Effect of Counterion Size on Knotted Polyelectrolyte Conformations.

Using Langevin dynamics simulations and a coarse-grained primitive model of electrolytes, we show that the behavior of knotted circular strong polyelectrolytes (PEs) in diluted aqueous solution is largely affected by the diameter of the counterions (CIs), σCI. Indeed, we observe that both gyration radius and knot length vary nonmonotonically with σCI, with both small and bulky CIs favoring knot localization, while medium-sized ones promote delocalized knots. We also show that the conformational change from delocalized to tight knots occurs via the progressive coalescence of the knot's essential crossings. The emerging conformers correspond to the minima of the free energy landscape profiled as a function of the knot length or PE size. We demonstrate that different conformational states can coexist, the transition between them appearing first-order-like and controlled by the enthalpic and entropic trade-off of the amount of CIs condensed on the PE. Such balance can be further altered by varying CI concentrations, thus providing an additional and more convenient tuning parameter for the system properties. Our results lay the foundation for achieving broader and more precise external adjustability of knotted PE size and shape by choosing the nature of its CIs. Thus, they offer new intriguing possibilities for designing novel PE-based materials that are capable of responding to changes in ionic solution properties.

Mercury Clathration-Driven Phase Transition in a Luminescent Bipyrazolate Metal-Organic Framework: A Multitechnique Investigation.

Mercury is one of the most toxic heavy metals. By virtue of its triple bond, the novel ligand 1,2-bis(1H-pyrazol-4-yl)ethyne (H2BPE) was expressly designed and synthesized to devise metal-organic frameworks (MOFs) exhibiting high chemical affinity for mercury. Two MOFs, Zn(BPE) and Zn(BPE)·nDMF [interpenetrated i-Zn and noninterpenetrated ni-Zn·S, respectively; DMF = dimethylformamide], were isolated as microcrystalline powders. While i-Zn is stable in water for at least 15 days, its suspension in HgCl2 aqueous solutions prompts its conversion into HgCl2@ni-Zn. A multitechnique approach allowed us to shed light onto the observed HgCl2-triggered i-Zn-to-HgCl2@ni-Zn transformation at the molecular level. Density functional theory calculations on model systems suggested that HgCl2 interacts via the mercury atom with the carbon-carbon triple bond exclusively in ni-Zn. Powder X-ray diffraction enabled us to quantify the extent of the i-Zn-to-HgCl2@ni-Zn transition in 100-5000 ppm HgCl2 (aq) solutions, while X-ray fluorescence and inductively coupled plasma-mass spectrometry allowed us to demonstrate that HgCl2 is quantitatively sequestered from the aqueous phase. Irradiating at 365 nm, an intense fluorescence is observed at 470 nm for ni-Zn·S, which is partially quenched for i-Zn. This spectral benchmark was exploited to monitor in real time the i-Zn-to-HgCl2@ni-Zn conversion kinetics at different HgCl2 (aq) concentrations. A sizeable fluorescence increase was observed, within a 1 h time lapse, even at a concentration of 5 ppb. Overall, this comprehensive investigation unraveled an intriguing molecular mechanism, featuring the disaggregation of a water-stable MOF in the presence of HgCl2 and the self-assembly of a different crystalline phase around the pollutant, which is sequestered and simultaneously quantified by means of a luminescence change. Such a case study might open the way to new-conception strategies to achieve real-time sensing of mercury-containing pollutants in wastewaters and, eventually, pursue their straightforward and cost-effective purification.

Asymmetric Phenyl Substitution: An Effective Strategy to Enhance the Photosensitizing Potential of Curcuminoids.

Curcumin has been demonstrated to exhibit photosensitized bactericidal activity. However, the full exploitation of curcumin as a photo-pharmaceutical active principle is hindered by fast deactivation of the excited state through the transfer of the enol proton to the keto oxygen. Introducing an asymmetry in the molecular structure through acting on the phenyl substituents is expected to be a valuable strategy to impair this undesired de-excitation mechanism competing with the therapeutically relevant ones. In this study, two asymmetric curcumin analogs were synthesized and characterized as to their electronic-state transition spectroscopic properties. Fluorescence decay distributions were also reconstructed. Their analysis confirmed the substantial stabilization of the fluorescent state with respect to the parent compound. Nuclear magnetic resonance experiments were performed with the aim of determining the structural features of the keto-enol ring and the strength of the keto-enol hydrogen bond. Electronic structure calculations were also undertaken to elucidate the effects of substitution on the features of the keto-enol semi-aromatic system and the proneness to proton transfer. Finally, their singlet oxygen-generation efficiency was compared to that of curcumin through the 9,10-dimethylanthracene fluorescent assay.

