Effect of Hydrophilicity or Hydrophobicity of Polyelectrolyte on the Interaction between Polyelectrolyte and Surfactants: Molecular Dynamics Simulations

Microstructures and Rheological Properties of Short-Side-Chain Perfluorosulfonic Acid in Water/2-Propanol

The viscosity and viscoelasticity of polyelectrolyte solutions with a single electrostatic interaction have been carefully studied experimentally and theoretically. Despite some theoretical models describe experimental results well, the influence of multiple interactions (electrostatic and hydrophobic) on rheological scaling is not yet fully resolved. Herein, we systematically study the microstructures and rheological properties of short-side-chain perfluorosulfonic acid (S-PFSA), the most promising candidate of a proton exchange membrane composed of a hydrophobic backbone with hydrophilic side-chains, in water/2-propanol. Small-angle X-ray scattering confirms that semiflexible S-PFSA colloidal particles with a length of ~38 nm and a diameter of 1-1.3 nm are formed, and the concentration dependence of the correlation length (ξ) obeys the power law ξ~c-0.5 consistent with the prediction of Dobrynin et al. By combining macrorheology with diffusing wave spectroscopy microrheology, the semidilute unentangled, semidilute entangled, and concentrated regimes corresponding to the scaling relationships ηsp~c0.5, ηsp~c1.5, and ηsp~c4.1 are determined. The linear viscoelasticity indicates that the entanglement concentration (ce) obtained from the dependence of ηsp on the polymer concentration is underestimated owing to hydrophobic interaction. The true entanglement concentration (cte) is obtained by extrapolating the plateau modulus (Ge) to the terminal modulus (Gt). Furthermore, Ge and the plateau width, τr/τe (τr and τe denote reptation time and Rouse time), scale as Ge~c2.4 and τr/τe~c4.2, suggesting that S-PFSA dispersions behave like neutral polymer solutions in the concentrated regime. This work provides mechanistic insight into the rheological behavior of an S-PFSA dispersion, enabling quantitative control over the flow properties in the process of solution coating.

On-Chip Control over Polyelectrolyte-Surfactant Complexation in Nonequilibrium Microfluidic Confinement

The goal of this work is to classify and quantify the factors that govern polyelectrolyte–surfactant complexation in microfluidic confinement and optimize the designs and operating modes of microfluidic reactors to offer additional advantages over the macroscopic synthesis of such complexes. We analyze and solve a system of governing convection–diffusion–reaction equations to conveniently represent these factors or their combinations as dimensionless similarity criteria. We discuss how these factors contribute to the on-chip control of the reaction initiation, the complex product distribution in a microfluidic device, and the phase behavior of the confined reacting flows and experimentally verify the results in microchips. This approach allows for designing microfluidic devices and setting their operating modes to avoid undesirable clogging by reaction products, control the initiation of the complexation reaction, and produce polyelectrolyte–surfactant aggregates with a broader size range and reduced dispersity.

Single-molecule studies reveal the distinction of strong and weak polyelectrolytes in aqueous solutions

Polyelectrolytes are an important class of functional polymers that have the advantages of both polymers and electrolytes due to the presence of charges, and have prospective applications in many fields. The charge of the backbone is an important factor affecting the properties of polyelectrolytes. Therefore, the complex interactions caused by the charges in polyelectrolyte solutions pose a challenge to the study of polyelectrolyte systems, and there is no consensus on the distinction between the behavior of strong and weak polyelectrolytes in solution. Based on single-molecule force spectroscopy (SMFS), the distinction of strong and weak polyelectrolytes is clarified for the first time at the single molecular level by comparing the single-chain elasticity in different environments. It is expected that the single-molecule study will provide the theoretical and experimental basis for the further application of polyelectrolytes.

