oxygens in these turns, a nd with the GLn sidechain. This interaction leads to stabilized structures in which the peptide backbone, specifically the peptide bonds in which L-Pr o participates, is protected against low-energy fragmentation during ESI-MS . (J Am Soc Mass Spectrom 2001, 12, 505-516) (C) 2001 American Society for Mass Spectrometry.