derived from this strategy include the space group, which is one of the first steps in a
conventional approach to solving a crystal structure, distance constraints from
1 H DQNMR, and chemical shifts. These experimental results are unified by
quantum-chemical calculations, enabling the verification of specific packing
models in silico and quantification of π-stacking effects. This approach can be
compared with that employed for solution structures of biomacromolecules through
distance constraints (nuclear Overhauser effect, NOE) and NMR chemical shifts
[171]. This, however, requires a large number of NOE constraints, whereas in a
crystalline solid the periodicity described by the space group gives access to the full
3D structure from only a few constraints. Thus, our strategy, which we propose to
term “multi-technique crystallography”, can be applied in general to provide
quantitative insights into the packing of semicrystalline polymers with specific
intermolecular packing features, such as hydrogen bonds or stacking of aromatic
moieties.
Based on work at the Max Planck Institute for Polymer Research [172, 173],
pulsed electron paramagnetic resonance (EPR) has experienced a remarkable
revival worldwide [174]. In particular, pulsed double electron–electron resonance
(DEER) spectroscopy in combination with site-directed spin labeling [175] is
extensively used today in studies of the structure of proteins, including their
function as carriers of small molecules, and of nucleic acids. Moreover, it is used
to probe large, complex biomacromolecules and their assemblies as well as 1361
protein folding [176].
4.3 Self-Assembly and Dynamics of Polypeptides
Polypeptides, i.e., macromolecules composed of amino acids, are probably the best
known example of molecular structures determined by intramolecular hydrogen
bonds [177]. Resembling biomacromolecules, they are considered for use in drug
delivery and gene therapy and thus have been subject of intensive studies [178]. In
addition, it is known that the superb performance of biological polypeptide-based
materials such as hair or spiders’ silk is due to a hierarchical superstructure over
several length scales, where structure control is exerted at every level of hierarchy
[179]. The two most common local conformations of polypeptides, known as
secondary structures, are the α-helix, stabilized by intramolecular hydrogen
bonds, and the β-sheet, stabilized by intermolecular bonds. These secondary
structures can be probed directly by solid-state NMR [163] and their packing can
be obtained by X-ray analysis [180]. Different chain conformations can also be
distinguished by simple circular dichroism (CD) measurements [177], employed in
Sect. 3.3.2. In addition, the α-helical structure posts a permanent dipole moment
along its backbone and can, therefore, be classified as type-A polymer in
Stockmayer’s classification [181]. This dipole moment can be measured precisely
using dielectric spectroscopy (DS) and can be used as a probe of the persistence
length of the secondary structure [182]. Over the years, we have studied various
168
K. Binder et al.
conventional approach to solving a crystal structure, distance constraints from
1 H DQNMR, and chemical shifts. These experimental results are unified by
quantum-chemical calculations, enabling the verification of specific packing
models in silico and quantification of π-stacking effects. This approach can be
compared with that employed for solution structures of biomacromolecules through
distance constraints (nuclear Overhauser effect, NOE) and NMR chemical shifts
[171]. This, however, requires a large number of NOE constraints, whereas in a
crystalline solid the periodicity described by the space group gives access to the full
3D structure from only a few constraints. Thus, our strategy, which we propose to
term “multi-technique crystallography”, can be applied in general to provide
quantitative insights into the packing of semicrystalline polymers with specific
intermolecular packing features, such as hydrogen bonds or stacking of aromatic
moieties.
Based on work at the Max Planck Institute for Polymer Research [172, 173],
pulsed electron paramagnetic resonance (EPR) has experienced a remarkable
revival worldwide [174]. In particular, pulsed double electron–electron resonance
(DEER) spectroscopy in combination with site-directed spin labeling [175] is
extensively used today in studies of the structure of proteins, including their
function as carriers of small molecules, and of nucleic acids. Moreover, it is used
to probe large, complex biomacromolecules and their assemblies as well as 1361
protein folding [176].
4.3 Self-Assembly and Dynamics of Polypeptides
Polypeptides, i.e., macromolecules composed of amino acids, are probably the best
known example of molecular structures determined by intramolecular hydrogen
bonds [177]. Resembling biomacromolecules, they are considered for use in drug
delivery and gene therapy and thus have been subject of intensive studies [178]. In
addition, it is known that the superb performance of biological polypeptide-based
materials such as hair or spiders’ silk is due to a hierarchical superstructure over
several length scales, where structure control is exerted at every level of hierarchy
[179]. The two most common local conformations of polypeptides, known as
secondary structures, are the α-helix, stabilized by intramolecular hydrogen
bonds, and the β-sheet, stabilized by intermolecular bonds. These secondary
structures can be probed directly by solid-state NMR [163] and their packing can
be obtained by X-ray analysis [180]. Different chain conformations can also be
distinguished by simple circular dichroism (CD) measurements [177], employed in
Sect. 3.3.2. In addition, the α-helical structure posts a permanent dipole moment
along its backbone and can, therefore, be classified as type-A polymer in
Stockmayer’s classification [181]. This dipole moment can be measured precisely
using dielectric spectroscopy (DS) and can be used as a probe of the persistence
length of the secondary structure [182]. Over the years, we have studied various
168
K. Binder et al.