Nano-Assembly of Quisinostat and Biodegradable Macromolecular Carrier Results in Supramolecular Complexes with Slow-Release Capabilities

Self-assembly of ionically charged small molecule drugs with water-soluble biodegradable polyelectrolytes into nano-scale complexes can potentially offer a novel and attractive approach to improving drug solubility and prolonging its half-life. Nanoassemblies of quisinostat with water-soluble PEGylated anionic polyphosphazene were prepared by gradient-driven escape of solvent resulting in the reduction of solvent quality for a small molecule drug. A study of binding, analysis of composition, stability, and release profiles was conducted using asymmetric flow field flow fractionation (AF4) and dynamic light scattering (DLS) spectroscopy. Potency assays were performed with WM115 human melanoma and A549 human lung cancer cell lines. The resulting nano-complexes contained up to 100 drug molecules per macromolecular chain and displayed excellent water-solubility and improved hemocompatibility when compared to co-solvent-based drug formulations. Quisinostat release time (complex dissociation) at near physiological conditions in vitro varied from 5 to 14 days depending on initial drug loading. Multimeric complexes displayed dose-dependent potency in cell-based assays and the results were analyzed as a function of complex concentration, as well as total content of drug in the system. The proposed self-assembly process may present a simple alternative to more sophisticated delivery modalities, namely chemically conjugated prodrug systems and nanoencapsulation-based formulations.

Effects of Micellization Behavior on the Interfacial Adsorption in Binary Anionic/Nonionic Surfactant Systems: A Molecular Simulation Study

A surfactant interfacial adsorption process is highly associated with its micellization behaviors in the water phase, which is of great fundamental and practical significance in enhanced oil recovery. In this paper, the typical anionic surfactant 1-dodecanesulfonic acid sodium (DAS) and nonionic surfactants octylphenol polyoxyethylene ether-n (OP-n, n = 1, 5, and 10) are introduced to investigate their micellization behavior and interfacial adsorption process via molecular dynamics simulation. Number density profiles reveal that the additional OP5 molecules in the water phase generate the mixed micelle with DAS molecules and greatly promote its interfacial adsorption. Interaction energy calculation is employed to confirm the interaction of anionic/nonionic surfactants in the mixed micelle. Then, the radial distribution function, solvent-accessible surface area, and solvation free energy are calculated to further explore and verify the adsorption mechanism of the mixed micelle. It is found that the nonionic surfactant obviously decreases the hydrophilicity of the mixed micelle in the water phase, which should be responsible for its intensive tendency of the interfacial adsorption.

Synergistic Effects of Alpha Olefin Sulfonate and Sodium Alginate on Inkjet Printing of Cotton/Polyamide Fabrics

Bicomponent or multicomponent fiber fabrics are important materials for manufacturing high-performance textiles. However, the printing and dyeing of these fabrics are very difficult because the dyeability of different fibers varies greatly. The present study investigated the inkjet printing performance of interwoven fabrics of cotton and polyamide 6. The surfactant alpha olefin sulfonate (AOS) was incorporated into the sodium alginate (SA) solution to pretreat the fabrics to improve the color effects of printed fabrics. The results indicate that fabric pretreatment using 5% alpha olefin sulfonate and 2% sodium alginate significantly enhanced the image colors through increasing the hydrophilicity of the film formed on polyamide fibers and changing the surface morphology of both the fibers. The molecules of AOS interacted with the macromolecules of SA to form the composite films, where the AOS concentration gradient increased outward and SA concentration gradient increased inward. The synergistic pretreatment of alpha olefin sulfonate and sodium alginate endowed the fabrics with high inkjet printing performance, satisfactory color fastnesses, and durability.

