Effect of Dipole Modifying Agents on the Surfactin Induced Conductance of Planar Lipid Bilayers

Abstract

We studied effects of compounds changing membrane dipole potential on membrane conductance induced by antimicrobial lipopeptide surfactin. Surfactin added on both sides of an artificial lipid bilayer from diphytanoyl phosphocholine in 1 M KCl (pH 6.5) produces an increase of the membrane conductance as a result of ion channel formation. Increasing a membrane dipole potential adding RH 421 to the bilayer bathing solution (10 µM , both sides) leads to ∼40 times increase of a steady-state conductance. At the same time, addition of phloretin (20 µM), known to decrease the dipole potential, results in decrease of the surfactin-induced conductance by ∼30 times. We note, that the effects of dipole modifiers on the surfactin-induced membrane conductance are clearly opposite to the effects observed with the same modifiers in case of syringomycin E-induced conductance of lipid bilayers [Ostroumova et al., Langmuir, 2008]. As we suggested earlier, the influence of dipole modifiers on syringomycin activity may be related to a promotion/retardation of a movement of positively charged syringomycin molecules in the direction of membrane hydrocarbon core. In contrast to syringomycin, surfactin is negatively charged. Hence, one can expect an inversion of the effect of dipole modifiers in case of surfactin. The obtained results are in agreement with the model proposed in [Ostroumova et al., Langmuir, 2008].The study was supported in part by the Russian Fund for Basic Research (09-04-48860), the State Program of Molecular and Cell Biology, the grant of St.-Petersburg Administration for young scientists, and the State contract (FAE Π-1372).