Large Hydrogen-Bonded Pre-nucleation (HSO₄^–)(H₂SO₄)_<i>m</i>(H₂O)_<i>k</i> and (HSO₄^–)(NH₃)(H₂SO₄)_<i>m</i>(H₂O)_<i>k</i> Clusters in the Earth’s Atmosphere

The importance of pre-nucleation cluster stability as the key parameter controlling nucleation of atmospheric airborne ions is well-established. In this Article, large ternary ionic (HSO₄^–)­(H₂SO₄)_<i>m</i>(NH₃)­(H₂O)_<i>n</i> clusters have been studied using Density Functional Theory (DFT) and composite ab initio methods. Twenty classes of clusters have been investigated, and thermochemical properties of common atmospheric (HSO₄^–)­(H₂SO₄)_<i>m</i>(NH₃)₀(H₂O)_<i>k</i> and (HSO₄^–)­(H₂SO₄)_<i>m</i>(NH₃)₁(H₂O)_<i>n</i> clusters (with <i>m</i>, <i>k</i>, and <i>n</i> up to 3) have been obtained. A large amount of new themochemical and structural data ready-to-use for constraining kinetic nucleation models has been reported. We have performed a comprehensive thermochemical analysis of the obtained data and have investigated the impacts of ammonia and negatively charged bisulfate ion on stability of binary clusters in some detail. The comparison of theoretical predictions and experiments shows that the PW91PW91/6-311++G­(3df,3pd) results are in very good agreement with both experimental data and high level ab initio CCSD­(T)/CBS values and suggest that the PW91PW91/6-311++G­(3df,3pd) method is a viable alternative to higher level ab initio methods in studying large pre-nucleation clusters, for which the higher level computations are prohibitively expensive. The uncertainties in both theory and experiments have been investigated, and possible ways of their reduction have been proposed

Large Hydrogen-Bonded Pre-nucleation (HSO₄^–)(H₂SO₄)_<i>m</i>(H₂O)_<i>k</i> and (HSO₄^–)(NH₃)(H₂SO₄)_<i>m</i>(H₂O)_<i>k</i> Clusters in the Earth’s Atmosphere

The importance of pre-nucleation cluster stability as the key parameter controlling nucleation of atmospheric airborne ions is well-established. In this Article, large ternary ionic (HSO₄^–)­(H₂SO₄)_<i>m</i>(NH₃)­(H₂O)_<i>n</i> clusters have been studied using Density Functional Theory (DFT) and composite ab initio methods. Twenty classes of clusters have been investigated, and thermochemical properties of common atmospheric (HSO₄^–)­(H₂SO₄)_<i>m</i>(NH₃)₀(H₂O)_<i>k</i> and (HSO₄^–)­(H₂SO₄)_<i>m</i>(NH₃)₁(H₂O)_<i>n</i> clusters (with <i>m</i>, <i>k</i>, and <i>n</i> up to 3) have been obtained. A large amount of new themochemical and structural data ready-to-use for constraining kinetic nucleation models has been reported. We have performed a comprehensive thermochemical analysis of the obtained data and have investigated the impacts of ammonia and negatively charged bisulfate ion on stability of binary clusters in some detail. The comparison of theoretical predictions and experiments shows that the PW91PW91/6-311++G­(3df,3pd) results are in very good agreement with both experimental data and high level ab initio CCSD­(T)/CBS values and suggest that the PW91PW91/6-311++G­(3df,3pd) method is a viable alternative to higher level ab initio methods in studying large pre-nucleation clusters, for which the higher level computations are prohibitively expensive. The uncertainties in both theory and experiments have been investigated, and possible ways of their reduction have been proposed

Adsorption of Lysine on Na-Montmorillonite and Competition with Ca²⁺: A Combined XRD and ATR-FTIR Study

Lysine adsorption at clay/aqueous interfaces plays an important role in the mobility, bioavailability, and degradation of amino acids in the environment. Knowledge of these interfacial interactions facilitates our full understanding of the fate and transport of amino acids. Here, X-ray diffraction (XRD) and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) measurements were used to explore the dynamic process of lysine adsorption on montmorillonite and the competition with Ca²⁺ at the molecular level. Density functional theory (DFT) calculations were employed to determine the peak assignments of dissolved lysine in the solution phase. Three surface complexes, including dicationic, cationic, and zwitterionic structures, were observed to attach to the clay edge sites and penetrate the interlayer space. The increased surface coverage and Ca²⁺ competition did not affect the interfacial lysine structures at a certain pH, whereas an elevated lysine concentration contributed to zwitterionic-type coordination at pH 10. Moreover, clay dissolution at pH 4 could be inhibited at a higher surface coverage with 5 and 10 mM lysine, whereas the inhibition effect was inconspicuous or undetected at pH 7 and 10. The presence of Ca²⁺ not only could remove a part of the adsorbed lysine but also could facilitate the readsorption of dissolved Si⁴⁺ and Al³⁺ and surface protonation. Our results provide new insights into the process of lysine adsorption and its effects on montmorillonite surface sites