Supramolecular assembly of Toll-like receptor 7/8 agonist into multimeric water-soluble constructs enables superior immune stimulation in vitro and in vivo

Resiquimod or R848 (RSQD) is a Toll-like receptor (TLR) 7/8 agonist which shows promise as vaccine adjuvant due to its potential to promote highly desirable cellular immunity. The development of this small molecule in the field to date has been largely impeded by its rapid in vivo clearance and lack of association with vaccine antigens. Here, we report a multimeric TLR 7/8 construct of nano-scale size, which results from a spontaneous self-assembly of RSQD with a water-soluble clinical-stage polymer - poly[di(carboxylatophenoxy)phosphazene] (PCPP). The formation of ionically paired construct (PCPP-R) and a ternary complex, which also includes Hepatitis C virus (HCV) antigen, has been demonstrated by dynamic lights scattering (DLS), turbidimetry, fluorescence spectroscopy, asymmetric flow field flow fractionation (AF4), and ¹H NMR spectroscopy methods. The resulting supramolecular assembly PCPP-R enabled superior immunostimulation in cellular assays (mouse macrophage reporter cell line) and displayed improved in vitro hemocompatibility (human erythrocytes). In vivo studies demonstrated that PCPP-R adjuvanted HCV formulation induced higher serum neutralization titers in BALB/c mice and shifted the response towards desirable cellular immunity, as evaluated by antibody isotype ratio (IgG2a/IgG1) and ex vivo analysis of cytokine secreting splenocytes (higher levels of interferon gamma (IFN-γ) single and tumor necrosis factor alpha (TNF-α)/IFN-γ double producing cells). The non-covalent multimerization approach stands in contrast to previously suggested RSQD delivery methods, which involve covalent conjugation or encapsulation, and offers a flexible methodology that can be potentially integrated with other parenterally administered drugs.

From Tunable DNA/Polymer Self-Assembly to Tailorable and Morphologically Pure Core-Shell Nanofibers

In reported experimental studies, DNA/polymer self-assemblies are usually kinetically trapped, leading to the encapsulation and irregular collapse of DNA chains within the resultant assemblies. In striking contrast, eukaryotic cells use tetrasome-to-nucleosome pathways to escape possible kinetic trapping for the formation of well-defined 10 nm chromatin fibers. Here, we report a novel pathway for DNA and amphiphilic diblock copolymer self-assembly inspired by the tetrasome pathway with highly controllable kinetics. The polymer is an A- b-B diblock copolymer with a hydrophilic and noninteractive block A and a hydrophobic and interactive block B. Below the critical water content for the micellization, B blocks wrap the backbone of a DNA chain by weak electrostatic interactions to form a linear DNA/polymer complex. With a gradual increase in the water content, the diblock copolymer unimers in the bulk solution tend to aggregate on the linear DNA/polymer complex, which induces the originally wrapped DNA chain, to change its conformation to wrap around the polymer aggregate, guiding and tailoring the self-assembly. Highly controllable kinetics is achieved via the reduced DNA/polymer electrostatic interactions and the high dynamics of the polymer chains in the system. DNA/polymer self-assembly leads to tailorable and morphologically pure core-shell nanofibers. Compared to the DNA/micelle self-assembly pathway described in our previous study, the present self-assembly pathway exhibits advantages for the fabrication of flexible nanofibers with lengths in micrometers and the potential for unique applications in preparing not only 2D networks at extremely low percolation thresholds but also chemiresistors with large on/off current ratios.

Complexation of Polyelectrolytes with Hydrophobic Drug Molecules in Salt-Free Solution: Theory and Simulations

The delivery and dissolution of poorly soluble drugs is challenging in the pharmaceutical industry. One way to significantly improve the delivery efficiency is to incorporate these hydrophobic small molecules into a colloidal polyelectrolyes(PE)-drug complex in their ionized states. Despite its huge application value, the general mechanism of PE collapse and complex formation in this system has not been well understood. In this work, by combining a mean-field theory with extensive molecular simulations, we unveil the phase behaviors of the system under dilute and salt-free conditions. We find that the complexation is a first-order-like phase transition triggered by the hydrophobic attraction between the drug molecules. Importantly, the valence ratio between the drug molecule and PE monomer plays a crucial role in determining the stability and morphology of the complex. Moreover, the sign of the zeta potential and the net charge of the complex are found to be inverted as the hydrophobicity of the drug molecules increases. Both theory and simulation indicate that the complexation point and complex morphology and the electrostatic properties of the complex have a weak dependence on chain length. Finally, the dynamics aspect of PE-drug complexation is also explored, and it is found that the complex can be trapped into a nonequilibrium glasslike state when the hydropobicity of the drug molecule is too strong. Our work gives a clear physical picture behind the PE-drug complexation phenomenon and provides guidelines to fabricate the colloidal PE-drug complex with the desired physical characteristics.