Interaction between surfaces decorated with like-charged pendants: Unravelling the interplay between energy and entropy leading to attraction.

HYPOTHESIS: The stronger motional coupling between monovalent counterions neutralizing homogeneously like-charged surfaces induced by an increase in charge density is known to foster inter-surface attraction. Compared to a uniformly distributed charge, point-like charges generate locally more intense fields, so that the correlation induced between counterions may be even stronger despite an identical total charge. It should thus be possible to induce surface attraction at lower charge densities than commonly expected. EXPERIMENTS: Monte Carlo simulations on primitive electrolyte models have been exploited to compute potential of mean force profiles and mobile ion densities for systems composed of two parallel surfaces bearing surface-tethered monovalent like-charged pendants as a function of the surface distance and pendant densities. FINDINGS: Surfaces bearing like-charged pendants are found to attract each other over a wide range of distances despite the presence of very low charge densities. Notwithstanding the attractive contribution to the inter-surface forces provided by electrostatic interactions, the entropic component of the system Helmholtz energy is found to play the key role in defining the overall magnitude. The latter finding appears justified by an increase in the relative delocalization of counterions upon decreasing the surface distance.

Inducing pH control over the critical micelle concentration of zwitterionic surfactants via polyacids adsorption: Effect of chain length and structure.

HYPOTHESIS: The critical concentration above which micelles form from zwitterionic surfactant solutions and their thermodynamic stability is affected by the interaction with weak Brønsted polyacid chains (An) via the formation of charged hydrogen bonds between the latter and anionic moieties. EXPERIMENTS: The interaction between zwitterionic micelles and polyacids capable of forming hydrogen bonds, and its dependence on the environmental pH and polymer structure, has been studied with constant-pH simulations and a restricted primitive model for all electrolytes. FINDINGS: At low pH, the formation of polyacid/micelle complexes is witnessed independently of the polymer size or structure, so that the concentration above which micelles form is substantially decreased compared to polyacid-free cases. Upon rising pH, polymer desorption takes place within a narrow range of pH values, its location markedly depending on the size and structure of polyacids, and on the relative disposition between headgroup charged moieties. Thus, the desorption onset for long linear polyacids (A60) interacting with sulphobetainic headgroups is roughly two pH units higher than for six decameric chains (6A10) adsorbed onto micelles bearing phosphorylcholinic headgroups. This effect, together with the preferential desorption of chain ends at intermediate pH, may be exploited for drug delivery purposes or building advanced metamaterials.

Tunable Knot Segregation in Copolyelectrolyte Rings Carrying a Neutral Segment.

We use Langevin dynamics simulations to study the knotting properties of copolyelectrolyte rings carrying neutral segments. We show that by solely tuning the relative length of the neutral and charged blocks, one can achieve different combinations of knot contour position and size. Strikingly, the latter is shown to vary nonmonotonically with the length of the neutral segment; at the same time, the knot switches from being pinned at the block's edge to becoming trapped inside it. Model calculations relate both effects to the competition between two adversarial mechanisms: the energy gain of localizing one or more of the knot's essential crossings on the neutral segment and the entropic cost of such localization. Tuning the length of the neutral segment sets the balance between the two mechanisms and hence the number of localized essential crossings, which in turn modulates the knot's size. This general principle ought to be useful in more complex systems, such as multiblock copolyelectrolytes, to achieve a more granular control of topological constraints.

Can oppositely charged polyelectrolyte stars form a gel? A simulational study.

