11 research outputs found
Characterisation of the hydrophobic collapse of polystyrene in water using free energy techniques
<p>We characterise the hydrophobic collapse of single polystyrene chains in water using molecular dynamics simulations. Specifically, we calculate the potential of mean force for the collapse of a single polystyrene chain in water using metadynamics, comparing the results between all atomistic with coarse-grained (CG) molecular simulation. We next explore the scaling behaviour of the collapsed globular shape at the minimum energy configuration, characterised by the radius of gyration, as a function of chain length. The exponent is close to one third, consistent with that predicted for a polymer chain in bad solvent. We also explore the scaling behaviour of the solvent accessible surface area (SASA) as a function of chain length, finding a similar exponent for both all atomistic and CG simulations. Furthermore, calculation of the local water density as a function of chain length near the minimum energy configuration suggests that intermediate chain lengths are more likely to form dewetted states, as compared to shorter or longer chain lengths.</p
Molecular Simulation of the Concentration-Dependent Interaction of Hydrophobic Drugs with Model Cellular Membranes
We report here the interactions between
a hydrophobic drug and
a model cellular membrane at the molecular level using all-atom molecular
dynamics simulations of paclitaxel, a hydrophobic cancer drug. The
calculated free energy of a single drug across the bilayer interface
displays a minimum in the outer hydrophobic zone of the membrane.
The transfer free energy shows excellent agreement with reported experimental
data. In two sets of long-time simulations of high concentrations
of drug in the membrane (12 and 11 mol %), the drugs display substantial
clustering and rotation with significant directional preference in
their diffusion. The main taxane ring partitions in the outer hydrophobic
zone, while the three phenyl rings prefer to be closer to the hydrophobic
core of the membrane. The clustering of the drug molecules, order
parameters of the lipid tails, and water penetration suggest that
the fluidity and permeability of the membrane are affected by the
concentration of drugs that it contains. Furthermore, at the high-concentration
limit, the free energy minimum shifts closer to the hydrophobic core
and the central barrier to cross the membrane decreases. Moreover,
the transfer free energy change substantially increases, suggesting
that increasing concentration facilitates drug partitioning into the
membrane
Molecular Dynamics Simulations of Supramolecular Anticancer Nanotubes
We
report here on long-time all-atomistic molecular dynamics simulations
of functional supramolecular nanotubes composed by the self-assembly
of peptide-drug amphiphiles (DAs). These DAs have been shown to possess
an inherently high drug loading of the hydrophobic anticancer drug
camptothecin. We probe the self-assembly mechanism from random with
∼0.4 μs molecular dynamics simulations. Furthermore,
we also computationally characterize the interfacial structure, directionality
of π–π stacking, and water dynamics within several
peptide-drug nanotubes with diameters consistent with the reported
experimental nanotube diameter. Insight gained should inform the future
design of these novel anticancer drug delivery systems
Molecular Dynamics Simulations of Polyelectrolyte Complexes
Polyelectrolyte
complexes (PECs) are currently of great interest
due to their applications toward developing new adaptive materials
and their relevance in membraneless organelles. These complexes emerge
during phase separation when oppositely charged polymers are mixed
in aqueous media. Peptide-based PECs are particularly useful toward
developing new drug delivery methods due to their inherent biocompatibility.
The underlying peptide sequence can be tuned to optimize specific
material properties of the complex, such as interfacial tension and
viscosity. Given their applicability, it would be advantageous to
understand the underlying sequence-dependent phase behavior of oppositely
charged peptides. Here, we report microsecond molecular dynamic simulations
to characterize the effect of hydrophobicity on the sequence-dependent
peptide conformation for model polypeptide sequences that were previously
reported by Tabandeh et al. These sequences are designed
with alternating chirality of the peptide backbone. We present microsecond
simulations of six oppositely charged peptide pairs, characterizing
the sequence-dependent effect on peptide size, degree of hydrogen
bonding, secondary structure, and conformation. This analysis recapitulates
sensible trends in peptide conformation and degree of hydrogen bonding,
consistent with experimentally reported results. Ramachandran plots
reveal that backbone conformation at the single amino acid level is
highly influenced by the neighboring sequence in the chain. These
results give insight into how subtle changes in hydrophobic side chain
size and chirality influence the strength of hydrogen bonding between
the chains and, ultimately, the secondary structure. Furthermore,
principal component analysis reveals that the minimum energy structures
may be subtly modulated by the underlying sequence
π–π Stacking Mediated Chirality in Functional Supramolecular Filaments
While a great diversity of peptide-based
supramolecular filaments
have been reported, the impact of an auxiliary segment on the chiral
assembly of peptides remains poorly understood. Herein we report on
the formation of chiral filaments by the self-assembly of a peptide-drug
conjugate containing an aromatic drug camptothecin (CPT) in a computational
study. We find that the chirality of the filament is mediated by the
π–π stacking between CPTs, not only by the well-expected
intermolecular hydrogen bonding between peptide segments. Our simulations
show that π–π stacking of CPTs governs the early
stages of the self-assembly process, while a hydrogen bonding network
starts at a relatively later stage to contribute to the eventual morphology
of the filament. Our results also show the possible presence of water
within the core of the CPT filament. These results provide very useful
guiding principles for the rational design of supramolecular assemblies
of peptide conjugates with aromatic segments
Effect of Nucleotide State on the Protofilament Conformation of Tubulin Octamers
At
the molecular level, the dynamic instability (random growth
and shrinkage) of the microtubule (MT) is driven by the nucleotide
state (GTP vs GDP) in the β subunit of the tubulin dimers at
the MT cap. Here, we use large-scale molecular dynamics (MD) simulations
and normal-mode analysis (NMA) to characterize the effect of a single
GTP cap layer on tubulin octamers composed of two neighboring protofilaments
(PFs). We utilize recently reported high-resolution structures of
dynamic MTs to simulate a GDP octamer both with and without a single
GTP cap layer. We perform multiple replicas of long-time atomistic
MD simulations (3 replicas, 0.3 μs for each replica, 0.9 μs
for each octamer system, and 1.8 μs total) of both octamers.
