11 research outputs found
Hydration effects turn a highly stretched polymer from an entropic into an energetic spring
Polyethylene glycol (PEG) is a structurally simple and nontoxic water-soluble polymer that is widely used in medical and pharmaceutical applications as molecular linker and spacer. In such applications, PEG’s elastic response against conformational deformations is key to its function. According to text-book knowledge, a polymer reacts to the stretching of its end-to-end separation by a decrease in entropy that is due to the reduction of available conformations, which is why polymers are commonly called entropic springs. By a combination of single-molecule force spectroscopy experiments with molecular dynamics simulations in explicit water, we show that entropic hydration effects almost exactly compensate the chain conformational entropy loss at high stretching. Our simulations reveal that this entropic compensation is due to the stretching-induced release of water molecules that in the relaxed state form double hydrogen bonds with PEG. As a consequence, the stretching response of PEG is predominantly of energetic, not of entropic, origin at high forces and caused by hydration effects, while PEG backbone deformations only play a minor role. These findings demonstrate the importance of hydration for the mechanics of macromolecules and constitute a case example that sheds light on the antagonistic interplay of conformational and hydration degrees of freedom
Adsorption mechanism and valency of catechol-functionalized hyperbranched polyglycerols
Nature often serves as a model system for developing new adhesives. In aqueous
environments, mussel-inspired adhesives are promising candidates.
Understanding the mechanism of the extraordinarily strong adhesive bonds of
the catechol group will likely aid in the development of adhesives. With this
aim, we study the adhesion of catechol-based adhesives to metal oxides on the
molecular level using atomic force microscopy (AFM). The comparison of single
catechols (dopamine) with multiple catechols on hyperbranched polyglycerols
(hPG) at various pH and dwell times allowed us to further increase our
understanding. In particular, we were able to elucidate how to achieve strong
bonds of different valency. It was concluded that hyperbranched polyglycerols
with added catechol end groups are promising candidates for durable surface
coatings
Mechanism of Reversible Peptide–Bilayer Attachment: Combined Simulation and Experimental Single-Molecule Study
The binding of peptides and proteins
to lipid membrane surfaces
is of fundamental importance for many membrane-mediated cellular processes.
Using closely matched molecular dynamics simulations and atomic force
microscopy experiments, we study the force-induced desorption of single
peptide chains from phospholipid bilayers to gain microscopic insight
into the mechanism of reversible attachment. This approach allows
quantification of desorption forces and decomposition of peptide–membrane
interactions into energetic and entropic contributions. In both simulations
and experiments, the desorption forces of peptides with charged and
polar side chains are much smaller than those for hydrophobic peptides.
The adsorption of charged/polar peptides to the membrane surface is
disfavored by the energetic components, requires breaking of hydrogen
bonds involving the peptides, and is favored only slightly by entropy.
By contrast, the stronger adsorption of hydrophobic peptides is favored
both by energy and by entropy and the desorption forces increase with
increasing side-chain hydrophobicity. Interestingly, the calculated
net adsorption free energies per residue correlate with experimental
results of single residues, indicating that side-chain free energy
contributions are largely additive. This observation can help in the
design of peptides with tailored adsorption properties and in the
estimation of membrane binding properties of peripheral membrane proteins
Peptide Desorption Kinetics from Single Molecule Force Spectroscopy Studies
We use a combined experimental/theoretical
approach to determine
the intrinsic monomeric desorption rate <i>k</i><sub>0</sub> of polytyrosine and polylysine homopeptides from flat surfaces.
