297 research outputs found
On the Molecular Origin of the Cooperative Coil-to-globule Transition of Poly(N-isopropylacrylamide) in Water
By means of atomistic molecular dynamics simulations we investigate the
behaviour of poly(N-isopropylacrylamide), PNIPAM, in water at temperatures
below and above the lower critical solution temperature (LCST), including the
undercooled regime. The transition between water soluble and insoluble states
at the LCST is described as a cooperative process involving an intramolecular
coil-to-globule transition preceding the aggregation of chains and the polymer
precipitation. In this work we investigate the molecular origin of such
cooperativity and the evolution of the hydration pattern in the undercooled
polymer solution. The solution behaviour of an atactic 30-mer at high dilution
is studied in the temperature interval from 243 to 323 K with a favourable
comparison to available experimental data. In the PNIPAM water soluble states
we detect a correlation between polymer segmental dynamics and diffusion motion
of bound water, occurring with the same activation energy. Simulation results
show that below the coil-to-globule transition temperature PNIPAM is surrounded
by a network of hydrogen bonded water molecules and that the cooperativity
arises from the structuring of water clusters in proximity to hydrophobic
groups. Differently, the perturbation of the hydrogen bond pattern involving
water and amide groups occurs above the transition temperature. Altogether
these findings reveal that even above the LCST PNIPAM remains largely hydrated
and that the coil-to-globule transition is related with a significant
rearrangement of the solvent in proximity of the surface of the polymer. The
comparison between the hydrogen bonding of water in the surrounding of PNIPAM
isopropyl groups and in bulk displays a decreased structuring of solvent at the
hydrophobic polymer-water interface across the transition temperature, as
expected because of the topological extension along the chain of such
interface
Numerical modelling of non-ionic microgels: an overview
Microgels are complex macromolecules. These colloid-sized polymer networks
possess internal degrees of freedom and, depending on the polymer(s) they are
made of, can acquire a responsiveness to variations of the environment
(temperature, pH, salt concentration, etc.). Besides being valuable for many
practical applications, microgels are also extremely important to tackle
fundamental physics problems. As a result, these last years have seen a rapid
development of protocols for the synthesis of microgels, and more and more
research has been devoted to the investigation of their bulk properties.
However, from a numerical standpoint the picture is more fragmented, as the
inherently multi-scale nature of microgels, whose bulk behaviour crucially
depends on the microscopic details, cannot be handled at a single level of
coarse-graining. Here we present an overview of the methods and models that
have been proposed to describe non-ionic microgels at different length-scales,
from the atomistic to the single-particle level. We especially focus on
monomer-resolved models, as these have the right level of details to capture
the most important properties of microgels, responsiveness and softness. We
suggest that these microscopic descriptions, if realistic enough, can be
employed as starting points to develop the more coarse-grained representations
required to investigate the behaviour of bulk suspensions
Interaction of Caffeine with Model Lipid Membranes
Caffeine is not only a widely consumed active stimulant, but it is also a model molecule commonly used in pharmaceutical sciences. In this work, by performing quartz-crystal microbalance and neutron reflectometry experiments we investigate the interaction of caffeine molecules with a model lipid membrane. We determined that caffeine molecules are not able to spontaneously partition from an aqueous environment, enriched in caffeine, into a bilayer. Caffeine could be however included in solid-supported lipid bilayers if present with lipids during self-assembly. In this case, thanks to surface-sensitive techniques, we determined that caffeine molecules are preferentially located in the hydrophobic region of the membrane. These results are highly relevant for the development of new drug delivery vectors, as well as for a deeper understanding of the membrane permeation role of purine molecules
Molecular insights on poly(N-isopropylacrylamide) coil-to-globule transition induced by pressure
