Aggregational behavior of aqueous dispersions of the antifungal lipopeptide iturin A

Iturin A, a lipopeptide: isolated from Bacillus subtilis, posses a strong a ntifungal activity, and has been devoted to a great deal of attention. Sinc e iturin is an amphiphilic compound with a great propensity to self-associa te in solution as well as inside the membrane, the question arises to wheth er its aggregational behavior is dependent on the concentration of the lipo peptide. In order to test this, the ability of iturin suspensions to encaps ulate water-soluble molecules has been examined. Iturin was dispersed at di fferent concentrations above its: critical micellar concentration, in a buf fer containing the water-soluble dye 5,6-carboxyfluorescein. For iturin A m icelles, a Stokes radius of 1.3 nm and on aggregational number of 7 was obt ained. The results shown in this work clearly demonstrate that iturin dispe rsions in water, at concentrations of 0.7, 1.4 and 3 mM, i.e. far above the critical micellar concentration (40 muM), are capable of encapsulating car boxyfluorescein, probably by adopting a type of aggregate different from th e micelle. Negative-staining electron microscopy shows the presence of vesi cles with an average size of 150 nm. By using C-14-iturin, it is shown that , at 3 mM concentration, 40% of the iturin molecules adopt this vesicular. state. It is proposed that iturin molecules form a fully interdigitated bil ayer, where each hydrocarbon tail span the entire hydrocarbon width of the bilayer, resulting in multilamellar vesicles capable of encapsulating an aq ueous compartment. The possible: implications of these results to the membr ane destabilizing effect of iturin A, are discussed according to the dynami c cone-shape of the iturin molecule. (C) 2001 Elsevier Science Inc. All rig hts reserved.