5 research outputs found
Effect of Surface Modification on Water Adsorption and Interfacial Mechanics of Cellulose Nanocrystals
With increasing environmental concerns about petrochemical-based
materials, the development
of high-performance polymer nanocomposites with sustainable filler
phases has attracted significant attention. Cellulose nanocrystals
(CNCs) are promising nanocomposite reinforcing agents due to their
exceptional mechanical properties, low weight, and bioavailability.
However, there are still numerous obstacles that prevent these materials
from achieving optimal performance, including high water adsorption,
poor nanoparticle dispersion, and filler properties that vary in response
to moisture. Surface modification is an effective method to mitigate
these shortcomings. We use computational approaches to obtain direct
insight into the water adsorption and interfacial mechanics of modified
CNC surfaces. Atomistic grand-canonical Monte Carlo simulations demonstrate
how surface modification of sulfated Na-CNCs impacts water adsorption.
We find that methylÂ(triphenyl)Âphosphonium (MePh<sub>3</sub>P<sup>+</sup>)-exchanged CNCs have lower water uptake than Na-CNCs, supporting
experimental dynamic vapor sorption measurements. The adsorbed water
molecules show orientational ordering when distributed around the
cations. Steered molecular dynamics simulations quantify tractionâseparation
behavior of CNCâCNC interfaces. We find that exchanging sodium
for MePh<sub>3</sub>P<sup>+</sup> effectively changes the surface
hydrophilicity, which in turn directly impacts interfacial adhesion
and tractionâseparation behavior. Our analysis provides guidelines
for controlling moisture effects in cellulose nanocomposites and nanocellulose
films through surface modifications
The Role of Structural Enthalpy in Spherical Nucleic Acid Hybridization
DNA hybridization onto DNA-functionalized
nanoparticle surfaces
(e.g., in the form of a spherical nucleic acid (SNA)) is known to
be enhanced relative to hybridization free in solution. Surprisingly,
via isothermal titration calorimetry, we reveal that this enhancement
is enthalpically, as opposed to entropically, dominated by âŒ20
kcal/mol. Coarse-grained molecular dynamics simulations suggest that
the observed enthalpic enhancement results from structurally confining
the DNA on the nanoparticle surface and preventing it from adopting
enthalpically unfavorable conformations like those observed in the
solution case. The idea that structural confinement leads to the formation
of energetically more stable duplexes is evaluated by decreasing the
degree of confinement a duplex experiences on the nanoparticle surface.
Both experiment and simulation confirm that when the surface-bound
duplex is less confined, i.e., at lower DNA surface density or at
greater distance from the nanoparticle surface, its enthalpy of formation
approaches the less favorable enthalpy of duplex formation for the
linear strand in solution. This work provides insight into one of
the most important and enabling properties of SNAs and will inform
the design of materials that rely on the thermodynamics of hybridization
onto DNA-functionalized surfaces, including diagnostic probes and
therapeutic agents
Size-Selective Nanoparticle Assembly on Substrates by DNA Density Patterning
The vision of nanoscale
self-assembly research is the programmable
synthesis of macroscale structures with controlled long and short-range
order that exhibit a desired set of properties and functionality.
However, strategies to reliably isolate and manipulate the nanoscale
building blocks based on their size, shape, or chemistry are still
in their infancy. Among the promising candidates, DNA-mediated self-assembly
has enabled the programmable assembly of nanoparticles into complex
architectures. In particular, two-dimensional assembly on substrates
has potential for the development of integrated functional devices
and analytical systems. Here, we combine the high-resolution patterning
capabilities afforded by electron-beam lithography with the DNA-mediated
assembly process to enable direct-write grayscale DNA density patterning.
This method allows modulation of the functionally active DNA surface
density to control the thermodynamics of interactions between nanoparticles
and the substrate. We demonstrate that size-selective directed assembly
of nanoparticle films from solutions containing a bimodal distribution
of particles can be realized by exploiting the cooperativity of DNA
binding in this system. To support this result, we study the temperature-dependence
of nanoparticle assembly, analyze the DNA damage by X-ray photoelectron
spectroscopy and fluorescence microscopy, and employ molecular dynamics
simulations to explore the size-selection behavior
Simulation and Experimental Assembly of DNAâGraft Copolymer Micelles with Controlled Morphology
Nanoparticles
formed through complexation of plasmid DNA and copolymers
are promising gene-delivery vectors, offering a wide range of advantages
over alternative delivery strategies. Notably, recent research has
shown that the shape of these particles can be tuned, which makes
it possible to gain understanding of their shape-dependent transfection
properties. Whereas earlier methods achieved shape tuning through
the use of block copolymers and variation of solvent polarity, here
we demonstrate through a combined experimental and computational approach
that the same degree of shape control can be achieved through the
use of graft copolymers that are easier to synthesize and provide
a wider range of parameters for shape control. Moreover, the approach
presented here does not require the use of organic solvents. The simulation
work provides insight into the mechanism governing the shape variation
as well as an effective model to guide further design of nonviral
gene-delivery vectors. Our experimental findings offer important opportunities
for the facile and large-scale synthesis of biocompatible gene-delivery
vectors with well-controlled shape and tunable transfection properties.
The in vitro study shows that both micelle shape and transfection
efficiency are strongly correlated with the key structural parameters
of the graft copolymer carriers
Interactions between Membranes and âMetaphilicâ Polypeptide Architectures with Diverse Side-Chain Populations
At physiological
conditions, most proteins or peptides can fold
into relatively stable structures that present on their molecular
surfaces specific chemical patterns partially smeared out by thermal
fluctuations. These nanoscopically defined patterns of charge, hydrogen
bonding, and/or hydrophobicity, along with their elasticity and shape
stability (folded proteins have Youngâs moduli of âŒ1
Ă 10<sup>8</sup> Pa), largely determine and limit the interactions
of these molecules, such as molecular recognition and allosteric regulation.
In this work, we show that the membrane-permeating activity of antimicrobial
peptides (AMPs) and cell-penetrating peptides (CPPs) can be significantly
enhanced using prototypical peptides with âmoltenâ surfaces:
metaphilic peptides with quasi-liquid surfaces and adaptable shapes.
These metaphilic peptides have a bottlebrush-like architecture consisting
of a rigid helical core decorated with mobile side chains that are
terminated by cationic or hydrophobic groups. Computer simulations
show that these flexible side chains can undergo significant rearrangement
in response to different environments, giving rise to adaptable surface
chemistry of the peptide. This quality makes it possible to control
their hydrophobicity over a broad range while maintaining water solubility,
unlike many AMPs and CPPs. Thus, we are able to show how the activity
of these peptides is amplified by hydrophobicity and cationic charge,
and rationalize these results using a quantitative mean-field theory.
Computer simulations show that the shape-changing properties of the
peptides and the resultant adaptive presentation of chemistry play
a key enabling role in their interactions with membranes