Conformational Properties of Sodium Polystyrenesulfonate in Water: Insights from a Coarse-Grained Model with Explicit Solvent

Adsorption Isotherm and Mechanism of Ca2+ Binding to Polyelectrolyte

Polyelectrolytes, such as poly(acrylic acid) (PAA), can effectively mitigate CaCO3 scale formation. Despite their success as antiscalants, the underlying mechanism of binding of Ca2+ to polyelectrolyte chains remains unresolved. Through all-atom molecular dynamics simulations, we constructed an adsorption isotherm of Ca2+ binding to sodium polyacrylate (NaPAA) and investigated the associated binding mechanism. We find that the number of calcium ions adsorbed [Ca2+]ads to the polymer saturates at moderately high concentrations of free calcium ions [Ca2+]aq in the solution. This saturation value is intricately connected with the binding modes accessible to Ca2+ ions when they bind to the polyelectrolyte chain. We identify two dominant binding modes: the first involves binding to at most two carboxylate oxygens on a polyacrylate chain, and the second, termed the high binding mode, involves binding to four or more carboxylate oxygens. As the concentration of free calcium ions [Ca2+]aq increases from low to moderate levels, the polyelectrolyte chain undergoes a conformational transition from an extended coil to a hairpin-like structure, enhancing the accessibility to the high binding mode. At moderate concentrations of [Ca2+]aq, the high binding mode accounts for at least one-third of all binding events. The chain's conformational change and its consequent access to the high binding mode are found to increase the overall Ca2+ ion binding capacity of the polyelectrolyte chain.

Concentration Dependent Single Chain Properties of Poly(sodium 4-styrenesulfonate) Subjected to Aromatic Interactions with Chlorpheniramine Maleate Studied by Diafiltration and Synchrotron-SAXS

The polyelectrolyte poly(sodium 4-styrenesulfonate) undergoes aromatic–aromatic interaction with the drug chlorpheniramine, which acts as an aromatic counterion. In this work, we show that an increase in the concentration in the dilute and semidilute regimes of a complex polyelectrolyte/drug 2:1 produces the increasing confinement of the drug in hydrophobic domains, with implications in single chain thermodynamic behavior. Diafiltration analysis at polymer concentrations between 0.5 and 2.5 mM show an increase in the fraction of the aromatic counterion irreversibly bound to the polyelectrolyte, as well as a decrease in the electrostatic reversible interaction forces with the remaining fraction of drug molecules as the total concentration of the system increases. Synchrotron-SAXS results performed in the semidilute regimes show a fractal chain conformation pattern with a fractal dimension of 1.7, similar to uncharged polymers. Interestingly, static and fractal correlation lengths increase with increasing complex concentration, due to the increase in the amount of the confined drug. Nanoprecipitates are found in the range of 30–40 mM, and macroprecipitates are found at a higher system concentration. A model of molecular complexation between the two species is proposed as the total concentration increases, which involves ion pair formation and aggregation, producing increasingly confined aromatic counterions in hydrophobic domains, as well as a decreasing number of charged polymer segments at the hydrophobic/hydrophilic interphase. All of these features are of pivotal importance to the general knowledge of polyelectrolytes, with implications both in fundamental knowledge and potential technological applications considering aromatic-aromatic binding between aromatic polyelectrolytes and aromatic counterions, such as in the production of pharmaceutical formulations.

Molecular dynamics insight into viscosity reduction of hydrolysed polyacrylamide by using carbon quantum dots

Hydrolysed polyacrylamide (HPAM) is widely used in many industrial fields where its rheological properties play a leading role. Recent discovery of the reduction of HPAM's viscosity by adding carbon quantum dots (CQDs), however, is controversial to the established theories. By using all atom molecular dynamics simulation with an OPLS-AA force field, this study aims to provide detailed molecular insight into such an uncommon phenomenon. The dynamic structures of the HPAM chain in the presence or absence of CQDs were clearly captured from the molecular aspect. The results reveal that the adsorption of CQD reduces the gyration radius of the HPAM chain, and it is the corresponding hydration effect that leads to the reduction of the viscosity. The amide rather than the carboxylate group along the HPAM chain is dominant in terms of the interaction with the CQDs, and the driven atoms depend on the surface where the polymer is adsorbed.

The Martini Model in Materials Science

The Martini model, a coarse-grained force field initially developed with biomolecular simulations in mind, has found an increasing number of applications in the field of soft materials science. The model's underlying building block principle does not pose restrictions on its application beyond biomolecular systems. Here, the main applications to date of the Martini model in materials science are highlighted, and a perspective for the future developments in this field is given, particularly in light of recent developments such as the new version of the model, Martini 3.

