Ionophorous and sequestering properties of surfactin, a biosurfactant from Bacillus subtilis

Surfactin is a bacterial lipopeptide containing two carboxylic groups. Its sequestering properties (surfactin binds calcium and magnesium ions) were determined in aqueous solution at pH 9.5. The calculated association constants were K = 1.5 × 105 M−1 for Ca2− and 1.9 × 104 M−1 for Mg2+. Its complexing properties were determined by the two-phase distribution system; the aqueous phase contained Ca2+ or Rb+ ions and the lipidic phase contained surfactin. At pH 9, surfactin forms a 1:1 complex with Ca2+ and a 2:1 complex with Rb+ and it shows a higher affinity for divalent cations than for monovalent cations. Surfactin is non-selective in binding monovalent cations but is slightly more effective in binding Ca2+ than Mg2+ or Ba2+. Surfactin is also a mobile carrier to transport monovalent and divalent cations across a solvent barrier and mediates the exchange between H− and inorganic cation.