Selective Na+/K+ Effects on the Formation of α-Cyclodextrin Complexes with Aromatic Carboxylic Acids: Competition for the Guest

The Effect of Alkali Iodide Salts in the Inclusion Process of Phenolphthalein in β-Cyclodextrin: A Spectroscopic and Theoretical Study

The formation of the inclusion complex between β-cyclodextrin (CD) and phenolphthalein (PP) was investigated by means of UV-Vis and FT-IR spectroscopies. The thermodynamic parameters were calculated in the absence and presence of LiI, KI, NaI and CsI iodide salts. The enthalpy change during the formation was found to be negative for all solutions with iodide salts. The enthalpy change was found to decrease in the sequence no salt > NaI > KI> CsI > LiI. Moreover, it was observed that with increasing salt concentration enthalpy decreases monotonically. The interaction between the two molecules was mostly attributed to hydrogen bonding and Van der Waals interactions. Thermodynamic properties revealed that electrostatic forces also contribute when LiI is present in solutions. A molecular docking study was performed to elucidate the docking between phenolphthalein and cyclodextrin. The FT-IR spectra of CD, PP and the CD-PP complex were recorded to establish the formation of the inclusion complex. Semi-empirical and DFT methods were utilized to study theoretically the complexation process and calculate the IR vibrational spectra. The adequate agreement between theoretical and experimental results supports the proposed structural model for the CD-PP complexation.

Thermal Properties of Multilayer Nanocomposites Based on Halloysite Nanotubes and Biopolymers

This paper reports a novel procedure to fabricate multilayer composite biofilms based on halloysite nanotubes (HNTs) and sustainable polymers. Among the biopolymers, the non-ionic (hydroxypropyl cellulose) and cationic (chitosan) molecules were selected. The nanocomposites were prepared by the sequential casting of ethanol solutions of hydroxypropyl cellulose and aqueous dispersions of chitosan/HNTs. The composition of the bio-nanocomposites was systematically changed in order to investigate the effect of the hydroxypropyl cellulose/HNTs ratio on the thermal properties of the films, which were investigated by differential scanning calorimetry (DSC) and thermogravimetry (TG). DSC studies were conducted in the static air (oxidative atmosphere), while TG measurements were carried out under nitrogen flow (inert atmosphere). The analysis of DSC data provided the enthalpy and the temperature for the oxidative degradation of the bio-nanocomposites. These results were helpful to estimate the efficacy of the well-compacted middle layer of HNTs as a flame retardant. TG experiments were performed at a variable heating rate and the collected data were analyzed by the Friedman’s method (non-isothermal thermogravimetric approach) with the aim of studying the kinetics of the hydroxypropyl cellulose degradation in the multilayer nanocomposites. This work represents an advanced contribution for designing novel sustainable nanocomposites with excellent thermal behavior as a consequence of their peculiar multilayer structure.

Determinants of the host-guest interactions between α-, β- and γ-cyclodextrins and group IA, IIA and IIIA metal cations: a DFT/PCM study

The most widely used native cyclodextrins are α-, β- and γ-cyclodextrins containing six, seven or eight α-d-glucopyranoside units in the ring, respectively. Although the ligation properties of these host molecules have been extensively studied, a number of questions regarding their metal binding and selectivity remain unaddressed: to what extent do the size and flexibility of the host α-, β- and γ-cyclodextrins influence their metal affinity/selectivity? Which metal is the most preferred binding partner of α-, β- and γ-cyclodextrins? How do the charge, size and preferred coordination number of the metal cation shape its interactions with the host cyclodextrin? Can the guest metal cation inflict structural alterations in the host molecule and, if so, how do these changes correlate with the metal's properties? In the present study, by employing density functional theory (DFT) calculations combined with polarizable continuum model (PCM) computations, we try to answer these questions by evaluating the thermodynamic parameters of the IA, IIA and IIIA group metal binding to α, β- and γ-cyclodextrins. We assess how the interaction between the two binding partners depends on (1) the size, valence state and preferred coordination number of the guest metal cations, (2) the size and flexibility of the host molecule, and (3) the dielectric properties of the environment. The series of group IA (Na⁺ and Rb⁺), IIA (Mg²⁺ and Sr²⁺) and IIIA (Al³+ and In³⁺) metal cations have been chosen for the task as they allow us to study the effect of various metal parameters (variable charge, ionic radius and coordination number) on the strength and form of the interactions with the host cyclodextrins.