To this end, single polypeptide molecules are covalently attached
to an AFM cantilever tip and desorbed from hydrophobic self-assembled
monolayers in two complementary experimental protocols. In the constant-pulling-velocity
protocol, the cantilever is moved at finite velocity away from the
surface and the distance at which the constant plateau force regime
ends and the polymer detaches is recorded. In the waiting-time protocol,
the cantilever is held at a fixed distance above the surface and the
time until the polymer detaches is recorded. The desorption plateau
force is varied between 10 and 90 pN, by systematically changing the
aqueous solvent quality via the addition of ethanol or salt. A simultaneous
fit of the experimental data from both protocols with simple two-state
kinetic polymer theory allows to unambiguously disentangle and determine
the model parameters corresponding to polymer contour length <i>L</i>, Kuhn length <i>a</i>, adsorption free energy
λ, and intrinsic monomeric desorption rate <i>k</i><sub>0</sub>. Crucial to our analysis is that a statistically significant
number of single-polymer desorption experiments are done with one
and the same single polymer molecule for different solvent qualities.
The surprisingly low value of about <i>k</i><sub>0</sub> ≈ 10<sup>5</sup> Hz points to significant cooperativity in
the desorption process of single polymers
Multivalent anchored and crosslinked hyperbranched polyglycerol monolayers as antifouling coating for titanium oxide surfaces
A set of new catecholic monolayer coatings was developed to improve the antifouling performance of TiO2 surfaces. To solve the problem of the weak charge-transfer interaction between a single catechol anchor and TiO2, multiple catechol groups were combined with hyperbranched polyglycerol (hPG) which is a distinct dendritic scaffold that exposes its multivalent anchor groups on the surface. Thus, multivalent catecholic hPGs can be easily prepared for surface modification. The immobilization of the compounds was monitored by quartz crystal microbalance with dissipation monitoring. Surface properties of the coatings were analyzed by water contact angle, X-ray photoelectron spectroscopy, and atomic force microscopy. The antifouling ability and stability were investigated by protein adsorption and cell adhesion. By increasing the number of catechol groups on the hPG scaffold, the stability and surface coverage could be significantly enhanced. Moreover, the inner-layer crosslinking of the coatings by grafting and initiating vinyl groups clearly improved their long-term stability. As a result, hPG with a catecholic functional degree of 10% (hPG-Cat10) and hPG with both catecholic and vinylic functional degree of 5% (hPG-Cat5-V5) were identified as the best catecholic hPGs to prepare bioinert and stable monolayer coatings on TiO2
Effect of Molecular Architecture on Single Polymer Adhesion
Several applications require strong
noncovalent adhesion of polymers to substrates. Graft and branched
polymers have proven superior to linear polymers, but the molecular
mechanism is still unclear. Here, this question is addressed on the
single molecule level with an atomic force microscopy (AFM) based
method. It is determined how the presence of side chains and their
molecular architecture influence the adhesion and the mobility of
polymers on solid substrates. Surprisingly, the adhesion of mobile
polymers cannot significantly be improved by side chains or their
architecture. Only for immobile polymers a significantly higher maximum
rupture force for graft, bottle-brush, and branched polymers compared
to linear chains is measured. Our results suggest that a combination
of polymer architecture and strong molecular bonds is necessary to
increase the polymer–surface contact area. An increased contact
area together with intrachain cohesion (e.g., by entanglements) leads
to improved polymer adhesion. These findings may prove useful for
the design of stable polymer coatings
Thermoswitchable Nanoparticles Based on Elastin-like Polypeptides
The design of biocompatible particles
with defined size on the nanometer scale has proven to be a challenging
task in current biomedical research. Here we present an approach toward
temperature-responsive nanoparticles by covalently cross-linking micelles
based on trimeric constructs of elastin-like polypeptides. These trimers
can be triggered to assemble into micelles by heating the solution
above a specific transition temperature (<i>T</i><sub>t</sub>) which was shown in previous studies. Here we show that the disassembly
of the micelles below the <i>T</i><sub>t</sub> can be prevented
by the incorporation of covalent cross-links in the core of the micelles.
This facilitates a temperature-triggered swelling and collapsing by
around 35% in diameter, as determined by dynamic light scattering.
Size distribution was confirmed by fluorescence correlation spectroscopy,
atomic force microscopy, and transmission electron microscopy. We
show switchable nanoparticles with reversible volume changes in the
temperature region between 30 and 40 °C, making these particles
promising candidates for switchable drug delivery carriers