Biosurfactant Complexation of Metals and Applications for Remediation - Biosurfactants: Research Trends and Applications - page 288

of surfactin with cations, especially divalent cations, reduces the hydrophilicity of
the head group by neutralizing the charged residues in the peptide ring, allowing the
surfactin-metal complex to integrate into hydrophobic structures (Shen et al., 2011).
This ionophore activity has garnered a great deal of interest in the medical field
due to its ability to cause hemolysis of erythrocytes, but has yet to be extensively
explored for potential remediation applications.
Similar to rhamnolipids, metals have been found to influence surfactin produc-
tion. Manganese added as MnSO 4 has been shown to increase the production of
surfactin roughly 10-fold. In the same study, additions of Fe resulted in a reduction of
surfactin produced on a per-biomass basis (Cooper et al., 1981). A more recent study
of surfactin production by a thermophilic B. subtilis showed varying responses to a
range of individual metal concentrations with the best productivity resulting from a
mixture of added metals (Makkar and Cameotra, 2002). It is not yet clear whether
the metal effects reported are a result of the metals alone or whether the presence/
absence of other medium components also influenced surfactin production.

o ther B iosurfaCtants
There are a variety of other biosurfactants produced but few are well-characterized
in terms of their interactions with metals. The following section provides some
examples of studies that have used these biosurfactants to complex metals in vari-
ous applications even though, in some cases, the strength of the interaction is not
characterized.

Siderolipids
Siderolipids (formerly flavolipids) are a recently described class of biosurfactants
produced by Pedobacter sp. MTN11 (formerly Flavobacterium sp. MTN11) (Bodour
et al., 2004) (Figure 11.5). The polar moiety of this anionic biosurfactant is composed
of a citric acid and two cadaverine molecules, and the nonpolar moiety is com-
posed of two branched chain acyl groups with 6-10 carbons each. It is reported to
complex Cd with a conditional stability constant of log K = 3.6 (Bodour et al., 2004),

OH

N

N

O

O

HO COOH

O

O

N

H

OH

FIGURE 11.5 Siderolipid (U9, U9) produced by Pedobacter sp. M T N11.