BULK AND SOLUTION PROPERTIES OF PEPTIDE-POLYMER CONJUGATES - Biologically-Responsive Hybrid Biomaterials - page 23

micelles (R H = 15 nm, as determined by DLS) from a mixture of equal parts
polyanionic PEG 113 - b -PAsp 18, 78 and polycationic PEG 113 - b -PLLys 18, 78 . Charge
neutralization drove the self-assembly of the polyion complex (PIC) core, while
phase separation necessitated the formation of the PEG corona. These forces
together produced the “chain length recognition” phenomenon, in which micelles
preferentially formed from chains with equivalent anionic and cationic segments
[71].
CD spectra obtained from aqueous solutions of PEG 113 - b -PMLG 20,50 and
lactose-modified PEG 75 - b -PMLG 32 revealed an associative tendency for the ŋ -
helical PMLG portion [72]. This tendency could provide the impetus for the
observed aggregation through an initial formation of planar bilayers followed by
closure of the bilayers into vesicles [73]. Although attributed to spherical
micelles, the 250 nm hydrodynamic radius determined by DLS could more
reasonably be assigned to vesicles or micellar aggregates.
Investigation of similarly sized aggregates by Cho et al . [74,75] revealed
aggregation numbers less than 100, a value inconsistent with typical vesicle-
produced aggregation numbers measuring in the thousands [76]. Despite the
known LCST behavior of the PNIPAM block, no size evolution was reported for
the PNIPAM-containing aggregates at that temperature. Zhang and collaborators
[77] reported micelle formation from poly(N-isopropylacrylamide)- b -
poly(lysine- co -glutamic acid). Since the copolypeptide portion was insoluble in
water between pH 6.0 and 10, it formed the core of micelles with hydrodynamic
radius measuring 1800 nm according to light scattering data.
TEM was performed on spherical aggregates measuring from 200 to
700 nm obtained from aqueous dissolution of a triblock copolymer composed
of two poly( L -alanine) or PBLG blocks flanking a central poly(2-
acryloyloxyethyllactoside) (PAELA) portion [78,79]. A decrease in aggregate
size corresponding to an increase in copolymer concentration suggested a
micellar structure, despite the TEM-observed compact structures similar to those
of multilamellar vesicles. Variation in initial polymer concentration was
reported to produce micelle morphologies ranging from spherical to wormlike
[80].
Large aggregates (R H = 425 nm, as determined by DLS) were obtained from
poly(acetyliminoethylene)- b -poly( L -phenylalanine) (PAI- b -PLPhe) at neutral pH
in 0.05 M phosphate buffer [81]. Infrared spectroscopy supplied a possible
explanation for the high aggregation rate, which was unexpected due to the
notably weak presence of hydrophobic L -phenylalanine: hydrogen bridging
between amino acids. Hydrophobic interactions may also have contributed to the
observed micellar clusters and coacervates.