We observe that a single GTP cap layer induces structural differences
in neighboring PFs, finding that one PF possesses a gradual curvature,
compared to the second PF which possesses a kinked conformation. This
results in either curling or splaying between these PFs. We suggest
that this is due to asymmetric strengths of longitudinal contacts
between the two PFs. Furthermore, using NMA, we calculate mechanical
properties of these octamer systems and find that octamer system with
a single GTP cap layer possesses a lower flexural rigidity
TCR Triggering by pMHC Ligands Tethered on Surfaces via Poly(Ethylene Glycol) Depends on Polymer Length
<div><p>Antigen recognition by T cells relies on the interaction between T cell receptor (TCR) and peptide-major histocompatibility complex (pMHC) at the interface between the T cell and the antigen presenting cell (APC). The pMHC-TCR interaction is two-dimensional (2D), in that both the ligand and receptor are membrane-anchored and their movement is limited to 2D diffusion. The 2D nature of the interaction is critical for the ability of pMHC ligands to trigger TCR. The exact properties of the 2D pMHC-TCR interaction that enable TCR triggering, however, are not fully understood. Here, we altered the 2D pMHC-TCR interaction by tethering pMHC ligands to a rigid plastic surface with flexible poly(ethylene glycol) (PEG) polymers of different lengths, thereby gradually increasing the ligands’ range of motion in the third dimension. We found that pMHC ligands tethered by PEG linkers with long contour length were capable of activating T cells. Shorter PEG linkers, however, triggered TCR more efficiently. Molecular dynamics simulation suggested that shorter PEGs exhibit faster TCR binding on-rates and off-rates. Our findings indicate that TCR signaling can be triggered by surface-tethered pMHC ligands within a defined 3D range of motion, and that fast binding rates lead to higher TCR triggering efficiency. These observations are consistent with a model of TCR triggering that incorporates the dynamic interaction between T cell and antigen-presenting cell.</p></div
FRET between streptavidin on plastic plates and IEkMCC tethered with PEG polymers.
<p>(A) Measured FRET efficiencies of IEkMCC tethered with six different PEG polymers. The intensity of DyLight 549 was captured before and after DyLight 649 was photobleached. The measured FRET efficiency () was calculated using the intensity of DyLight 549 before () and after () DyLight 649 photobleaching ( ). The averaged values of two measurements were plotted with standard deviations. (B) After normalization, the measured FRET efficiencies match those calculated based on the Flory radius () of the PEG polymers. The of the PEG polymer of subunits and unit length was calculated using , where is 0.28 nm <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0112292#pone.0112292-Oesterhelt1" target="_blank">[32]</a>. Theoretical FRET efficiency () was calculated using the equation , where the Förster distance () of the DyLight 549-DyLight 649 donor-acceptor pair is 5 nm and the distance between the pMHC ligand and streptavidin is of the PEG polymer plus the pMHC radius of 2 nm. The FRET efficiencies were normalized by dividing the FRET efficiencies by the FRET efficiency of PEG 88.</p
PEG linkers and properties.
1<p>PEG contour length is calculated based on the PEO unit length of 0.28 nm in water <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0112292#pone.0112292-Oesterhelt1" target="_blank">[32]</a>.</p>2<p>The Flory radius () of the PEG polymer of subunits and unit length was calculated using , where is 0.28 nm.</p><p>PEG linkers and properties.</p
T cell activation by IEkMCC tethered with PEG polymers of different lengths.
<p>(A) T cell IL2 production in response to IEkMCC-PEG ligands of varying coating densities after 6 hours of stimulation. Data are representative of three independent experiments. The percent of T cells producing IL2 was determined by intracellular staining and flow cytometry. Three experiments using T cells from three different mice were performed (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0112292#pone.0112292.s008" target="_blank">Fig. S8</a> for flow cytometry plots). The percent of T cells producing IL2 was normalized to the highest value in each experiment. The data points are averages of the normalized values with standard errors of the means. (B) The rate of T cell response to IEkMCC ligands tethered with PEG polymers of different lengths. T cell IL2 production in response to stimulation on 96 well plates coated with 110 pM IEkMC-PEG ligands. T cells were harvested every hour for 6 hours and levels of IL2 expression were assayed by flow cytometry. Three experiments using T cells from three different mice were performed (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0112292#pone.0112292.s009" target="_blank">Fig. S9</a> for flow cytometry plots). The percent of T cells producing IL2 was normalized to the highest value in each experiment. The data points are averages of the normalized values with standard errors of the means. (C) The rates of T cell IL2 responses to IEkMCC ligands tethered with PEG polymers were extracted from the slope of linear fitting curves in Fig. 4B and plotted against the Flory radius of the polymers. The linear regressions and equations for deriving the rates are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0112292#pone.0112292.s007" target="_blank">Fig. S7</a>.</p