Ionic Strength-Responsive Binding between Nanoparticles and Proteins

Electrostatic interaction is a strong, dominant nonspecific interaction which was extensively studied in protein–nanoparticle (NP) interactions [Lounis, F. M.; J. Phys. Chem. B 2017, 121, 2684−2694; Tavares, G. M.; Langmuir 2015, 31, 12481–12488; Antonov, M.; Biomacromolecules 2010, 11, 51–59], whereas the role of hydrophobic interaction arising from the abundant hydrophobic residues of globule proteins upon protein–NP binding between the proteins and charged nanoparticles has rarely been studied. In this work, a series of positively charged magnetic nanoparticles (MNPs) were prepared via atom transfer radical polymerization and surface hydrophobicity differentiation was achieved through postpolymerization quaternization by different halohydrocarbons. The ionic strength- and hydrophobicity-responsive binding of these MNPs toward β-lactoglobulin (BLG) was studied by both qualitative and quantitative methods including turbidimetric titration, dynamic light scattering, and isothermal titration calorimetry. Judged from the critical binding pH and binding constant for MNP–BLG complexation, the dependence of binding affinity on surface hydrophobicity exhibited an interesting shift with increasing ionic strength, which means that the MNPs with higher surface hydrophobicity exhibits weaker binding affinity at lower ionic strength but stronger affinity at higher ionic strength. This interesting observation could be attributed to the difference in ionic strength responsiveness for hydrophobic and electrostatic interactions. In this way, the well-tuned binding pattern could be achieved with optimized binding affinity by controlling the surface hydrophobicity of MNPs and ionic strength, thus endowing this system with great potential to fabricate separation and delivery system with high selectivity and efficiency

Visible-Light-Induced Catalytic Transfer Hydrogenation of Aromatic Aldehydes by Palladium Immobilized on Amine-Functionalized Iron-Based Metal–Organic Frameworks

Visible-light-induced selective transfer hydrogenation of aromatic aldehyde to the corresponding alcohol was achieved by using Pd nanocatalyst supported on amine-functionalized iron-based metal–organic frameworks [Pd/MIL-101­(Fe)-NH₂] with triethylamine (TEA) as an electron donor and HCOOH as a proton source. The Pd/MIL-101­(Fe)-NH₂, obtained by an in situ photodeposition method, showed homogeneously and highly dispersed Pd nanoparticles (NPs) with a uniform size throughout the MIL-101­(Fe)-NH₂ support due to an effective stabilization role of amine groups on the backbone linkage of MIL-101­(Fe)-NH₂. The resulting Pd/MIL-101­(Fe)-NH₂ exhibited excellent catalytic performance toward a visible-light-induced transfer hydrogenation of benzaldehyde by producing a benzyl alcohol yield of 77% with a full benzaldehyde conversion in the presence of TEA-HCOOH. In addition to benzaldehyde, biomass-based renewable platform molecules such as furfural and 5-hydroxymethylfurfural (HMF) were successively converted into the corresponding alcohols with the yields of 29% for furfuryl alcohol and 27% for 2,5-dihydroxymethylfuran (DHMF), respectively, which are the highest yields reported so far by visible-light-induced transfer hydrogenation method. Our experimental investigation reveals that a preliminary photoirradiation promotes in situ photodeposition of Pd salt [MIL-101­(Fe)-NH₃]⁺·1/2­[PdCl₄]^2– to form Pd catalyst Pd/MIL-101­(Fe)-NH₂ in the presence of TEA-HCOOH, and a further photoirradiation successively triggers Pd/MIL-101­(Fe)-NH₂-promoted transfer hydrogenation of aldehyde, again, with the help of TEA-HCOOH. Our theoretical research based on density functional theory (DFT) further confirms a dual function of amine group in the Pd/MIL-101­(Fe)-NH₂ for Pd NPs stabilization as well as for enhancement of the electron density of the Pd center upon light adsorption. The photocatalytic system of Pd nanocatalyst and TEA-HCOOH thus demonstrates an environmentally friendly and efficient strategy for aldehyde hydrogenation by using renewable solar energy as a driving force