Biopolymer-Targeted Adsorption onto Halloysite Nanotubes in Aqueous Media

Studies on the adsorption of biopolymers onto halloysite nanotubes (HNTs) in water were conducted. Three polymers with different charges-anionic (pectin), neutral (hydroxypropyl cellulose), and cationic (chitosan)-were chosen. The thermodynamic parameters for the adsorption of polymers onto the HNT surface were determined by isothermal titration calorimetry (ITC). The experimental data were interpreted based on a Langmuir adsorption model. The standard variations in free energy, enthalpy, and entropy of the process were obtained and discussed. Turbidimetry was used to evaluate the stability of functionalized nanoparticles in water. The ζ-potential clarified the surface charge properties of functionalized nanotubes upon polymer adsorption. The interaction of modified nanotubes with polymers led to the formation of a colloidal system with tunable stability and surface properties, which offers different perspectives on new applications of these dispersions, such as carriers for substances to be released in response to external stimuli.

Quantifying non-covalent binding affinity using mass spectrometry: a systematic study on complexes of cyclodextrins with alkali metal cations

RATIONALE

To date, the quantification of binding affinities for non-covalent complexes between cyclodextrin (CD) and alkali cations including Li(+) , Na(+) , K(+) , Rb(+) , and Cs(+) has not been investigated in detail by electrospray ionization mass spectrometry (ESI-MS) due to the unknown ionization efficiencies of the different species. In this study, the binding constants of CD-Cs(+) complexes were determined by an improved mass spectrometric titration methodology, which was based only on the peak intensities of equilibrium CD. Hence, the discrepancy of ionization efficiencies of CD, alkaline cation and their complex would not affect the measurement. Then the obtained lgKa values were provided as references for competitive ESI-MS. The binding constants for complexes of α-, β- or γ-CD with Li(+) , Na(+) , K(+) and Rb(+) could be derived directly and quickly.

METHODS

The lgKa values between α-, β- or γ-CD and Cs(+) data were processed by curve fitting. These lgKa values were provided as references for competitive ESI-MS. In addition, linear fit equations for complexes of α-, β- or γ-CD with Cs(+) were derived. Through the linear fit equations of competitive ESI-MS, the binding constants for complexes of Li(+) , Na(+) , K(+) and Rb(+) with α-, β- or γ-CD were acquired.

RESULTS

Results showed that the binding constant (lgKa ) values for the complexes of Cs(+) with α-, β- and γ-cyclodextrins were 3.94, 3.88 and 3.80, respectively, revealing that the binding strength decreased with the increase in diameter of cyclodextrins. The competitive ESI-MS results showed a clear trend of decreasing affinity for complexes of cyclodextrins in the order of Li(+) , Na(+) , K(+) , Rb(+) .

CONCLUSIONS

The binding constants of non-covalent cyclodextrin-alkali cation complexes have been systematically studied by an improved mass spectrometric titration and competitive ESI-MS. Also, the structural features of the complexes were discussed. Our results are valuable for better understanding of mechanisms driving inclusion chemistry under well-defined conditions.

Bioinspired graphene nanopores with voltage-tunable ion selectivity for Na(+) and K(+)

Biological protein channels have many remarkable properties such as gating, high permeability, and selectivity, which have motivated researchers to mimic their functions for practical applications. Herein, using molecular dynamics simulations, we design bioinspired nanopores in graphene sheets that can discriminate between Na(+) and K(+), two ions with very similar properties. The simulation results show that, under transmembrane voltage bias, a nanopore containing four carbonyl groups to mimic the selectivity filter of the KcsA K(+) channel preferentially conducts K(+) over Na(+). A nanopore functionalized by four negatively charged carboxylate groups to mimic the selectivity filter of the NavAb Na(+) channel selectively binds Na(+) but transports K(+) over Na(+). Surprisingly, the ion selectivity of the smaller diameter pore containing three carboxylate groups can be tuned by changing the magnitude of the applied voltage bias. Under lower voltage bias, it transports ions in a single-file manner and exhibits Na(+) selectivity, dictated by the knock-on ion conduction and selective blockage by Na(+). Under higher voltage bias, the nanopore is K(+)-selective, as the blockage by Na(+) is destabilized and the stronger affinity for carboxylate groups slows the passage of Na(+) compared with K(+). The computational design of biomimetic ion-selective nanopores helps to understand the mechanisms of selectivity in biological ion channels and may also lead to a wide range of potential applications such as sensitive ion sensors, nanofiltration membranes for Na(+)/K(+) separation, and voltage-tunable nanofluidic devices.