We present a Langevin molecular dynamics study of an equimolar mixture of monodispersed oppositely charged di-block four-armed polyelectrolyte stars. We used an implicit solvent coarse-grained representation of the polyelectrolyte stars, and varied the length of the terminal charged blocks that reside on each arm. By varying the polymer concentration, we computed PV diagrams and determined the free-swelling equilibrium concentration with respect to a pure water reservoir as a function of the charged block length. We investigated various structural properties of the resulting equilibrium structures, like the number of ionic bonds, dangling arms, isolated stars, and cluster sizes. The ionic bonds featured a broad distribution of the number of arms involved and also displayed a distribution of net charges peaked around the neutral ionic bond. Our main result is that for charged block length equal to 4 and 5 ionized beads the resulting macro-aggregate spans the box and forms a network phase. Furthermore, we investigated the restructuring dynamics of ionic bonds; the results suggested both short bond lifetimes and a high frequency of ballistic association/dissociation events. Bonds result strong enough to yield a stable gel phase, but they are still weak enough to allow network restructuring under thermal fluctuations.

On the distribution of hydrophilic polyelectrolytes and their counterions around zwitterionic micelles: the possible impact on the charge density in solution.

Despite their charge neutrality, micelles composed of surfactants with zwitterionic headgroups selectively accumulate anions at their hydrophobic core/solution interphase due to electrostatic interactions if headgroup positive moieties are the innermost. This tendency may be markedly enhanced if polyions substitute simple ions. To investigate this possibility, solutions composed of zwitterionic micelles and hydrophilic polyanions have been investigated with Monte Carlo simulations representing the studied systems via primitive electrolyte models. Structural and energetic properties are obtained to highlight the impact of connecting simple ions into polyions on the interactions between electrolytes and micelles. Despite the latter, polyanions conserve their conformational properties. A marked increase in the concentration of charged species inside the micellar corona is, instead, found when polyions are present independently of their charge sign or the headgroup structure. Thus, polyelectrolytes act as "shuttle" for all charged species, with the potential of increasing reactions rates involving the latter due to mass effects. Besides, results for the polyions/micelles mixing free energy and Helmholtz energy profiles indicate that the critical micelle concentration is impacted minimally by hydrophilic polyelectrolytes, an outcome agreeing with experiments. This finding is entirely due to weak enthalpic effects while mixing hydrophilic polyions and micelles. A strong reduction in the screening of the micelle negative charge, acquired following the adsorption of anions in the corona and due to counterions layering just outside it (the so called "chameleon effect"), is forecasted when polyanions substitute monovalent anions.

Interface Counterion Localization Induces a Switch between Tight and Loose Configurations of Knotted Weak Polyacid Rings despite Intermonomer Coulomb Repulsions.

Stochastic simulations have been used to investigate the conformational behavior of knotted weak polyacid rings as a function of pH. Different from the commonly expected ionization-repulsion-expansion scheme upon increasing pH, theoretical results suggest a nonmonotonic behavior of the gyration radius Rg2. Polyelectrolyte recontraction at high ionization is induced by the weakening of Coulomb repulsion due to counterions (CIs) localizing at the interphase between the polymer and solvent, and the more marked it appears, the more complex is the knot topology. Compared with strong polyelectrolytic species of identical ionization, weak polyacids present tighter knots due to their ability to localize neutral monomers inside the tangled part. Increasing the solvent Bjerrum length enhances CIs localization, lowering the pH at which polyacids start decreasing their average size. A similar effect is also obtained by increasing the amount of "localizable" cations by adding salts.

Monte Carlo study of the effects of macroion charge distribution on the ionization and adsorption of weak polyelectrolytes and concurrent counterion release.

HYPOTHESIS: Adsorption of weak polyelectrolytes onto charged nanoparticles, and concurrent effects such as spatial partitioning of ions may be influenced by details of the polyelectrolyte structure (linear or star-like) and size, by the mobility of the nanoparticle surface charge, or the valence of the nanoparticle counterions. EXPERIMENTS: Ionization and complexation of weak polyelectrolytes on spherical macroions with monovalent and divalent countrions has been studied with constant-pH Monte Carlo titrations and primitive electrolyte models for linear and star-like polymers capable, also, of forming charged hydrogen bonds. Nanoparticles surface charge has been represented either as a single colloid-centered total charge (CCTC) or as surface-tethered mobile monovalent spherical charges (SMMSC). FINDINGS: Differences in the average number of adsorbed polyelectrolyte arms and their average charge, and in the relative amount of macroion counterions (m-CI's) released upon polymer adsorption are found between CCTC and SMMSC nanoparticles. The amount of the counterions released also depends on the polymer structure. As CCTC adsorbs a lower number of star-like species arms, the degree of condensation of polymer counterions (p-CI's) onto the polyelectrolyte is also substantially higher for the CCTC colloid, with a concurrent decrease of the osmotic coefficient values.

