In a recent study using simple CW EPR spectroscopy [22], new light could be shed
on the dehydration mechanism in LCST-polypeptides [109]. It was shown that
hydrophilic (backbone) and hydrophobic (side chain) hydration layers of ELPs can
exist in a coupled state or a decoupled state (Fig. 8). The decoupled hydration state
consists of hydrophobic and hydrophilic hydration layers that respond independently
to temperature whereas the coupled hydration state is characterized by a common,
cooperative dehydration of both hydration layers. The authors could show that the
primary sequence of an ELP can be tuned to exhibit either of the hydration layer
coupling modes. Charged side chains lead to decoupling, whereas strongly hydrophobic side chains trigger stronger interaction between hydrophilic and hydrophobic
hydration, leading to coupling of both layers. These results indicate that ELPs are the
first identified class of polymers that exhibit a first-order inverse phase transition on
nanoscopic length scales. These findings are important for the understanding and
further use of ELPs in applications such as drug delivery and may also provide
insights into the role of hydration layers in governing the structure–function relationship of intrinsically disordered proteins, as discussed above.
4.2 Columnar Stacks
Columnar stacks are the structure-determining feature of discotic liquid crystals
(DLCs) [24]. As noted in the “Introduction”, the disc-shaped aromatic core units
rotate around the column axis, which can conveniently be studied by NMR via
1
H–
13
C dipole–dipole or
2
H quadrupole coupling. Moreover, imperfections of the
parallel packing within the column lead to a reduction in the dynamic order S to
values below 0.5. Such disorder was indeed observed early on for the extended
hexabenzocoronene (HBC) units with alkyl chains attached, whereas the smaller
triphenylene moieties lead to much narrower DLC phase ranges, but are much better
packed [110]. In fact, the high charge-carrier mobility in a highly ordered helical
columnar structure derived from a triphenylene derivative [111] generated a remarkable interest in the semiconducting, photoconducting, and other electronic properties
of columnar liquid crystal materials. By incorporating a phenylene ring between the
HBC core and the alkyl chain, the order within the column of HBC could be greatly
improved [112] and, together with perylenediimide (PDI), was used to generate
highly efficient self-organized thin films for organic photovoltaics [113].
Indeed, PDI derivatives are attractive in all-organic photovoltaic solar cells and
field-effect transistors. These applications rely on the high charge carrier mobilities
that made PDI the best n-type semiconductors available to date [113]. PDIs have an
elongated shape, and can therefore display considerable dynamics even in the frozen,
crystal-like state. This was observed in a triethyleneglycol (TEG)-substituted PDI
[114]. From X-ray scattering, we found that the PDI building blocks assemble into
columns arranged in a hexagonal unit cell with a lattice parameter of 2.23 nm. The
meridional reflections in the wide-angle region are assigned to the π-stacking distance
of 0.34 nm between individual molecules in the stacks. Additional weak and diffuse
off-meridional reflections show a d-spacing of 0.70 nm, i.e., twice the simple
Probing Macromolecular and Supramolecular Structure, Dynamics, and Function. . .
311
on the dehydration mechanism in LCST-polypeptides [109]. It was shown that
hydrophilic (backbone) and hydrophobic (side chain) hydration layers of ELPs can
exist in a coupled state or a decoupled state (Fig. 8). The decoupled hydration state
consists of hydrophobic and hydrophilic hydration layers that respond independently
to temperature whereas the coupled hydration state is characterized by a common,
cooperative dehydration of both hydration layers. The authors could show that the
primary sequence of an ELP can be tuned to exhibit either of the hydration layer
coupling modes. Charged side chains lead to decoupling, whereas strongly hydrophobic side chains trigger stronger interaction between hydrophilic and hydrophobic
hydration, leading to coupling of both layers. These results indicate that ELPs are the
first identified class of polymers that exhibit a first-order inverse phase transition on
nanoscopic length scales. These findings are important for the understanding and
further use of ELPs in applications such as drug delivery and may also provide
insights into the role of hydration layers in governing the structure–function relationship of intrinsically disordered proteins, as discussed above.
4.2 Columnar Stacks
Columnar stacks are the structure-determining feature of discotic liquid crystals
(DLCs) [24]. As noted in the “Introduction”, the disc-shaped aromatic core units
rotate around the column axis, which can conveniently be studied by NMR via
1
H–
13
C dipole–dipole or
2
H quadrupole coupling. Moreover, imperfections of the
parallel packing within the column lead to a reduction in the dynamic order S to
values below 0.5. Such disorder was indeed observed early on for the extended
hexabenzocoronene (HBC) units with alkyl chains attached, whereas the smaller
triphenylene moieties lead to much narrower DLC phase ranges, but are much better
packed [110]. In fact, the high charge-carrier mobility in a highly ordered helical
columnar structure derived from a triphenylene derivative [111] generated a remarkable interest in the semiconducting, photoconducting, and other electronic properties
of columnar liquid crystal materials. By incorporating a phenylene ring between the
HBC core and the alkyl chain, the order within the column of HBC could be greatly
improved [112] and, together with perylenediimide (PDI), was used to generate
highly efficient self-organized thin films for organic photovoltaics [113].
Indeed, PDI derivatives are attractive in all-organic photovoltaic solar cells and
field-effect transistors. These applications rely on the high charge carrier mobilities
that made PDI the best n-type semiconductors available to date [113]. PDIs have an
elongated shape, and can therefore display considerable dynamics even in the frozen,
crystal-like state. This was observed in a triethyleneglycol (TEG)-substituted PDI
[114]. From X-ray scattering, we found that the PDI building blocks assemble into
columns arranged in a hexagonal unit cell with a lattice parameter of 2.23 nm. The
meridional reflections in the wide-angle region are assigned to the π-stacking distance
of 0.34 nm between individual molecules in the stacks. Additional weak and diffuse
off-meridional reflections show a d-spacing of 0.70 nm, i.e., twice the simple
Probing Macromolecular and Supramolecular Structure, Dynamics, and Function. . .
311