BULK AND SOLUTION PROPERTIES OF PEPTIDE-POLYMER CONJUGATES - Biologically-Responsive Hybrid Biomaterials - page 36

Lamellar phase-separated morphologies have been observed for a large
number of peptide-based block copolymers. In comparison to conventional, fully
amorphous diblock copolymers, these lamellar structures are typically observed
over a much broader range of compositions. This behavior in bulk, as in solution,
can be easily explained with a consideration of the flat interface generated by the
rod-rod packing. In addition to these more conventional morphologies, structural
investigation of peptide-based block copolymers has also revealed novel phase-
separated structures, previously unknown for fully amorphous diblock
copolymers. These observations underscore the fact that the solid-state structure
formation of peptide-based block copolymers is not solely dictated by phase
separation (as is the case for amorphous diblock copolymers), but is also
influenced by other factors such as intra- and intermolecular hydrogen bonding
and chain conformation. While much of the early interest in peptide-polymer
conjugates was driven by their potential application as membrane materials or for
the development of antithrombogenic surfaces, more recent studies reveal that
these materials can also possess interesting mechanical properties [61] and can
also be used as templates for nanofabrication and devices [94].
Mainly over the last 10 years, different structures have been obtained from
solution-based assembly, including spherical aggregates, wormlike micelles, and
vesicles. Depending on its hydrophilicity, the polypeptide chain can be found in
the core or shell of the structure. In both situations, the ability of the peptide
chains to stabilize a well-defined conformation (generally ŋ -helix or Ȳ -sheet)—
and to reversibly shift this secondary structure in response to stimuli—allows
increased control over morphologies and so-called “smart” properties. For
instance, the use of charged hydrophilic polypeptide chains (such as PLGA and
PLLys) in shell-forming micelles or vesicles allows the formation of stimuli-
responsive systems with a higher amplitude and tolerance to the presence of salts
as compared to classical polyelectrolytes. Recent work from the groups of
Deming [117], Lecommandoux [118], and Hadjichristidis [122] also evidenced
that the use of a hydrophobic polypeptide chain in ŋ -helical (or rod-like)
conformation drives the self-assembly process into vesicle formation,
independent of the copolymer composition and structure (diblock or triblock).
The block copolymer structures currently under development are
progressively more complex. They may include stimuli-responsiveness to
temperature, pH, or both, possibly in addition to biological function. For
instance, Lin et al. [101] reported micelles with a poly(propylene oxide) core
encased in a mixed shell of poly( L -glutamic acid) and polyethylene glycol.
Micellar radii increased at lowered temperatures and decreased with decreased
pH. Morphological changes due to pH variation were shown to be directly