Biomedical Engineering Reference
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Fig. 2.9 Conductance change during static capture. Measured conductance change upon molecu-
lar capture for both bare (
dark points
) and RecA-coated (
light points
) dsDNA under various salt
concentrations (
top
). The relation is found to be linear for each (
bottom
) and can be described well
by fitting to (
2
)(
solid lines
)[
13
]
0.38 nC/m. As with the force spectroscopy measurements, these analyses show
that a reduction from the known line charge density of bare dsDNA (
1.1 nC/m) is
necessary to have quantitative agreement between model and experiment. A fit to the
conductance changes measured for RecA-coated dsDNA, meanwhile, yields an
apparent line charge density of
1.6
0.6 nC/m. When compared with the theo-
retical nucleoprotein filament charge of
1.8 nC/m (calculated by simple charge
summation of the RecA amino acids with the underlying dsDNA [
13
]), this result
indicates that a charge reduction is
not
necessary to achieve quantitative agreement
for RecA-dsDNA.
Force spectroscopy is also performed on the protein-coated molecule under
multiple ionic strengths (Fig.
2.10a
). Several things can be noted from these results.
First, the slopes of RecA-dsDNA force curves can be easily distinguished from
those of bare dsDNA under all examined conditions. This points toward the ability
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