5 research outputs found
Synthesis and Characterization of Well-Defined PEGylated Polypeptoids as Protein-Resistant Polymers
Well-defined
polypeptoids bearing oligomeric ethylene glycol side
chains (PNMeÂ(OEt)<sub><i>n</i></sub>G, <i>n</i> = 1–3) with a controlled molecular weight (3.26–28.6
kg/mol) and narrow molecular weight distribution (polydispersity index,
PDI = 1.03–1.10) have been synthesized by ring-opening polymerization
of the corresponding <i>N</i>-carboxyanhydrides having oligomeric
ethylene glycol side chains (MeÂ(OEt)<sub><i>n</i></sub>-NCA, <i>n</i> = 1–3) using primary amine initiators. Kinetic
studies of polymerization revealed a first-order dependence on the
monomer concentration, consistent with living polymerization. The
obtained PEGylated polypeptoids are highly hydrophilic with good water
solubility (>200 mg/mL) and are amorphous, with a glass transition
temperature in the −41.1 to +46.4 °C range that increases
with increasing molecular weight and decreasing side chain length.
DLS and SANS analyses revealed no appreciable adsorption of lysozyme
to PNMeOEtG. PNMeOEtG having different molecular weights exhibited
minimal cytotoxicity toward HEp2 cells. These combined results suggest
the potential use of PEGylated polypeptoids as protein-resistant materials
in biomedical and biotechnological fields
Solution Self-Assemblies of Sequence-Defined Ionic Peptoid Block Copolymers
A series of amphiphilic ionic peptoid
block copolymers where the
total number (1 or 3) and position of ionic monomers along the polymer
chain are precisely controlled have been synthesized by the submonomer
method. Upon dissolution in water at pH = 9, the amphiphilic peptoids
self-assemble into small spherical micelles having hydrodynamic radius
in ∼5–10 nm range and critical micellar concentration
(CMC) in the 0.034–0.094 mg/mL range. Small-angle neutron scattering
(SANS) analysis of the micellar solutions revealed unprecedented dependence
of the micellar structure on the number and position of ionic monomers
along the chain. It was found that the micellar aggregation number
(<i>N</i><sub>agg</sub>) and the micellar radius (<i>R</i><sub>m</sub>) both increase as the ionic monomer is positioned
progressively away from the junction of the hydrophilic and hydrophobic
segments along the polymer chain. By defining an ionic monomer position
number (<i>n</i>) as the number of monomers between the
junction and the ionic monomer, <i>N</i><sub>agg</sub> exhibited
a power law dependence on <i>n</i> with an exponent of ∼1/3
and ∼3/10 for the respective singly and triply charged series.
By contrast, <i>R</i><sub>m</sub> exhibited a weaker dependence
on the ionic monomer position by a power law relationship with an
exponent of ∼1/10 and ∼1/20 for the respective singly
and triply charged series. Furthermore, <i>R</i><sub>m</sub> was found to scale with <i>N</i><sub>agg</sub> in a power-law
relationship with an exponent of 0.32 for the singly charged series,
consistent with a weakly charged ionic star-like polymer model in
the unscreened regime. This study demonstrated a unique method to
precisely tailor the structure of small spherical micelles based on
ionic block copolymers by controlling the sequence and position of
the ionic monomer
Nanoscale Lipid/Polymer Hybrid Vesicles: Effects of Triblock Copolymer Composition and Hydrophilic Weight Fraction
Hybrid vesicles resulting from combined self-assembly of polymers
and lipids have gained interest in recent years as drug delivery systems,
biosensors, and model systems for structural scaffolds as artificial
organelles. So far, in the literature, more attention has been given
to study how the discrepancy of chemical compositions and hydrophobic
segment sizes between polymers and lipids affect the vesicle’s
nature. In this study, we focused on how the hydrophilic blocks impact
the vesicle’s morphology. Therefore, we developed hybrid lipid/polymer
vesicle systems with lipid 1,2-dioleoyl-sn-glycero-3-phosphocholine
(DOPC) and amphiphilic triblock copolymers composed of poly(ethylene
glycol)–poly(dimethylsiloxane)–poly(ethylene glycol)
(PEG–PDMS–PEG). Three types of hybrid vesicle systems
were investigated in a systematic manner by changing the hydrophilic
block length and the polymer–lipid composition. From the data
obtained from cryo-transmission electron microscopy (cryo-TEM) and
small-angle neutron scattering, we observed that the hydrophilic mass
fraction of an amphiphilic polymer can affect the membrane thickness,
size polydispersity, and lamellarity of polymer/lipid hybrid vesicles
Dynamics of Phospholipid Membranes beyond Thermal Undulations
We
investigated the molecular dynamics of unilamellar liposomes
by neutron spin echo spectroscopy. We report the first experimental
evidence of a short-range motion at the length scale of the size of
the headgroup of a lipid. The associated mean squared displacement
shows a <i>t</i><sup>0.26</sup> dependence in the pico-
to nanosecond region that indicates another process beyond the predictions
of the Zilman–Granek (ZG) model (<i>t</i><sup>0.66</sup>) and translational diffusion (<i>t</i><sup>1</sup>). A
comparison with theory shows that the observed low exponent is associated
with a non-Gaussian transient trapping of lipid molecules in a local
area and supports the continuous time random walk model. The analysis
of the mean squared displacement leads to the important conclusion
that the friction at the interface between water and liposomes plays
a minor role. Center of mass diffusion of liposomes and transient
trapping of lipids define the range in which the ZG model can be applied
to analyze membrane fluctuations
Monitoring the Internal Structure of Poly(<i>N</i>‑vinylcaprolactam) Microgels with Variable Cross-Link Concentration
The combination of a set of complementary
techniques allows us
to construct an unprecedented and comprehensive picture of the internal
structure, temperature dependent swelling behavior, and the dependence
of these properties on the cross-linker concentration of microgel
particles based on <i>N</i>-vinylcaprolactam (VCL). The
microgels were synthesized by precipitation polymerization using different
amounts of cross-linking agent. Characterization was performed by
small-angle neutron scattering (SANS) using two complementary neutron
instruments to cover a uniquely broad Q-range with one probe. Additionally
we used dynamic light scattering (DLS), atomic force microscopy (AFM),
and differential scanning calorimetry (DSC). Previously obtained nuclear
magnetic resonance spectroscopy (NMR) results on the same PVCL particles
are utilized to round the picture off. Our study shows that both the
particle radius and the cross-link density and therefore also the
stiffness of the microgels rises with increasing cross-linker content.
Hence, more cross-linker reduces the swelling capability distinctly.
These findings are supported by SANS and AFM measurements. Independent
DLS experiments also found the increase in particle size but suggest
an unchanged cross-link density. The reason for the apparent contradiction
is the indirect extraction of the parameters via a model in the evaluation
of DLS measurements. The more direct approach in AFM by evaluating
the cross section profiles of observed microgel particles gives evidence
of significantly softer and more deformable particles at lower cross-linker
concentrations and therefore verifies the change in cross-link density.
DSC data indicate a minor but unexpected shift of the volume phase
transition temperature (VPTT) to higher temperatures and exposes a
more heterogeneous internal structure of the microgels with increasing
cross-link density. Moreover, a change in the total energy transfer
during the VPT gives evidence that the strength of hydrogen bonds
is significantly affected by the cross-link density. A strong and
reproducible deviation of the material density of the cross-linked
microgel polymer chains toward a higher value compared to the respective
linear chains has yet to be explained