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before the last large peak. Control experiments without N
S yielded
no small peaks. These small peaks had similar shape and slope
whose critical characteristics discriminated them from thermal
noises, and thus were assigned as perylene
π
−
perylene unfolding or
π
-stack rupturing. The observed curves could be categorized into
three types: (a) curves having two consecutive small peaks and one
large peak, (b) curves displaying only one small peak and one large
peak, and (c) curves exhibiting one large peak only. Figure 5.15a
−
c
exemplify typical force
extension curves obtained from the AFM
pulling experiments. Repeating the AFM pulling experiment many
times, we obtained the statistical occurrence of each category: type
(a), doubly folded states, 34% (146/420); type (b), singly folded
state, 43% (179/420); and type (c), unfolded state, 23% (95/420).
Figure 5.15d reveals the unfolding force distribution while rupturing
the N
−
π
S and the force required to dislodge DNA duplex at a loading
rate of 0.54
m/s. Data indicate that the most probable unfolding
force to actuate perylene
µ
−
perylene unfolding was 19 pN, and to
dislodge DNA duplex was 35 pN. The N
π
S rupturing histograms and
the DNA rupturing histograms can be clearly resolved, suggesting
the DNA
−
π
DNA unzipping does not occur before the
-rupturing
event in most experiments.
Figure 5.15
AFM pulling at a 0.54
µ
m/s speed induces
π
-stack unfolding
and DNA duplex bursting; the force
−
extension curves
(a
c) and force histograms (d) clearly reveal the weak
noncovalent forces at picoNewton levels. The blue histogram
and arrows correspond to the perylene attractions, and the
red histogram and arrows to the DNA hybridization. Type
(a) curves have two small peaks; type (b), one small peak;
type (c) curve, no small peak. (d) Gaussian fitting gave the
most probable rupturing force of 19 and 35 pN for rupturing
perylene
−
π
-stacks and dsDNA, respectively.
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