Chemistry Reference
In-Depth Information
2.3
Rod-Coil Diblock Copolymers Based on Peptide Rods
Polymers with a stiff helical rod-like structure have many advantages over
other synthetic polymers because they possess stable secondary structures
due to cooperative intermolecular interactions. Examples of polymers with
helical conformation are polypeptides in which the two major structures
include
-helical secondary structure enforces
a rod-like structure, in which the polypeptide main chain is coiled and
forms the inner part of the rod [59]. This rod-like feature is responsible for
the formation of the thermotropic and lyotropic liquid crystalline phases.
Polypeptide molecules with
α
-helices and
β
-sheets. The
α
-helical conformation in solution are arranged
with their long axes parallel to each other to give rise to a nematic liquid
crystalline phase. However, even long chain polypeptides can exhibit a lay-
ered supramolecular structure when they have a well-defined chain length.
For example, the monodisperse poly(
α
,l-glutamic acid) prepared by bacterial
synthetic methods assembles into smectic ordering on length scales of tens of
nanometers [60, 61].
Incorporation of an elongated coil-like block to this helical rod system
in a single molecular architecture may be an attractive way of creating new
supramolecular structures due to its ability to segregate incompatible seg-
ments of individual molecules. The resulting rod-coil copolymers based on
a polypeptide segment may also serve as models providing insight into the
ordering of complicated biological systems. Low molecular weight block
copolymers consisting of poly(
α
-benzyl-l-glutamate)withDPof10or20
and polystyrene with DP of 10 were synthesized by Klok, Lecommandoux,
and a coworker [62]. Both the rod-coil polymers were observed to exhibit
thermotropic liquid-crystalline phases with assembled structures that differ
from the lamellar structures. Incorporation of a polypeptide segment into
a polystyrene segment was observed to induce a significant stabilization of
the
γ
-helical secondary structure, as confirmed by FT-IR spectra. However,
small-angle X-ray diffraction patterns indicated that
α
-helical polypeptides
do not seem to assemble into hexagonal packing for the rod-coil copoly-
mer with ten
α
-benzyl-l-glutamate repeating units. The amorphous character
of the polystyrene coil is thought to frustrate a regular packing of the
γ
-
helical fraction of the short polypeptide segments. Increasing the length of
the polypeptide segment to a DP of 20 gives rise to a strong increase in
the fraction of diblock copolymers with
α
-helical polypeptide segment. By
studying this block copolymer with small-angle X-ray analysis, a 2D hex-
agonal columnar supramolecular structure was observed with a hexagonal
packing of the polypeptide segments adopting an 18/5
α
α
-helical conform-
ation with a lattice constant of 16A. The authors proposed a packing model
for the formation of the double-hexagonal organization. In this model, the
rod-coil copolymers are assembled in a hexagonal fashion into infinitely long
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