Multimerization and Aggregation of Native-State Insulin: Effect of Zinc

The aggregation of insulin is complicated by the coexistence of various multimers, especially in the presence of Zn²⁺. Most investigations of insulin multimerization tend to overlook aggregation kinetics, while studies of insulin aggregation generally pay little attention to multimerization. A clear understanding of the starting multimer state of insulin is necessary for the elucidation of its aggregation mechanism. In this work, the native-state aggregation of insulin as either the Zn–insulin hexamer or the Zn-free dimer was studied by turbidimetry and dynamic light scattering, at low ionic strength and pH near pI. The two states were achieved by varying the Zn²⁺ content of insulin at low concentrations, in accordance with size-exclusion chromatography results and literature findings (Tantipolphan, R.; Romeijn, S.; Engelsman, J. d.; Torosantucci, R.; Rasmussen, T.; Jiskoot, W. J. Pharm. Biomed. 2010, 52, 195). The much greater aggregation rate and limiting turbidity (τ_∞) for the Zn–insulin hexamer relative to the Zn-free dimer was explained by their different aggregation mechanisms. Sequential first-order kinetic regimes and the concentration dependence of τ_∞ for the Zn–insulin hexamer indicate a nucleation and growth mechanism, as proposed by Wang and Kurganov (Wang, K.; Kurganov, B. I. Biophys. Chem. 2003, 106, 97). The pure second-order process for the Zn-free dimer suggests isodesmic aggregation, consistent with the literature. The aggregation behavior at an intermediate Zn²⁺ concentration appears to be the sum of the two processes

Aquivion–Carbon Composites with Tunable Amphiphilicity for Pickering Interfacial Catalysis

A key demand in biomass conversion is how to achieve high reactivity with immiscible reagents with the use of neither cosolvents nor additives. Pickering interfacial catalysis encompassing the design of amphiphilic catalysts behaving concomitantly as emulsifiers offers an elegant solution. In this study, we prepared a systematic series of amphiphilic Aquivion–carbon composites by the hydrothermal carbonization of guar gum with Aquivion perfluorosulfonic superacid. By tuning the Aquivion–carbon composition, materials with tunable hydrophilic-lipophilic properties could be prepared, showing high versatility for conducting biphasic reactions without stirring. In particular, an optimal formulation based on 5:1 Aquivion–carbon could be developed, showing high activity in the transesterification reaction of glyceryl trioleate with methanol at 100 °C with good reusability due to the genesis of stable Pickering emulsions

pH-Dependent Aggregation and Disaggregation of Native β‑Lactoglobulin in Low Salt

The aggregation of β-lactoglobulin (BLG) near its isoelectric point was studied as a function of ionic strength and pH. We compared the behavior of native BLG with those of its two isoforms, BLG-A and BLG-B, and with that of a protein with a very similar pI, bovine serum albumin (BSA). Rates of aggregation were obtained through a highly precise and convenient pH/turbidimetric titration that measures transmittance to ±0.05 %T. A comparison of BLG and BSA suggests that the difference between pH_max (the pH of the maximum aggregation rate) and pI is systematically related to the nature of protein charge asymmetry, as further supported by the effect of localized charge density on the dramatically different aggregation rates of the two BLG isoforms. Kinetic measurements including very short time periods show well-differentiated first and second steps. BLG was analyzed by light scattering under conditions corresponding to maxima in the first and second steps. Dynamic light scattering (DLS) was used to monitor the kinetics, and static light scattering (SLS) was used to evaluate the aggregate structure fractal dimensions at different quench points. The rate of the first step is relatively symmetrical around pH_max and is attributed to the local charges within the negative domain of the free protein. In contrast, the remarkably linear pH dependence of the second step is related to the uniform reduction in global protein charge with increasing pH below pI, accompanied by an attractive force due to surface charge fluctuations

Effect of Heparin on Protein Aggregation: Inhibition versus Promotion

The effect of heparin on both native and denatured protein aggregation was investigated by turbidimetry and dynamic light scattering (DLS). Turbidimetric data show that heparin is capable of inhibiting and reversing the native aggregation of bovine serum albumin (BSA), β-lactoglobulin (BLG), and Zn–insulin at a pH near pI and at low ionic strength <i>I</i>; however, the results vary with regard to the range of pH, <i>I</i>, and protein–heparin stoichiometry required to achieve these effects. The kinetics of this process were studied to determine the mechanism by which interaction with heparin could result in inhibition or reversal of native protein aggregates. For each protein, the binding of heparin to distinctive intermediate aggregates formed at different times in the aggregation process dictates the outcome of complexation. This differential binding was explained by changes in the affinity of a given protein for heparin, partly due to the effects of protein charge anisotropy as visualized by electrostatic modeling. The heparin effect can be further extended to include inhibition of denaturing protein aggregation, as seen from the kinetics of BLG aggregation under conditions of thermally induced unfolding with and without heparin

Multi-Stimuli-Responsive Amphiphilic Assemblies through Simple Postpolymerization Modifications

A strategy to construct different stimuli-responsive polymers from postpolymerization modifications of a single polymer scaffold via thiol–disulfide exchange has been developed. Here, we report on a random copolymer that enables the design and syntheses of a series of dual or multi-stimuli-responsive nanoassemblies using a simple postpolymerization modification step. The reactive functional group involves a side chain monopyridyl disulfide unit, which rapidly and quantitatively reacts with various thiols under mild conditions. Independent and concurrent incorporation of physical, chemical, or biologically responsive properties have been demonstrated. We envision that this strategy may open up opportunities to simplify the synthesis of multifunctional polymers with broad implications in a variety of biological applications