Silica Ouzo Effect: Amphiphilic Drugs Facilitate Nanoprecipitation of Polycondensed Mercaptosilanes

Amphiphilic drugs are therapeutic agents whose molecular structures contain both hydrophobic and hydrophilic portions. Here we report a systematic study on how amphiphilic drugs can assist in silica nanoprecipitation. 3-Mercaptopropyltrimethoxysilane (MPTMS) was used as the sole silica material and 12 amphiphilic drugs spanning a wide spectrum of therapeutic categories were included. MPTMS polycondensation was conducted in a DMSO-based organic phase. After a sufficient time, particle formation was induced by injecting a small amount of the organic phase into a water solution containing various amphiphiles. The results show that all amphiphilic drugs studied exerted concentration-dependent facilitating effect on nanoparticle formation. Under certain preparation conditions, the particle solution showed physical stability over a long period and the formed particles could be as small as 100 nm. By systematically varying drug concentrations and injection volumes, the ability of each amphiphile to promote nanoprecipitation can be quantified and compared, based on two novel indices: the area under the critical volume-concentration curve (AUC) and the critical stabilization concentration (CSC). We demonstrate that both ability indices significantly correlated with the drug's log P and critical micelle concentrations (CMC). Furthermore, we have optimized the aging and particle purification condition and extensively characterized our system through comprehensive TEM and zeta-potential measurements, as well as determinations for drug entrapment and release. In conclusion, we have established a quantitative structure-activity relationship for amphiphilic small-molecular drugs in their ability to interact with poly(mercaptopropyl)silsesquioxane species and form nanoparticles via solvent shifting. We speculate that both hydrophobic and electrostatic interactions play important roles in the formation and stabilization of nanoparticles.

Surfactant induced aggregation behavior of Merocyanine-540 adsorbed on polymer coated positively charged gold nanoparticles

Surfactant induced aggregation behavior of Merocyanine 540 adsorbed on polymer (PDD) coated gold nanoparticles (AuNP) is reported. The absorption band of the dye shifts to higher energy in the presence of free polymer and polymer coated AuNP implying aggregation. Addition of a negatively charged surfactant (SDS) induces multiple bands in the extinction spectrum of the dye adsorbed on nanoparticle surface. The highest (460nm) and lowest (564nm) energy bands of the dye become prominent at 10 and >50μM SDS concentrations respectively (dye: 10μM; AuNP: 100-200pM). Based on earlier results the high energy band is likely to originate from dye aggregates and the low energy band is likely to originate from dye monomers. This is attributed to the interplay between polymer-surfactant and polymer-dye interactions at the AuNP surface. The extinction spectra of dye adsorbed at AuNP surface remain unaffected in the presence of a positively charged (CTAB) or a neutral surfactant (Tx-100), at low surfactant concentrations. However at higher surfactant concentrations (>60μM) dye aggregation takes place which is attributed to dye-surfactant interactions. The fluorescence intensity of the dye quenched significantly but its lifetime increased in the presence of polymer coated AuNP. This is attributed to aggregation and reduction in the photoisomerization rate of the dye adsorbed on AuNP surface.

Use of isothermal titration calorimetry to study surfactant aggregation in colloidal systems

BACKGROUND

Isothermal titration calorimetry (ITC) is a general technique that allows for precise and highly sensitive measurements. These measurements may provide a complete and accurate thermodynamic description of association processes in complex systems such as colloidal mixtures.