Impact of Charge Correlation, Chain Rigidity, and Chemical Specific Interactions on the Behavior of Weak Polyelectrolytes in Solution.

In this work, we performed titration simulations of weak linear polyelectrolytes via the Monte Carlo method and the constant pH ensemble aiming to understand how polyelectrolyte concentration, chain rigidity, and the formation of intra- and inter-chain charged hydrogen bonds (c-H-bonds) impact on ionization and conformations of polyacidic species, counterions (CIs) distribution, and system Helmholtz energy. Increasing polyelectrolyte concentration resulted in enhanced acidity for all cases investigated due to the increased screening of chain charges by CIs and, when possible, the formation of interchain c-H-bonds. Our simulations also evidenced that polyelectrolytes able to form c-H-bonds can populate simultaneously two conformational states (aggregated and unfolded) in a range of pH, the transition between the two appearing first order-like. To better understand how properties of two polyelectrolytic chains are modified by their relative distance, we performed window sampling (WS) simulations, which highlighted nontrivial features in the ionization and conformational behaviors. As byproducts of WS simulations, we obtained also the potential of mean force between two chains; from this, it emerges that the reversible work needed to reach a specific interchain distance does not always increase with the pH, especially for c-H-bonds forming semirigid chains brought at short distances.

Escherichia coli as a Model for the Description of the Antimicrobial Mechanism of a Cationic Polymer Surface: Cellular Target and Bacterial Contrast Response.

In this study, we use Escherichia coli as a model to investigate the antimicrobial mechanism of a film made of a copolymer based on monomethylether poly(ethylene glycol), methyl methacrylate, and 2-dimethyl(aminoethyl) methacrylate, whose surface is active towards Gram-negative and Gram-positive bacteria. The polymer contains not quaternized amino groups that can generate a charged surface by protonation when in contact with water. For this purpose, we adopted a dual strategy based on the analysis of cell damage caused by contact with the polymer surface and on the evaluation of the cell response to the surface toxic action. The lithic effect on the protoplasts of E. coli showed that the polymer surface can affect the structure of cytoplasmic membranes, while assays of calcein leakage from large unilamellar vesicles at different phospholipid compositions indicated that action on membranes does not need a functionally active cell. On the other hand, the significant increase in sensitivity to actinomycin D demonstrates that the polymer interferes also with the structure of the outer membrane, modifying its permeability. The study on gene expression, based on the analysis of the transcripts in a temporal window where the contact with the polymer is not lethal and the damage is reversible, showed that some key genes of the synthesis and maintenance of the outer membrane structure ( fabR, fadR, fabA, waaA, waaC, kdsA, pldA, and pagP), as well as regulators of cellular response to oxidative stress ( soxS), are more expressed when bacteria are exposed to the polymer surface. All together these results identified the outer membrane as the main cellular target of the antimicrobial surface and indicated a specific cellular response to damage, providing more information on the antimicrobial mechanism. In this perspective, data reported here could play a pivotal role in a microbial growth control strategy based not only on the structural improvements of the materials but also on the possibility of intervening on the cellular pathways involved in the contrast reaction to these and other polymers with similar mechanisms.

Synthesis and Spectroscopic Characterization of 2-(het)Aryl Perimidine Derivatives with Enhanced Fluorescence Quantum Yields.