Differential geometry based solvation model II: Lagrangian formulation

Solvation is an elementary process in nature and is of paramount importance to more sophisticated chemical, biological and biomolecular processes. The understanding of solvation is an essential prerequisite for the quantitative description and analysis of biomolecular systems. This work presents a Lagrangian formulation of our differential geometry based solvation models. The Lagrangian representation of biomolecular surfaces has a few utilities/advantages. First, it provides an essential basis for biomolecular visualization, surface electrostatic potential map and visual perception of biomolecules. Additionally, it is consistent with the conventional setting of implicit solvent theories and thus, many existing theoretical algorithms and computational software packages can be directly employed. Finally, the Lagrangian representation does not need to resort to artificially enlarged van der Waals radii as often required by the Eulerian representation in solvation analysis. The main goal of the present work is to analyze the connection, similarity and difference between the Eulerian and Lagrangian formalisms of the solvation model. Such analysis is important to the understanding of the differential geometry based solvation model. The present model extends the scaled particle theory of nonpolar solvation model with a solvent-solute interaction potential. The nonpolar solvation model is completed with a Poisson-Boltzmann (PB) theory based polar solvation model. The differential geometry theory of surfaces is employed to provide a natural description of solvent-solute interfaces. The optimization of the total free energy functional, which encompasses the polar and nonpolar contributions, leads to coupled potential driven geometric flow and PB equations. Due to the development of singularities and nonsmooth manifolds in the Lagrangian representation, the resulting potential-driven geometric flow equation is embedded into the Eulerian representation for the purpose of computation, thanks to the equivalence of the Laplace-Beltrami operator in the two representations. The coupled partial differential equations (PDEs) are solved with an iterative procedure to reach a steady state, which delivers desired solvent-solute interface and electrostatic potential for problems of interest. These quantities are utilized to evaluate the solvation free energies and protein-protein binding affinities. A number of computational methods and algorithms are described for the interconversion of Lagrangian and Eulerian representations, and for the solution of the coupled PDE system. The proposed approaches have been extensively validated. We also verify that the mean curvature flow indeed gives rise to the minimal molecular surface and the proposed variational procedure indeed offers minimal total free energy. Solvation analysis and applications are considered for a set of 17 small compounds and a set of 23 proteins. The salt effect on protein-protein binding affinity is investigated with two protein complexes by using the present model. Numerical results are compared to the experimental measurements and to those obtained by using other theoretical methods in the literature.

Adsorption of fatty acids on iron (hydr)oxides from aqueous solutions

The interaction of iron (hydr)oxides with fatty acids is related to many industrial and natural processes. To resolve current controversies about the adsorption configurations of fatty acids and the conditions of the maximum hydrophobicity of the minerals, we perform a detailed study of the adsorption of sodium laurate (dodecanoate) on 150 nm hematite (α-Fe(2)O(3)) particles as a model system. The methods used include in situ FTIR spectroscopy, ex situ X-ray photoelectron spectroscopy (XPS), measurements of the adsorption isotherm and contact angle, as well as the density functional theory (DFT) calculations. We found that the laurate adlayer is present as a mixture of inner-sphere monodentate mononuclear (ISMM) and outer-sphere (OS) hydration shared complexes independent of the solution pH. Protonation of the OS complexes does not influence the conformational order of the surfactant tails. One monolayer, which is filled through the growth of domains and is reached at the micellization/precipitation edge of laurate, makes the particles superhydrophobic. These results contradict previous models of the fatty acid adsorption and suggest new interpretation of literature data. Finally, we discovered that the fractions of both the OS laurate and its molecular form increase in D(2)O, which can be used for interpreting complex spectra. We discuss shortcomings of vibrational spectroscopy in determining the interfacial coordination of carboxylate groups. This work advances the current understanding of the oxide-carboxylate interactions and the research toward improving performance of fatty acids as surfactants, dispersants, lubricants, and anticorrosion reagents.