By using extensive all-atom molecular dynamics simulations of an atactic linear polymer chain, we provide microscopic insights into poly(N-isopropylacrylamide) (PNIPAM) coil-to-globule transition addressing the roles played by both temperature and pressure. We detect a coil-to-globule transition up to large pressures, showing a reentrant behavior of the critical temperature with increasing pressure in agreement with experimental observations. Furthermore, again confirming the experimental findings, we report the existence at high pressures of a new kind of globular state. It is characterized by a more structured hydration shell that is closer to PNIPAM hydrophobic domains, as compared to the globular state observed at atmospheric pressure. Our results highlight that temperature and pressure induce a PNIPAM coil-to-globule transition through different molecular mechanisms, opening the way for a systematic use of both thermodynamic variables to tune the location of the transition and the properties of the associated swollen/collapsed states
Simultaneous quantification of antioxidants paraxanthine and caffeine in human saliva by electrochemical sensing for CYP1A2 phenotyping
The enzyme CYP1A2 is responsible for the metabolism of numerous antioxidants in the body, including caffeine, which is transformed into paraxanthine, its main primary metabolite. Both molecules are known for their antioxidant and pro-oxidant characteristics, and the paraxanthine-to-caffeine molar ratio is a widely accepted metric for CYP1A2 phenotyping, to optimize dose\u2013 response effects in individual patients. We developed a simple, cheap and fast electrochemical based method for the simultaneous quantification of paraxanthine and caffeine in human saliva, by differential pulse voltammetry, using an anodically pretreated glassy carbon electrode. Cyclic voltammetry experiments revealed for the first time that the oxidation of paraxanthine is diffusion controlled with an irreversible peak at ca. +1.24 V (vs. Ag/AgCl) in a 0.1 M H2 SO4 solution, and that the mechanism occurs via the transfer of two electrons and two protons. The simultaneous quantification of paraxanthine and caffeine was demonstrated in 0.1 M H2 SO4 and spiked human saliva samples. In the latter case, limits of detection of 2.89 \ub5M for paraxanthine and 5.80 \ub5M for caffeine were obtained, respectively. The sensor is reliable, providing a relative standard deviation within 7% (n = 6). Potential applicability of the sensing platform was demonstrated by running a small scale trial on five healthy volunteers, with simultaneous quantification by differential pulse voltammetry (DPV) of paraxanthine and caffeine in saliva samples collected at 1, 3 and 6 h postdose administration. The results were validated by ultra-high pressure liquid chromatography and shown to have a high correlation factor (r = 0.994)
Pd@TiO2/carbon nanohorn electrocatalysts: reversible CO2 hydrogenation to formic acid
Direct conversion of carbon dioxide to formic acid at thermodynamic equilibrium is an advantage of enzymatic catalysis, hardly replicated by synthetic analogs, but of high priority for carbon-neutral energy schemes. The bio-mimetic potential of totally inorganic Pd@TiO2 nanoparticles is envisioned herein in combination with Single Walled Carbon NanoHorns (SWCNHs). The high surface nano-carbon entanglement templates a wide distribution of \u201chard-soft\u201d bimetallic sites where the small Pd nanoparticles (1.5 nm) are shielded within the TiO2 phase (Pd@TiO2), while being electrically wired to the electrode by the nanocarbon support. This hybrid electrocatalyst activates CO2 reduction to formic acid at near zero overpotential in the aqueous phase (onset potential at E < 120.05 V vs. RHE, pH = 7.4), while being able to evolve hydrogen via sequential formic acid dehydrogenation. The net result hints at a unique CO2 \u201ccircular catalysis\u201d where formic acid versus H2 selectivity is addressable by flow-reactor technology
Protein-like dynamical transition of hydrated polymer chains
Combining elastic incoherent neutron scattering experiments at different
resolutions and molecular dynamics simulations, we report the observation of a
protein-like dynamical transition in Poly(N-isopropylacrylamide) chains. We
identify the onset of the transition at a temperature Td of about 225~K. Thanks
to a novel global fit procedure, we find quantitative agreement between
measured and calculated polymer mean-squared displacements at all temperatures
and time resolutions. Our results confirm the generality of the dynamical
transition in macromolecular systems in aqueous environments, independently of
the internal polymer topology
Simultaneous Planck, Swift, and Fermi observations of X-ray and gamma-ray selected blazars
We present simultaneous Planck, Swift, Fermi, and ground-based data for 105
blazars belonging to three samples with flux limits in the soft X-ray, hard
X-ray, and gamma-ray bands. Our unique data set has allowed us to demonstrate
that the selection method strongly influences the results, producing biases
that cannot be ignored. Almost all the BL Lac objects have been detected by
Fermi-LAT, whereas ~40% of the flat-spectrum radio quasars (FSRQs) in the
radio, soft X-ray, and hard X-ray selected samples are still below the
gamma-ray detection limit even after integrating 27 months of Fermi-LAT data.
