BULK AND SOLUTION PROPERTIES OF PEPTIDE-POLYMER CONJUGATES - Biologically-Responsive Hybrid Biomaterials - page 31

potential route for mimicking glycoprotein structure and function. DLS showed a
homogeneous population of 45 nm hydrodynamic radius. SANS and TEM
supported a hollow vesicular structure with a wall thickness of around 20 nm.
Copolypeptide vesicles
Inclusion of a polypeptide segment in the vesicle membrane offers advantages
such as stimuli-responsive secondary structure and biocompatibility. Besides
pairing a polypeptide segment with a synthetic polypeptide, researchers have
reported the formation of vesicles from copolymers consisting solely of
polypeptides, without the presence of a synthetic polymer block. This section of
the review summarizes efforts in the formation of vesicles from such
copolypeptide systems.
Evolution of the aggregate morphology of phosphate-head-group-bearing
PMLG 10 occurred in water, as observed by Doi et al . [114,115] Within an hour of
initial preparation, inter-chain hydrogen bonding drove a shift from globular
structures (measuring 50 to 100 nm in diameter, as determined by TEM) to
fibrous aggregates. After twenty-four hours, hydrophobic interactions promoted
the formation of 4-nm-thick twisted ribbons through anti-parallel stacking of the
Ȳ -sheet conformation adopted by PMLG 10 -P (as determined by CD and Fourier
transform infrared (FTIR) spectroscopy.)
Unilamellar vesicles were formed in water from the zwitterionic material
PLGA 15 - b -PLLys 15 [116]. According to DLS data, the initial hydrodynamic
radius near 110 nm increased to 175 nm with an increase in pH from 3 to 12. At
intermediate pH, free chains in solution were observed. Based on NMR analysis
and zeta-potential measurements, the size change was attributed to a structural
inversion of the membrane. Authors thus evidenced for the first time the
formation of “schizophrenic vesicles” that could be reversibly produced as a
function of pH in pure water. Formation of hollow spheres rather than micelles
may be attributed to the low interfacial curvature induced by the presence of a
rod-like polypeptide in the hydrophobic portion of the block copolymer
membrane.
This was also evidenced by the range of structures produced by PLLeu 10-75 - b -
PELLys 60-200 [117]. The ŋ -helical tendencies of either non-ionic residue - the L -
leucine and the ethylene glycol-modified L -lysine - produced sheets, irregular
structures, and vesicles measuring less than 1 Ⱥ m in diameter.
The poly( L -glutamate) brush portion of PLLeu 180 - b -PLGA 180 provided
electrosteric stability to selectively precipitated particles suspended in water or in
0.15 M phosphate saline buffer (pH 7.4) [118]. Particle shape varied according