Perimidines are a particularly versatile family of heterocyclic compounds, whose properties are exploited in several applications ranging from industrial to medicinal chemistry. The molecular structure of perimidine incorporates a well-known efficient fluorophore, i.e.: 1,8-diaminonaphthalene. The high fluorescence quantum yield shared by most naphthalene derivatives, has enabled their use as stains for bio-imaging and biophysical characterizations. However, fluorescence is dramatically depressed in perimidine as well as in the few of its derivatives analysed so far to this respect. The use of perimidine-like molecules in life sciences might be notably fostered by enhancement of their fluorescence emission. Even more excitingly, the concomitance of both biologically active moieties and a fluorophore in the same molecular structure virtually discloses application of perimidines as drug compounds in state-of-art theranostics protocols. However, somewhat surprisingly, relatively few attempts were made until now in the direction of increasing the performances of perimidines as fluorescent dyes. In this work we present the synthesis and spectroscopic characterization of four perimidine derivatives designed to this aim, two of which result to be endowed with fluorescence quantum yields comparable to 1,8-diaminonaphthalene. A rationalization for such improved behaviour has been attempted employing TD-DFT calculations, which have unravelled the interrelations among bond structure, lone pair conjugation, local electron density changes and fluorescence quantum yield.

Assessment of the Effects of Anisotropic Interactions among Hydrogen Molecules and Their Isotopologues: A Diffusion Monte Carlo Investigation of Gas Phase and Adsorbed Clusters.

Despite the fact that the para-hydrogen molecule (p-H2) and its isotopomers (o-D2 and p-T2) are commonly modeled as spherical objects due to the large separation between rotational states, there may be situations (e.g. adsorption in pores and on surfaces) in which such an approximation neglects important degrees of freedom (i.e. the rotational ones) and introduces uncontrolled biases in the predicted properties. To better understand when approximating such molecules as spheres introduces shortcomings in their representation, we employed diffusion Monte Carlo to simulate small/medium-sized molecular aggregates, either isolated in space or experiencing external model potentials, to compute energetic quantities and distribution functions. These were chosen to mimic situations possibly occurring in real systems, in which orientational isotropy is broken. The comparison between isolated clusters with molecules described as rigid rotors with a 4D potential or as spheres interacting via Adiabatic Hindered Rotor models shows that neither energetic nor structural quantities are affected by reducing the systems dimensionality. The orientational degrees of freedom of the rotors remains largely uncoupled from translational ones whatever the molecular mass. The same happens for rotors interacting with a frozen hydrogen molecule in the vicinity of a repulsive surface. Deviating from such behavior are molecular aggregates interacting with potentials mimicking the presence of ionic adsorption sites inside porous materials. Such difference is ascribable to the markedly anisotropic and longer ranged nature of those interactions, both features being relevant in defining the adsorption energy of the molecular species.

Out of Equilibrium Self-Assembly of Janus Nanoparticles: Steering It from Disordered Amorphous to 2D Patterned Aggregates.

Solvent evaporation driven self-assembly of Janus nanoparticles (J-NPs) has been simulated employing lattice-gas models to investigate the possible emergence of new superlattices. Depending on the chemical nature of NP faces (hence solvophilicity and relative interaction strength), zebra-like or check-like patterns and micellar agglomerates can be obtained. Vesicle-like aggregates can be produced by micelle-based corrals during heterogeneous evaporation. Patterns formed during aggregation appear to be robust against changes in evaporation modality (i.e., spinodal or heterogeneous) or interaction strengths, and they are due to a strictly nanoscopic orientation of single J-NPs in all cases. Due to the latter feature, the aggregate size growth law N(t) ∝ ta has its exponent a markedly depending on the chemical nature of the J-NPs involved in spite of the unvaried growth mechanism. We interpret such a finding as connected to the increasingly stricter orientation pre-requirements for successful (binding) NP landing upon going from isotropic (a ≃ 0.50), to "zebra" (a ≃ 0.38), to "check" (a ≃ 0.23), and finally to "micelle" (a = 0.15-0.17) pattern forming NPs.

Quest for Inexpensive Hydrogen Isotopic Fractionation: Do We Need 2D Quantum Confining in Porous Materials or Are Rough Surfaces Enough? The Case of Ammonia Nanoclusters.