The radio to sub-mm spectral slope of blazars is quite flat up to ~70GHz, above
which it steepens to ~-0.65. BL Lacs have significantly flatter spectra
than FSRQs at higher frequencies. The distribution of the rest-frame
synchrotron peak frequency (\nupS) in the SED of FSRQs is the same in all the
blazar samples with =10^13.1 Hz, while the mean inverse-Compton peak
frequency, , ranges from 10^21 to 10^22 Hz. The distributions of \nupS
and of \nupIC of BL Lacs are much broader and are shifted to higher energies
than those of FSRQs and strongly depend on the selection method. The Compton
dominance of blazars ranges from ~0.2 to ~100, with only FSRQs reaching values
>3. Its distribution is broad and depends strongly on the selection method,
with gamma-ray selected blazars peaking at ~7 or more, and radio-selected
blazars at values ~1, thus implying that the assumption that the blazar power
is dominated by high-energy emission is a selection effect. Simple SSC models
cannot explain the SEDs of most of the gamma-ray detected blazars in all
samples. The SED of the blazars that were not detected by Fermi-LAT may instead
be consistent with SSC emission. Our data challenge the correlation between
bolometric luminosity and \nupS predicted by the blazar sequence.Comment: Version accepted by A&A. Joint Planck, Swift, and Fermi
collaborations pape
Planck 2013 results. XXII. Constraints on inflation
We analyse the implications of the Planck data for cosmic inflation. The Planck nominal mission temperature anisotropy measurements, combined with the WMAP large-angle polarization, constrain the scalar spectral index to be ns = 0:9603 _ 0:0073, ruling out exact scale invariance at over 5_: Planck establishes an upper bound on the tensor-to-scalar ratio of r < 0:11 (95% CL). The Planck data thus shrink the space of allowed standard inflationary models, preferring potentials with V00 < 0. Exponential potential models, the simplest hybrid inflationary models, and monomial potential models of degree n _ 2 do not provide a good fit to the data. Planck does not find statistically significant running of the scalar spectral index, obtaining dns=dln k = 0:0134 _ 0:0090. We verify these conclusions through a numerical analysis, which makes no slowroll approximation, and carry out a Bayesian parameter estimation and model-selection analysis for a number of inflationary models including monomial, natural, and hilltop potentials. For each model, we present the Planck constraints on the parameters of the potential and explore several possibilities for the post-inflationary entropy generation epoch, thus obtaining nontrivial data-driven constraints. We also present a direct reconstruction of the observable range of the inflaton potential. Unless a quartic term is allowed in the potential, we find results consistent with second-order slow-roll predictions. We also investigate whether the primordial power spectrum contains any features. We find that models with a parameterized oscillatory feature improve the fit by __2 e_ _ 10; however, Bayesian evidence does not prefer these models. We constrain several single-field inflation models with generalized Lagrangians by combining power spectrum data with Planck bounds on fNL. Planck constrains with unprecedented accuracy the amplitude and possible correlation (with the adiabatic mode) of non-decaying isocurvature fluctuations. The fractional primordial contributions of cold dark matter (CDM) isocurvature modes of the types expected in the curvaton and axion scenarios have upper bounds of 0.25% and 3.9% (95% CL), respectively. In models with arbitrarily correlated CDM or neutrino isocurvature modes, an anticorrelated isocurvature component can improve the _2 e_ by approximately 4 as a result of slightly lowering the theoretical prediction for the ` <_ 40 multipoles relative to the higher multipoles. Nonetheless, the data are consistent with adiabatic initial conditions
BeyondPlanck II. CMB map-making through Gibbs sampling
We present a Gibbs sampling solution to the map-making problem for CMB
measurements, building on existing destriping methodology. Gibbs sampling
breaks the computationally heavy destriping problem into two separate steps;
noise filtering and map binning. Considered as two separate steps, both are
computationally much cheaper than solving the combined problem. This provides a
huge performance benefit as compared to traditional methods, and allows us for
the first time to bring the destriping baseline length to a single sample. We
apply the Gibbs procedure to simulated Planck 30 GHz data. We find that gaps in
the time-ordered data are handled efficiently by filling them with simulated
noise as part of the Gibbs process. The Gibbs procedure yields a chain of map
samples, from which we may compute the posterior mean as a best-estimate map.
The variation in the chain provides information on the correlated residual
noise, without need to construct a full noise covariance matrix. However, if
only a single maximum-likelihood frequency map estimate is required, we find
that traditional conjugate gradient solvers converge much faster than a Gibbs
sampler in terms of total number of iterations. The conceptual advantages of
the Gibbs sampling approach lies in statistically well-defined error
propagation and systematic error correction, and this methodology forms the
conceptual basis for the map-making algorithm employed in the BeyondPlanck
framework, which implements the first end-to-end Bayesian analysis pipeline for
CMB observations.Comment: 11 pages, 10 figures. All BeyondPlanck products and software will be
released publicly at http://beyondplanck.science during the online release
conference (November 18-20, 2020). Connection details will be made available
at the same website. Registration is mandatory for the online tutorial, but
optional for the conferenc
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