[34]. In the liquid crystalline phase, the disks rotate around the column axis.
A particularly simple way of characterizing such restricted molecular dynamics is
provided by the dynamic order parameter S, 0 S 1. It is defined as the ratio
between the motionally averaged and the static anisotropic NMR interaction, e.g.,
dipole–dipole coupling, anisotropic chemical shift, or quadrupole coupling
[14]. For the rotation of disks in a perfectly packed column, S ¼ 0.5 for
13 C–
1 H
dipole–dipole coupling or
2 H quadrupole coupling, centered around the C–H (C–D)
bond direction. Imperfections of the packing in the liquid crystalline phase, where
disks can be inclined to the column axis, lead to reduction of S below 0.5 and values
as low as 0.15 have been found [35]. Thus, S provides both dynamic and structural
information. In general, solid state NMR yields site selective information about the
amplitude and time scales of molecular motions over broad ranges of length and
time; for a recent review see [30].
The information about the structure and dynamics of polymers that EPR can
provide is very similar [19, 21]. For synthetic polymers, biopolymers, and supramolecular assemblies, nitroxide spin probes and spin labels are particularly useful [36]. In
solution, their EPR spectra are governed by the g-factor and the hyperfine splitting
(denoted as a) to the
14
N nucleus of the NO group. In solution, the former determines
the frequency of the center of the triplet arising from the hyperfine coupling. Both
parameters are sensitive to the electronic environment. In the solid state, the anisotropy of the g-tensor leads to broad characteristic EPR line shapes, similar to those in
solid state NMR. Likewise, the EPR spectra are averaged by rotational motions, yet on
time scales in the nanosecond scale rather than the microsecond scale that is relevant
in NMR [14, 19]. This motional averaging has been exploited extensively in macromolecular science, due to the pioneering work of J. Freed [37]. Moreover, similar to
NMR, the dipole–dipole couplings between electron and nuclear spins can be
exploited to determine intermolecular distances below 1 nm. The much stronger
couplings between two electron spins can probe distances up to about 8–10 nm.
Both types of measurements are achieved by pulsed electron–nuclear or
electron–electron double resonance techniques [23, 24], respectively.
3 Structure and Dynamics of Macromolecular Systems
Governed by Their Local Conformations
3.1 Conformational Memory in Synthetic Polymers
In flexible polymers, the chains tend to form random coils and the local conformations allow isotropic rotational motions of the residues by a combination of angular
fluctuations and conformational transitions [14, 38]. Stiff macromolecules with
flexible side groups, however, lack conformational freedom within the backbone,
298
H.W. Spiess
A particularly simple way of characterizing such restricted molecular dynamics is
provided by the dynamic order parameter S, 0 S 1. It is defined as the ratio
between the motionally averaged and the static anisotropic NMR interaction, e.g.,
dipole–dipole coupling, anisotropic chemical shift, or quadrupole coupling
[14]. For the rotation of disks in a perfectly packed column, S ¼ 0.5 for
13 C–
1 H
dipole–dipole coupling or
2 H quadrupole coupling, centered around the C–H (C–D)
bond direction. Imperfections of the packing in the liquid crystalline phase, where
disks can be inclined to the column axis, lead to reduction of S below 0.5 and values
as low as 0.15 have been found [35]. Thus, S provides both dynamic and structural
information. In general, solid state NMR yields site selective information about the
amplitude and time scales of molecular motions over broad ranges of length and
time; for a recent review see [30].
The information about the structure and dynamics of polymers that EPR can
provide is very similar [19, 21]. For synthetic polymers, biopolymers, and supramolecular assemblies, nitroxide spin probes and spin labels are particularly useful [36]. In
solution, their EPR spectra are governed by the g-factor and the hyperfine splitting
(denoted as a) to the
14
N nucleus of the NO group. In solution, the former determines
the frequency of the center of the triplet arising from the hyperfine coupling. Both
parameters are sensitive to the electronic environment. In the solid state, the anisotropy of the g-tensor leads to broad characteristic EPR line shapes, similar to those in
solid state NMR. Likewise, the EPR spectra are averaged by rotational motions, yet on
time scales in the nanosecond scale rather than the microsecond scale that is relevant
in NMR [14, 19]. This motional averaging has been exploited extensively in macromolecular science, due to the pioneering work of J. Freed [37]. Moreover, similar to
NMR, the dipole–dipole couplings between electron and nuclear spins can be
exploited to determine intermolecular distances below 1 nm. The much stronger
couplings between two electron spins can probe distances up to about 8–10 nm.
Both types of measurements are achieved by pulsed electron–nuclear or
electron–electron double resonance techniques [23, 24], respectively.
3 Structure and Dynamics of Macromolecular Systems
Governed by Their Local Conformations
3.1 Conformational Memory in Synthetic Polymers
In flexible polymers, the chains tend to form random coils and the local conformations allow isotropic rotational motions of the residues by a combination of angular
fluctuations and conformational transitions [14, 38]. Stiff macromolecules with
flexible side groups, however, lack conformational freedom within the backbone,
298
H.W. Spiess