Biosurfactant Complexation of Metals and Applications for Remediation - Biosurfactants: Research Trends and Applications - page 287

than for monovalent cations. Monovalent cations have a choice of two binding sites
on surfactin, each of which has different affinity for the cation. For example, for Rb +
there is a site with high affinity (association constant of 71 M −1 ) and a site with low
affinity (association constant of 11 M −1 ). Accordingly, surfactin shows a 1:1 molar
ratio with Ca 2+ and a 2:1 molar ratio with Rb + (Shen et al., 2011).
Other lipopeptides have also been shown to bind metals. Lichenysin is a lipopep-
tide produced by Bacillus licheniformis that structurally resembles surfactin. It has
seven amino acids attached to a C13-C15 fatty acid tail. It differs from surfactin
through the substitution of a glutaminyl residue for glutamic acid in the number one
peptide position. This reduces the molecular charge from −2 in surfactin to −1 in
lichenysin. Lichenysin has been reported to bind Ca 2+ and Mg 2+ fourfold and 10-fold,
respectively, more strongly than surfactin (Grangemard et al., 2001). The affinity for
Ca 2+ over Mg 2+ is only twofold higher for lichenysin in comparison to 10-fold for
surfactin. This may result from the replacement of the more specific “claw” bind-
ing structure of surfactin with a less discriminate binding structure in lichenysin.
Lichenysin also forms a 2:1 molar ratio with Ca +2 instead of the 1:1 reported for
surfactin (Grangemard et al., 2001).
Viscosin is a lipopeptide produced by a variety of Pseudomonas sp. and is char-
acterized by a fatty acid tail connected to two amino acids and a seven-member
cyclic peptide. Viscosin has a conditional stability constant of 5.87 with Cd 2+
(Saini et al., 2008).
Effects of Metals on Lipopeptides
It has been reported that solution cations (Mg 2+ , Mn 2+ , Ca 2+ , Ba 2+ , Li + , Na + , K + , and
Rb + ) can reduce the critical micelle concentration of surfactin from 4- to 12-fold,
depending on the cation and concentration (Thimon et al., 1992). The shape of the
aggregates is affected by both pH and cation concentration and type (mono- vs.
divalent) (Shen et al., 2011; Thimon et al., 1992). Higher pH and lower cation con-
centration both result in smaller and rounder aggregates while lower pH and higher
cation concentration result in aggregates that are rodlike to lamellar. These find-
ings can be explained by the neutralization of charge in the surfactin head group
by added cations. When the charge in the peptide ring is neutralized by a cation,
the effective size of the head-group ring is reduced, allowing molecules to interact
more closely. This results in a reduction of curvature in the aggregate, which favors
larger rodlike and lamellar structures. A divalent cation can completely neutralize
the charge of the surfactin head group, while a monovalent cation only partially neu-
tralizes the charge and so the effect is not as great (the same concept applies to any
charged surfactant molecule).
Surfactin effectively attaches to and integrates with lipid membranes. Its ability
to do so is highly enhanced when combined with metals. Surfactin has been shown
to produce ion-conducting pores in artificial lipid membranes when complexed
with a metal cation. Further, surfactin has been shown to carry metal ions across
a hydrocarbon barrier phase from one aqueous phase to another (Sheppard et al.,
1991; Thimon et al., 1993). This ability can be attributed to its interaction with the
metal cations; surfactin will not exhibit ionopore formation if not allowed to interact
with cations first (Thimon et al., 1993). Recent data suggest that the complexation