Dynamics of Human Serum Albumin Corona Formation on Gold Nanorods with Different Surface Ligands In Silico

The interaction between human serum albumin (HSA) and nanoparticles (NPs) to form HSA corona has widely been studied since endogenous functions of albumin are highly attractive for drug delivery. However, a full understanding of the molecular dynamics and factors behind the formation of HSA corona, including interactions between HSA and different surface ligands and between neighboring HSA molecules, resulting in conformational change of HSA is presently lacking. Here, we assembled 14 HSA molecules around gold nanorods (AuNRs) with different surface chemistries (bare gold surface, cetyltrimethylammonium bromide (CTAB), polystyrene sulfonate (PSS), and polydiallyldimethylammonium chloride (PDADMAC)) in silico and examined the dynamics of HSA corona formation using coarse-grained molecular dynamics for 300 ns of simulation. We observed that PDADMAC, being more flexible than PSS, resulted in all HSA molecules moving toward AuNR-PDADMAC, while the instability of CTAB on AuNR resulted in fewer HSA molecules moving toward AuNR-CTAB compared to AuNR-PSS. HSA molecules around AuNR-PDADMAC also exhibited the largest conformational change in terms of their radius of gyration (R_g) and root mean square deviation (RMSD). In the absence of surface ligands, HSA molecules around the bare AuNR were susceptible to steric hindrance with conformational change observed in terms of their RMSD but not their R_g unlike that of HSA molecules around AuNR-PDADMAC. The insights gained from the inclusion of neighboring HSA molecules in the simulation of corona formation could be more representative than examining a single adsorbed HSA molecule on AuNRs with different surface passivations.

Studying the physics of charged macromolecules by single molecule fluorescence spectroscopy

It is well documented that conventional methods such as dynamic light scattering have encountered difficulties in characterizing charged macromolecules and, therefore, it is desirable that new methods and techniques are introduced. With the ultra-high sensitivity, single molecule fluorescence spectroscopy has successfully lowered the detection limit considerably and enabled measurement under extreme dilution conditions-around the concentration of 10^-9M-at which the effect of inter-chain electrostatic repulsion is suppressed. Furthermore, the excellent spatial and temporal resolution as well as the capacity of molecular recognition of these methods help in obtaining rich information of charged macromolecules. This paper summarizes the applications of single molecule fluorescence spectroscopy, especially fluorescence correlation spectroscopy and photon counting histogram, in the studies on charged macromolecules in aqueous solutions and plenty of new information has been revealed on the molecular conformation, counterion distribution, and a few important governing factors. The powerfulness and effectiveness of single molecule fluorescence spectroscopy make it promising in the investigations of charged macromolecules.

Screening lengths and osmotic compressibility of flexible polyelectrolytes in excess salt solutions

We report results of small angle neutron scattering measurements made on sodium polystyrene sulfonate in aqueous salt solutions. The correlation length (ξ) and osmotic compressibility are measured as a function of polymer (c) and added salt (cS) concentrations, and the results are compared with scaling predictions and the random-phase approximation (RPA). In Dobrynin et al.'s scaling model the osmotic pressure consists of a counter-ion contribution and a polymer contribution. The polymer contribution is found to be two orders of magnitude smaller than expected from the scaling model, in agreement with earlier observations made on neutral polymers in good solvent condition. RPA allows the determination of single-chain dimensions in semidilute solutions at high polymer and added salt concentrations, but fails for cS≤ 2 M. The χ parameter can be modelled as the sum of an intrinsic contribution (χ0) and an electrostatic term: χ∼χ0 + K'/√cS, where χ0 > 0.5 is consistent with the hydrophobic nature of the backbone of NaPSS. The dependence of χelec∼ 1/√cS disagrees with the random-phase approximation (χelec∼ 1/cs), but agrees with the light scattering results in dilute solution and Dobrynin et al.'s scaling treatment of electrostatic excluded volume.

Polyelectrolyte stiffness on gold nanorods mediates cell membrane damage

Charge and surface chemistry of gold nanorods (AuNRs) are often considered the predictive factors for cell membrane damage. Unfortunately, extensive research on AuNR passivated with polyelectrolyte (PE) ligand shell (AuNR-PE) has hitherto left a vital knowledge gap between the mechanical stability of the ligand shell and the cytotoxicity of AuNR-PEs. Here, the agreement between unbiased coarse-grained molecular dynamics (CGMD) simulation and empirical outcomes on hemolysis of red blood cells by AuNR-PEs demonstrates for the first time, a direct impact of the mechanical stability of the PE shell passivating the AuNRs on the lipid membrane rupture. Such mechanical stability is ultimately modulated by the rigidity of the PE components. The CGMD simulation results also reveal the mechanism where the PE chain adsorbs near the surface of the lipid bilayer without penetrating the hydrophobic core of the bilayer, which allows the hydrophobic AuNR core to be in direct contact with the hydrophobic interior of the lipid bilayer, thereby perforating the lipid membrane to induce membrane damage.