SCOPE OF THE REVIEW

This review will address uses of ITC for studies of surfactant aggregation to form micelles, with emphasis on the thermodynamic studies of homologous surfactant series. We will also review studies on surfactant association with polymers of different molecular characteristics and with colloidal particles.

GENERAL SIGNIFICANCE

ITC studies on the association of different homologous series of surfactants provide quantitative information on independent contribution from their apolar hydrocarbon chains and polar headgroups to the different thermodynamic functions associated with micellization (Gibbs energy, enthalpy and entropy). Studies on surfactant association to polymers by ITC provide a comprehensive description of the association process, including examples in which particular features revealed by ITC were elucidated by using ancillary techniques such as light or X-ray scattering measurements. Examples of uses of ITC to follow surfactant association to biomolecules such as proteins or DNA, or nanoparticles are also highlighted. Finally, recent theoretical models that were proposed to analyze ITC data in terms of binding/association processes are discussed.

MAJOR CONCLUSIONS

This review stresses the importance of using direct calorimetric measurements to obtain and report accurate thermodynamic data, even in complex systems. These data, whenever possible, should be confirmed and associated with other ancillary techniques that allow elucidation of the nature of the transformations detected by calorimetric results, providing a complete description of the process under scrutiny.

Molecular Dynamics Simulations of Adsorption of Poly(acrylic acid) and Poly(methacrylic acid) on Dodecyltrimethylammonium Chloride Micelle in Water: Effect of Charge Density

We have investigated the interaction of dodecyltrimethylammonium chloride (DoTA) micelle with weak polyelectrolytes, poly(acrylic acid) and poly(methacrylic acid). Anionic as well as un-ionized forms of the polyelectrolytes were studied. Polyelectrolyte-surfactant complexes were formed within 5-11 ns of the simulation time and were found to be stable. Association is driven purely by electrostatic interactions for anionic chains whereas dispersion interactions also play a dominant role in the case of un-ionized chains. Surfactant headgroup nitrogen atoms are in close contact with the carboxylic oxygens of the polyelectrolyte chain at a distance of 0.35 nm. In the complexes, the polyelectrolyte chains are adsorbed on to the hydrophilic micellar surface and do not penetrate into the hydrophobic core of the micelle. Polyacrylate chain shows higher affinity for complex formation with DoTA as compared to polymethacrylate chain. Anionic polyelectrolyte chains show higher interaction strength as compared to corresponding un-ionized chains. Anionic chains act as polymeric counterion in the complexes, resulting in the displacement of counterions (Na(+) and Cl(-)) into the bulk solution. Anionic chains show distinct shrinkage upon adsorption onto the micelle. Detailed information about the microscopic structure and binding characteristics of these complexes is in agreement with available experimental literature.

Coarse-Grained Simulation of Polycation/DNA-Like Complexes: Role of Neutral Block

Complexes formed by polycations and DNA are of great research interest because of their prospective application in gene therapy. Whereas the applications of multiblock based polycation generally exhibit promising features, a thorough understanding on the effect of neutral block incorporated in polycation is still lacking. By using coarse-grained dynamics simulation with the help of a simple model for solvent mediated interaction, we perform a theoretical study on the physicochemical properties of various polyplexes composed of a single DNA-like polyanion chain and numbers of linear polycationic chains with different modifications. By analyzing various properties, we find the hydrophobic/hydrophilic modifications of linear polycations may bring an improvement on one aspect of the properties as gene carrier but also involve a trade-off with another one. In particular, polycation with a hydrophobic middle block and a hydrophilic tail block display distinct advantages among di- and triblock linear polycations as gene carrier, while careful design of the hydrophobic block should be made to reduce the zeta potential. The simulation results are compared with available experimental data displaying good agreements.

