Biomedical Engineering Reference
In-Depth Information
citric acid, and fulvic acid (ranging from log K = 1.2 to 4.5), all commonly found
in soil (Ochoa-Loza et al., 2001). This study provided evidence that a biosurfactant
could offer a versatile and powerful tool for dealing with heavy metal contamination.
Following this discovery, it was shown that monorhamnolipid forms strong com-
plexes with many elements in addition to Cd with the following conditional sta-
bility constant sequence (from strongest, log K = 10.30, to weakest, 0.96): Al
3+
>
Cu
2+
> Pb
2+
> Cd
2+
> Zn
2+
> Fe
3+
> Hg
2+
> Ca
2+
> Co
2+
> Ni
2+
> Mn
2+
> Mg
2+
> K
+
(Ochoa-Loza et al., 2001). This sequence shows that naturally occurring, abundant
metal cations (Ca
2+
, Mg
2+
) have lower stability constants than trace elements of
environmental concern (Pb
2+
, Cd
2+
, Hg
2+
). These results, combined with the fact that
monorhamnolipid complexed metals more strongly than other organic ligands that
might compete in the soil environment, suggested that monorhamnolipid was a good
candidate for metal removal applications.
Insight into the nature of the interactions of monorhamnolipid with metals has
recently been elucidated at the molecular level using a combination of
1
H NMR,
FTIR, and H/D exchange mass spectrometry (Schalnat, 2012). About 600 MHz
1
H
NMR studies of monorhamnolipid solutions containing Pb
2+
provide the surprising
result that the carboxylate moiety is only weakly involved in the metal complexation.
This assertion is supported by the insignificant chemical shift difference observed
for the methylene protons immediately adjacent to the carboxylic acid in the absence
and presence of metal cations in solution. For strong carboxylate-metal ion complex-
ation, chemical shift changes of >2 ppm are typically observed (Bodor et al., 2002).
In contrast, chemical shift changes for these methylene protons for the monorhamno-
lipids with Pb
2+
are only 0.012-0.013 ppm, far smaller than the 2 ppm shift expected
for carboxylate binding.
Given the large formation constants measured for these metal complexes as noted
earlier, one must conclude that strong complexes are formed by binding of the metal
cation to other parts of the monorhamnolipid molecule. Indeed, the small chemical
shift changes observed are similar to those for metal-crown ether and carbohydrate-
metal complexes (Ferrari et al., 2005; Karkhaneei et al., 2001; Pankiewicz et al.,
2005; Rondeau et al., 2003; Rouhollahi et al., 1994). This similarity implies that
strong binding might occur by the involvement of multiple atoms in the monorham-
nolipid, possibly through the formation of a binding pocket involving the carbox-
ylate and the rhamnose sugar. Indeed, an energy-minimized molecular mechanics
model of the C10:C10 monorhamnolipid shown in Figure 11.3a documents hydrogen-
bonding interactions between two of the rhamnose sugar hydroxyls and the carbox-
ylic acid. This leads to the formation of an oxygen-rich cavity that might serve as a
metal cation binding pocket in which shared coordination of metal cations can occur
in much the same way as in a crown ether.
FTIR spectroscopy provides further evidence for metal complexation in a bind-
ing pocket involving both the carboxylate and the sugar hydroxyls. Fruitful spectral
results can be found in two frequency regions of the spectrum. In one frequency
region, the frequency difference between the ν
as
(COO
−
) and ν
s
(COO
−
) bands (∆ν) of
carboxylate species is sensitive to chemical environment and is useful for insight into
metal cation binding. The value of ∆ν is different for free carboxylates compared to
those complexed to metal cations, thus providing an indicator of coordination that
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