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Fe
2
O
3
thin film
cytochromes
blue: wh
o
le cell on F
e
(III) minera
l
green: cytochrome on Fe(III) mineral
WLC of t
w
o outer m
e
mbrane cyt
o
chromes
Gold substrate
0
100
200
300
400
500
600
distance (nm)
Figure 14.3.
(Left) Schematic of mineral thin-ilm probe used on protein molecules
that were puriied from
S. oneidensis
. (Right) Force spectra for Fe
2
O
3
-probe on OmcA
or MtrC (light and dark green).
33
Blue (both light and dark) curves correspond to force
spectra for the
S. oneidensis
-FeOOH pair. Black curves correspond to the modelled
force-extension relationship for a monomer (shorter curve whose extended length is
~300 nm) and dimer of MtrC or OmcA as determined by the worm-like chain model.
14.4.4 Tuning into a Force-Signature to Idenfy Specific Proteins
The aforementioned example shows how force spectra can provide the critical
piece of information that allows us to understand phenomena that occur
within the space between a microorganism and mineral surface. But, force
spectra such as these embody only one-dimensional events that occur at a
very speciic location on a microorganism (or mineral). If a one-dimensional
force spectrum yields a distinct force-signature, then it can be mapped
in three-dimensional space to actually show the location (and number) of
particular macromolecules across the surface of a living bacterium. Such
information is virtually impossible to gain with any other instrument or
technique. Optical microscopy lacks the resolution to detect single-molecule
events, and high-resolution electron microscopy cannot be performed on
living cells in solution.
As noted earlier, a distinct force-signature was observed when MtrC or
OmcA formed a bond between
and a Fe(III)-containing mineral.
Therefore, we attempted to use AFM to collect an image that shows the
positions of MtrC or OmcA molecules on the surface of a living cell of
S. oneidensis
S.
oneidensis
as it was resting on the surface of an Fe(III) mineral immersed in a
maps for OmcA.
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