Surfactant-polymer interactions: molecular architecture does matter

Polymer-surfactant mixtures are found in many industrial formulations, and hence there is a significant interest in understanding, at a molecular level, how the self-assembly of surfactant is affected by oppositely-charged polyelectrolytes (PEs). We use self-consistent field modeling and show that the modes of interaction of PEs strongly depend on the architecture of the PE on the segmental level. Hydrophilic cationic PEs with their charge proximal to the linear backbone are expected to bind electrostatically to the outsides of the coronas of the spherical micelles of anionic surfactants, such as sodium laureth sulphate (SLES). As a result, the surfactant aggregation number increases, but at the same time the colloidal stability deteriorates, due to bridging of the PEs between micelles. PEs with their charge somewhat displaced from the backbone by way of short hydrophobic spacers, are expected to be present inside a micelle at the core-corona boundary. In this case the aggregation number decreases, yet the colloidal stability is retained. Hence, SLES tends to remove hydrophilic PEs from an aqueous solution, whereas it solubilizes more hydrophobic ones. The binding isotherm shows that the uptake of PEs remains typically below charge compensation and in this case the spherical micelle topology remains the preferred state.

A dimension map for molecular aggregates

A pair of gyradius ratios, defined from the principal radii of gyration, are used to generate a dimension map that describes the geometry of molecular aggregates in water and in organic solvents. Molecular dynamics simulations were performed on the aggregation of representative biomolecules and polyaromatic compounds to demonstrate application of the dimension map. It was shown that molecular aggregate data on the dimension map were bounded by two boundary curves, and that the map could be separated into three regions representing three groups of structures: one-dimensional rod-like structures; two-dimensional planar structures or short-cylinder-like structures; and three-dimensional sphere-like structures. Examining the location of the aggregates on the dimension map and how the location changes with solvent type and solute material parameter provides a simple yet effective way to infer the aggregation manner and to study solubility and mechanism of aggregation.

Complete dissociation and reassembly behavior as studied by using poly(ethylene glycol)-block-poly(glutamate sodium) and kanamycin A

Kanamycin A, an amino modified sugar, can interact with poly(ethylene glycol)-block-poly(glutamate sodium) (PEG114-PGlu64) via electrostatic interactions (with PGlu) and hydrogen bonding (with PEG). The interplay of these two forces determines the assembly process and the resulting structure. In deionized water, kanamycin A and PEG114-PGlu64 form a spherical structure at [+]/[-] = 3.5. This structure dissociates instantly and completely in the presence of 30 mM NaCl. However, a new structure is reassembled in about 2 hours. A similar phenomenon is observed when the buffer pH is increased from 7.8 to 8.3. We attribute the distinct dissociation/reassembly process to the reestablishment of the balance between electrostatic interactions and hydrogen bonding. The dissociation/reassembly process in response to environmental changes offers a novel approach to release the loaded cargo in a controlled manner.

Charging and uncharging a neutral polymer in solution: a small-angle neutron scattering investigation

Aqueous formulations containing polymers and surfactants find several applications in pharmaceutics, coatings, inks, and home products. The association between polymers and surfactants contributes greatly to the function of these complex fluids, however, the effects of polar organic solvents, ubiquitous in formulations, remain mostly unexplored. We have analyzed small angle neutron scattering (SANS) data to determine the conformation of a "model" nonionic polymer, poly(ethylene oxide) (PEO), in aqueous solutions as affected by the presence of an ionic surfactant, sodium dodecyl sulfate (SDS), and subsequent addition of short-chain alcohol (ethanol or 2-propanol). PEO chains (MW = 90,000) are Gaussian in dilute aqueous solutions, but become polyelectrolyte-like upon the addition of 30 mM SDS, with about 6 SDS micelles bound to each PEO chain. Micelles associated with polymer are similar in structure and interactions to micelles that form in aqueous solutions in the absence of polymer. Addition of alcohol alters both the polymer and micelle structure and interactions, leads to detachment of micelles from the polymer, and the PEO chains regain their Gaussian conformation. 2-Propanol is more effective than ethanol in influencing the polymer conformation and the properties of SDS micelles in aqueous solutions, either in the presence or in the absence of PEO. This study contributes fundamental insights on polymer and surfactant organization in solution, as well as new, quantitative information on systems that are widely used in practice.