We study the adsorption energetics and quantum properties of the molecular hydrogen isotopes H2, D2, and T2 onto the surface of rigid ammonia nanoclusters with quantum simulations and accurate model potential energy surfaces (PES). A highly efficient diffusion Monte Carlo (DMC) algorithm for rigid rotors allowed us to accurately define zero-point adsorption energies for the three isotopes, as well as the degree of translational and rotational delocalization that each affords on the surface. From the data emerges that the quantum adsorption energy (Eads) of T2 can be up to twice the one of H2 at 0 K, suggesting the possibility of exploiting some form of solid ammonia to selectivity separate hydrogen isotopes at low temperatures (≃20 K). This is discussed by focusing on the structural motif that may be more effective for the task. The analysis of the contributions to Eads, however, surprisingly indicates that the average kinetic energy (Ekin) and rotation energy (Erotkin) of T2 can also be, respectively, 2 times and 20 times higher than those of H2; this finding markedly deviates from what is predicted for hydrogen molecules inside carbon nanotubes (CNT) or metallic-organic frameworks (MOF), where Ekin and Erotkin is higher for H2 due to the unavoidable effects of confinement and hindrance to its rotational motion. The rationale for these differences is provided by the geometrical distributions for the rigid rotors, which reveal an increasingly stronger coupling between rotational and translational degrees of freedom upon increasing the isotopic mass. This effect has never been observed before on adsorbing surfaces (e.g., graphite) and is induced by a strongly anisotropic and anharmonic bowl-like potential experienced by the rotors.

Dynamics of photoexcited Ba(+) cations in (4)He nanodroplets.

We present a joint experimental and theoretical study on the desolvation of Ba(+) cations in (4)He nanodroplets excited via the 6p ← 6s transition. The experiments reveal an efficient desolvation process yielding mainly bare Ba(+) cations and Ba(+)Hen exciplexes with n = 1 and 2. The speed distributions of the ions are well described by Maxwell-Boltzmann distributions with temperatures ranging from 60 to 178 K depending on the excitation frequency and Ba(+) Hen exciplex size. These results have been analyzed by calculations based on a time-dependent density functional description for the helium droplet combined with classical dynamics for the Ba(+). In agreement with experiment, the calculations reveal the dynamical formation of exciplexes following excitation of the Ba(+) cation. In contrast to experimental observation, the calculations do not reveal desolvation of excited Ba(+) cations or exciplexes, even when relaxation pathways to lower lying states are included.

Vibrationally Inelastic Collision Between Li2(ν = 0) and Li: Direct and Postponed Elongation Mechanisms.

The mechanism for vibrational inelastic excitation during the collision between Li2(ν = 0) and Li was investigated exploiting classical trajectory simulations over a potential energy surface generated by fitting valence full configuration interaction calculations employing a large basis set. From the trajectory results, it emerges that the vibrational excitation in noncapture collisions presents uniquely a forward-scattered projectile for the highest levels of excitation (ΔE(0 → ν') ≃ Ecoll). For lower ν', a minor contribution presenting a backward-scattered projectile appears, which, however, has its major contribution coming from a "slingshot"-like (orbiting) mechanism exploiting the attractive features of the Li3 potential energy surface rather than a direct recoil.

On the convergence of diffusion Monte Carlo in non-Euclidean spaces. I. Free diffusion.

We develop a set of diffusion Monte Carlo algorithms for general compactly supported Riemannian manifolds that converge weakly to second order with respect to the time step. The approaches are designed to work for cases that include non-orthogonal coordinate systems, nonuniform metric tensors, manifold boundaries, and multiply connected spaces. The methods do not require specially designed coordinate charts and can in principle work with atlases of charts. Several numerical tests for free diffusion in compactly supported Riemannian manifolds are carried out for spaces relevant to the chemical physics community. These include the circle, the 2-sphere, and the ellipsoid of inertia mapped with traditional angles. In all cases, we observe second order convergence, and in the case of the sphere, we gain insight into the function of the advection term that is generated by the curved nature of the space.

On the convergence of diffusion Monte Carlo in non-Euclidean spaces. II. Diffusion with sources and sinks.

We test the second order Milstein method adapted to simulate diffusion in general compact Riemann manifolds on a number of systems characterized by nonconfining potential energy surfaces of increasing complexity. For the 2-sphere and more complex spaces derived from it, we compare the Milstein method with a number of other first and second order approaches. In each case tested, we find evidence that demonstrate the versatility and relative ease of implementation of the Milstein method derived in Part I.