M. Rautenbach et al., The interaction of analogues of the antimicrobial lipopeptide, iturin A(2), with alkali metal ions, BIO MED CH, 8(11), 2000, pp. 2539-2548
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.