Formation and dissociation of the acridine orange dimer as a tool for studying polyelectrolyte-surfactant interactions

Steady-state and time-resolved fluorescence and UV-vis techniques were used to study the formation and dissociation of acridine orange dimer in order to investigate hyaluronan-acridine orange, hyaluronan-CTAB (cetyltrimethylammonium bromide), polystyrenesulfonate-acridine orange, and polystyrenesulfonate-CTAB interactions in aqueous solution. Steady-state and time-resolved fluorescence and the dimer:monomer absorbance ratio of acridine orange (AO) were used to determine dimer formation on polymer chains of polyelectrolytes. Acridine orange clearly formed dimers on polystyrensulfonate chains as well as on hyaluronan, but we show that the electrostatic interaction is much weaker in the case of hyaluronan. After the addition of surfactant, we observed an enhancement of fluorescence intensity, indicating the dissociation of AO dimers into monomers and the replacement of acridine orange on polymer chains by surfactant molecules. Importantly, we show that surfactant molecules bind to polymer chains before the critical micelle concentration is reached and form the so-called "bottle-brush" structure.

Universal binding behavior for ionic alkyl surfactants with oppositely charged polyelectrolytes

Oppositely charged polyelectrolyte-surfactant mixtures are ubiquitous in biology and the basis of numerous consumer healthcare products. Despite their broad use, however, a rational approach to their formulation remains challenging because of the complicated association mechanisms. Through compilation and analysis of literature reports and our own research, we have developed a semiempirical correlation of the binding strength of surfactants to polyelectrolytes in salt-free mixtures as a function of the polyion linear charge density and the surfactant hydrophobicity. We have found that the cooperative binding strength increases as the square of the polyelectrolyte's linear charge density and in proportion to the surfactant's hydrophobicity, such that a quantitative relationship holds across a broad range of polyelectrolytes. Deviations from the correlation reveal the role of system-specific interactions not considered in the analysis. This engineering relationship aids in the rational design of oppositely charged polyelectrolyte-surfactant formulations for consumer products and biomedicines by enabling the prediction of binding strengths in polyelectrolyte-surfactant mixtures based on mesoscale parameters determined from the chemical composition.

Exploring the desumoylation process of SENP1: a study combined MD simulations with QM/MM calculations on SENP1-SUMO1-RanGAP1

The small ubiquitin-related modifier (SUMO)-specific protease (SENP) processes SUMOs to mature forms and deconjugates them from various modified substrates. Loss of the equilibrium from desumoylation catalyzed by abnormal SENP1 is associated with cancers and transcription factor activity. In spite of the significant role of SENP1, the molecular basis of its desumoylation remains unclear. Here, MD simulations and QM/MM methods are combined to investigate the catalytic mechanism of desumoylation. The results showed that substrate SUMO1-RanGAP1 fitted into the catalytic pocket of SENP1 by the break of internal hydrophobic interactions and the isomerization of isopeptide from trans to cis. After that, the nucleophilic sulfur anion of Cys603 in SENP1 attacked the carbonyl carbon of Gly97 of SUMO1 to trigger the reaction, and then a tetrahedral intermediate and an acyl-enzyme intermediate were generated in turn, leading to the final release of enzyme SENP1 and two products, free SUMO1 and RanGAP1. In the process, nucleophilic attack was identified as the rate-determining step with a potential energy barrier of 20.2 kcal/mol. These results are in agreement with experimental data from mutagenesis and other experiments. Our findings elucidate the catalytic mechanism of SENP1 with its substrate and may provide a better understanding of SENP desumoylation. In particular, we have identified key residues in SENP1 needed for desumoylation that might be beneficial for the design of novel inhibitors of SENP1-related diseases.