Molecular weight dependence of chain conformation of strong polyelectrolytes

Using sodium polystyrene sulfonate (NaPSS) and quarternized poly 4-vinylpyridine (QP4VP) as model systems, the chain conformation of polyelectrolytes under finite salt concentrations is investigated at a single molecular level. By fluorescence correlation spectroscopy (FCS), the hydrodynamic radius (R _h) of the samples with the molecular weight ranging more than one order of magnitude was measured. The variations of R _h as a function of molecular weight reveal the molecular weight dependence: under moderate salt concentrations (such as 10^-4 and 0.1M), the shorter chains of both NaPSS and QP4VP take the rod-like conformation, while the longer chains take the coiled conformation (random coil or swelled random coil conformation, respectively). At high enough salt levels, both the charged chains take the coiled conformations. Photon counting histogram (PCH) measurements of the local pH value at the vicinity of the NaPSS chain expose the higher extent of counterion adsorption for longer chains as well as higher salt concentrations, telling that the charge regularization process is the major governing factor.

Aqueous size-exclusion chromatography of polyelectrolytes on reversed-phase and hydrophilic interaction chromatography columns

The size-exclusion separation of a water-soluble polyelectrolyte polymer, sodium polystyrene sulfonate (NaPSS), was demonstrated on common reversed-phase (C_18, C₄, phenyl, and cyano) and hydrophilic interaction chromatography (HILIC) columns. The effect of common solvents - acetonitrile (ACN), tetrahydrofuran (THF), and methanol (MeOH), used as mobile phase modifiers - on the elution of NaPSS and the effect of column temperature (within a relatively narrow range corresponding to typical chromatographic conditions, i.e., 10 °C-60 °C) on the partition coefficient, K_SEC, were also investigated. Non-size-exclusion chromatography (non-SEC) effects can be minimized by the addition of an electrolyte and an organic modifier to the mobile phase, and by increasing the column temperature (e.g., to 50 °C or 60 °C). Strong solvents such as THF and ACN are more successful in the reduction of such effects than is the weaker solvent MeOH. The best performance is seen on medium polarity and polar stationary phases, such as cyanopropyl- and diol-modified silica (HILIC), where the elution of the NaPSS polyelectrolyte is by a near-ideal SEC mechanism. Hydrophobic stationary phases, such as C₁₈, C₄, and phenyl, require a higher concentration of a strong solvent modifier (THF) in the mobile phase to reduce non-SEC interactions of the solute with the stationary phase.

Dynamic Feature of Incipient Polymer Collapse below the Theta Point

We study the dynamics of a polymer chain with gradually changing the solvent quality from good to poor by dissipative particle dynamics simulation. We find several spectral modes related to internal motions of intrachain interaction. Approaching the coil-to-globule transition point, all fast modes of spectrum ω > 1 (ns)^-1 disappear. There is only a slow mode at ω ≈ 0.66 (ns)^-1. Moreover, the spectral density at this slow mode reaches a maximum value at the transition point. We suggest that, at the transition point, the chain conformation relaxes to the most probable distribution only by the slow mode. There is a critical slowing down of internal motion with passing through the transition point.

Conformation transitions of a single polyelectrolyte chain in a poor solvent: a replica-exchange lattice Monte-Carlo study

The thermodynamic behaviors of a strongly charged polyelectrolyte chain in a poor solvent are studied using replica-exchange Monte-Carlo simulations on a lattice model, focusing on the effects of finite chain length and the solvent quality on the chain conformation and conformation transitions. The neutralizing counterions and solvent molecules are considered explicitly. The thermodynamic quantities that vary continuously with temperature over a wide range are computed using the multiple histogram reweighting method. Our results suggest that the strength of the short-range hydrophobic interaction, the chain length, and the temperature of the system, characterized by ε, N, and T, respectively, are important parameters that control the conformations of a charged chain. When ε is moderate, the competition between the electrostatic energy and the short-range hydrophobic interaction leads to rich conformations and conformation transitions for a longer chain with a fixed length. Our results have unambiguously demonstrated the stability of the n-pearl-necklace structures, where n has a maximum value and decreases with decreasing temperature. The maximum n value increases with increasing chain length. Our results have also demonstrated the first-order nature of the conformation transitions between the m-pearl and the (m-1)-pearl necklaces. With the increase of ε, the transition temperature increases and the first-order feature becomes more pronounced. It is deduced that at the thermodynamic limit of infinitely long chain length, the conformational transitions between the m-pearl and the (m-1)-pearl necklaces may remain first order when ε > 0 and m = 2 or 3. Pearl-necklace conformations cannot be observed when either ε is too large or N is too small. To observe a pearl-necklace conformation, the T value needs to be carefully chosen for simulations performed at only a single temperature.