Biology Reference
In-Depth Information
5.3.3
NMR Studies of Folding and Self-Assembling
Phenomena
Of particular significance is that all oligomers adopt as folded
nanostructures within the concentration range studied by NMR
and UV
M to 0.1 M) in most solvents. Because
the oligomers exist as the folded structures, the folding
−
vis spectroscopy (1
µ
unfolding
equilibrium constant cannot be determined because the unfolded
species cannot be easily measured. In a good solvent such as
1,1,2,2-tetrachloroethane (TCE), minor amount of unfolded species
of the dimer can be detected. Since folded and unfolded species
are exchanging rapidly, the equilibrium constant
−
= 6.1 was
determined using Eq. 5.4 and the low-concentration NMR data (<1
mM). Similarly,
K
fold
K
= 6.2 was estimated from the intensity ratio of
fold
0
1 transitions from the electronic absorption spectra.
Another significant observation is that the folded nanostructures
can further self-assemble into larger structures like
→
0 and 0
→
2A:2A
or
2A:2A:2A
and the critical concentration for folded oligomers to
self-assemble is again
1 mM, which is essentially same as the
critical concentration of the monomer.
C
=
∼
C
dd
d d
-
1
obs
K
=
(5.4)
fold
-
obs
2
] > 1 mM), the
Ha and Hb resonance peaks shift further upfield. The same forces
that drive the oligomer to fold now drive aggregation into larger
nanostructures (Scheme 5.5). These two processes have similar
enthalpies; it is the entropic barriers that enable folding preceding
self-assemblies.
As the oligomer concentrations increase ([
A
0
The self-assembly formation constant (
K
) can be determined
SA
by solving an over-simplified equal-
K
power series using observed
SA
chemical shift (
) at various concentrations (Fig. 5.1a). Fitting
this power series (solid lines in Fig. 5.1a) to the experimental data,
we obtained
d
obs
−
−
K
= 31 M
1
in TCE and
K
= 90 M
1
in CHCl
at 20
SA
SA
3
°
C. Figure 5.2 shows the van't Hoff plots [ln
K
=
∆
S
0
/
R
−
∆
H
0
/(
RT
)]
of the variable temperature UV
−
vis and
1
H NMR data for both the
folding (
) processes. As expected,
enthalpies for folding and self-assembling are comparable with
K
) and the self-assembling (
K
fold
SA
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