NMR under fast magic angle spinning (MAS) offered an attractive way to elucidate
the packing and local dynamics of the building blocks in supramolecular assemblies
[15]; for a review of the early examples see [16].
In the early days of magnetic resonance, NMR and EPR spectroscopy were
developed in parallel and often by the same people [17, 18]. Later on, the two
techniques largely separated, but recent developments in microwave technology
have allowed spectroscopists to use pulse methods in EPR as well [19] and it is
rewarding to see the two “sister spectroscopies” merge again. In fact, the current
revival of EPR (ESR) spectroscopy in macromolecular science [20–22] is largely due
to the development of pulsed methods by groups active in both solid state NMR and
EPR [23, 24]. Using these techniques, together with site-directed spin labeling [25],
the structure of biomacromolecules and supramolecular assemblies can now be
probed on the nanometer scale, which nicely augments the subnanometer information
provided by NMR. A singular advantage of MR methods is the fact that structure
determination does not require single crystals, as needed for X-ray diffraction or
neutron scattering [26]. Therefore, MR can be applied to condensed matter in all
forms: liquids, crystalline solids, disordered solids, liquid crystals, and even gases.
This chapter collects a few recent studies on the structure and dynamics of
macromolecular and supramolecular systems, largely based on the author’s group
in collaboration with other more synthesis-oriented colleagues. For additional reading
we refer to a recent perspective article [27] and recent reviews [28–30]
2 Solid State NMR and Pulsed EPR Techniques
for Analyzing Structure and Dynamics
Signals originating from hydrogen-bonded protons are well separated in
1
H MAS
NMR spectra, typically resonating between 8 and 20 ppm [11, 16]. Therefore, the
1
H
chemical shift provides semiquantitative information about the strength of the hydrogen bonds. In addition, the
1
H chemical shift is also a sensitive probe of so-called ring
currents associated with aromatic moieties [16]. They are observed as a low field shift
compared to the corresponding liquid state signal and may thereby serve as a direct
hint for π–π interactions. Likewise, the low field shift can be simply related to the
packing via so-called nucleus independent chemical shift (NICS) maps [31]. This
augments the well-known sensitivity of
13
C NMR chemical shifts to local conformation [6], known as the “γ-gauche effect”. Detailed packing information is obtained
from distance measurements between specific proton sites at adjacent building blocks
via high resolution double quantum (DQ) solid state NMR under MAS [16, 28]. This
is particularly important for supramolecular assemblies involving aromatic groups
and functional polymers for organic electronics [32, 33].
Solid state NMR, however, is probably even more powerful for probing the time
scale and geometry of rotational motions [14]. For instance, disk-shaped aromatics
often stack into columnar structures as part of discotic liquid crystals (DLC)
Probing Macromolecular and Supramolecular Structure, Dynamics, and Function. . .
297
the packing and local dynamics of the building blocks in supramolecular assemblies
[15]; for a review of the early examples see [16].
In the early days of magnetic resonance, NMR and EPR spectroscopy were
developed in parallel and often by the same people [17, 18]. Later on, the two
techniques largely separated, but recent developments in microwave technology
have allowed spectroscopists to use pulse methods in EPR as well [19] and it is
rewarding to see the two “sister spectroscopies” merge again. In fact, the current
revival of EPR (ESR) spectroscopy in macromolecular science [20–22] is largely due
to the development of pulsed methods by groups active in both solid state NMR and
EPR [23, 24]. Using these techniques, together with site-directed spin labeling [25],
the structure of biomacromolecules and supramolecular assemblies can now be
probed on the nanometer scale, which nicely augments the subnanometer information
provided by NMR. A singular advantage of MR methods is the fact that structure
determination does not require single crystals, as needed for X-ray diffraction or
neutron scattering [26]. Therefore, MR can be applied to condensed matter in all
forms: liquids, crystalline solids, disordered solids, liquid crystals, and even gases.
This chapter collects a few recent studies on the structure and dynamics of
macromolecular and supramolecular systems, largely based on the author’s group
in collaboration with other more synthesis-oriented colleagues. For additional reading
we refer to a recent perspective article [27] and recent reviews [28–30]
2 Solid State NMR and Pulsed EPR Techniques
for Analyzing Structure and Dynamics
Signals originating from hydrogen-bonded protons are well separated in
1
H MAS
NMR spectra, typically resonating between 8 and 20 ppm [11, 16]. Therefore, the
1
H
chemical shift provides semiquantitative information about the strength of the hydrogen bonds. In addition, the
1
H chemical shift is also a sensitive probe of so-called ring
currents associated with aromatic moieties [16]. They are observed as a low field shift
compared to the corresponding liquid state signal and may thereby serve as a direct
hint for π–π interactions. Likewise, the low field shift can be simply related to the
packing via so-called nucleus independent chemical shift (NICS) maps [31]. This
augments the well-known sensitivity of
13
C NMR chemical shifts to local conformation [6], known as the “γ-gauche effect”. Detailed packing information is obtained
from distance measurements between specific proton sites at adjacent building blocks
via high resolution double quantum (DQ) solid state NMR under MAS [16, 28]. This
is particularly important for supramolecular assemblies involving aromatic groups
and functional polymers for organic electronics [32, 33].
Solid state NMR, however, is probably even more powerful for probing the time
scale and geometry of rotational motions [14]. For instance, disk-shaped aromatics
often stack into columnar structures as part of discotic liquid crystals (DLC)
Probing Macromolecular and Supramolecular Structure, Dynamics, and Function. . .
297