Biomedical Engineering Reference
In-Depth Information
C4
C7
΄
C3
C2
Pb
2+
(a)
(b)
FIGURE 11.3
(See color insert.)
(a) An energy-minimized molecular mechanics model
of monorhamnolipid (C10, C10) showing the oxygen-rich cavity that may serve as a cation
binding pocket. (b) A model showing how a Pb
2+
ion might interact with the binding pocket
of monorhamnolipid (C10, C10).
is sensitive to coordination type (Colthup et al., 1990; Mehrotra and Bohra, 1983;
Palacios et al., 2004; Strathmann and Myneni, 2004). Due to symmetry differences
between the free ionic species and monodentate metal carboxylate complexes, large
increases in ∆ν are observed. However, bidentate chelation, even weak bidentate
coordination in which the interaction strength between the two carboxylate oxygen
atoms and the metal cation is unequal, does not alter overall symmetry, and, there-
fore, does not significantly alter the spectral behavior from that of the ionic form, or
gives rise to small decreases in ∆ν. For the free monorhamnolipid, ∆ν is on the order
of 160 cm
−1
. For the Pb
2+
complex, this ∆ν decreases slightly to 154 cm
−1
, consistent
with bidendate or weak (i.e., asymmetric) bidentate binding.
Involvement of the sugar hydroxyls in the metal cation binding is also indicated
in the FTIR spectra in the second useful spectral region. Sugar vibrational modes
are known to undergo frequency shifts upon metal coordination (Tajmir-Riahi,
1985, 1989; Tian et al., 2000). For monorhamnolipid-Pb
2+
complexes, the most
significant spectral changes occur for the [δ(COH) + ν(C-O)] band, observed at
1233 cm
−1
for the monorhamnolipid-Pb
2+
complex but at 1191 cm
−1
for the free
monorhamnolipid, and the [δ
ip
(OH) + ν(C-O)] band, observed at 1210 cm
−1
for
the monorhamnolipid-Pb
2+
complex but at 1170 cm
−1
for free monorhamnolipid.
This shift to higher frequency is indicative of an increase in bond strength with
a concomitant decrease in bond length upon Pb
2+
coordination. Collectively, the
data in these two spectral regions support involvement of both the carboxylate and
the sugar in metal cation binding.
Further evidence for the existence of a binding pocket was sought through
gas-phase hydrogen-deuterium exchange (HDX) ESI-MS experiments. This
approach is predicated on changes in the rate of H/D exchange based on the
acidity/basicity of labile hydrogen atoms in addition to the details of the three-
dimensional structure of the molecule that dictates accessibility of these labile
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