The interaction of analogues of the antimicrobial lipopeptide, iturin A(2), with alkali metal ions

Electrospray mass spectrometry was employed as a tool in this first study o n the molecular interaction between the alkali metal ions and antifungal li popeptide iturin A, and some analogues. Cationisation by sodium and signal intensity of lipopeptide species depended on sodium concentration, but was independent of sample solvent, carrier solvent polarity and sample pH betwe en 4 and 11. 8-Beta, a linear analogue of iturin A(2) (8-Beta; beta -aminot etradecanoyl-NYNQPNS), and its shorter linear lipopeptide analogues, associ ated either one or two alkali metal cations, while the N-->C cyclic peptide s associated with only one cation. The chirality of the beta -NC14 residue had a limited influence on the cationisation. It was observed that 8-Beta c ontained at least four interaction sites for a cation of which two, the C-t erminal carboxylate and the side-chain of tyrosine, can take part in ionic interaction with a cation. It is proposed that the remaining two interactio n centres of alkali metal ions are within the two type II beta -turns found in conformation of natural iturin A. This was corroborated by the diminish ed capacity of the shorter peptides, in which one of the p-turns was elimin ated to bind a second larger cation. All the lipopeptides showed the same o rder of alkali metal ion selectivity: Na+ > K+ > Rb+. These results indicat ed a size limitation in the interaction cavity or cavities. The absence of, or observation of only low abundance, di-cationised complexes of cyclic pe ptides the indicated association of the cation in the interior of the pepti de ring. It is thus hypothesised that alkali metal ions can bind in one of the two p-turns in the natural iturin A molecule. (C) 2000 Elsevier Science Ltd. All rights reserved.