Sorption of poly(hexamethylenebiguanide) on cellulose: mechanism of binding and molecular recognition

Antimicrobial agents such as poly(hexamethylene biguanide) (PHMB) find application in medical, apparel, and household textile sectors; although it is understood that certain concentrations need to be applied to achieve suitable performance, there has been very little work published concerning the interactions of the polymer and its adsorption mechanism on cellulose. In this paper, such physical chemistry parameters are examined and related to computational chemistry studies. Adsorption isotherms were constructed: at low concentrations, these were typical Langmuir isotherms; at higher concentrations, they were more indicative of Freundlich isotherms, attributed to a combination of electrostatic and hydrogen-bonding forces, which endorsed computational chemistry proposals. At lower concentrations, electrostatic interactions between PHMB and carboxylic acid groups in the cellulose dominate with a contribution to binding through hydrogen bonding; as the concentration of PHMB increases, hydrogen bonding with cellulose becomes increasingly dominant. At high PHMB concentrations, observations of increasing PHMB adsorption are attributed to monolayer aggregation and multilayer stacking of PHMB through electrostatic interactions with counterions and hydrogen bonding of biguanide groups.     

Sorption of ammonia and nitrogen on cellulose acetate

Adsorption isotherms of ammonia and nitrogen on cellulose acetate in the temperature range 303–333 K are measured by reversed-phase chromatography. It is established that ammonia specifically interacts with the polymer matrix of the sorbent. The experimental data are described in terms of the Laatikainen-Lindstrom quasi-chemical model of vapor sorption. The enthalpies of sorption are determined, and their dependences on the ammonia and nitrogen concentrations in the sorbent are established.     

Sorption of gold, silver and mercury on cellulose

Summary Distribution coefficients of Au³⁺ on cellulose in 0.1 M NaNO₃ are rather high in the pH range 2 to 8 (D 5·10³ ml/g). They are appreciably lower in presence of NaCl or NaCN. Distribution coefficients of Ag⁺ are relatively low in 0.1 M NaNO₃, appreciably higher in presence of NH₄NO₃ (D 10³ ml/g in weakly acid media) and low in presence of NaCl, Na₂S₂O₃ or NaCN. Distrubution coefficients of Hg²⁺ are low in 0.1 M NaNO₃, and in presence of NaCN, but higher in 0.5 M NaCl (D 2·10² ml/ g). Responsible for the equilibria are mainly the special interactions between the predominant species with the sorbent under the given conditions, and less the complex equilibria in solution.

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Sorption von Gold, Silber und Quecksilber an Cellulose

Zusammenfassung Die Verteilungskoeffizienten von Au³⁺ an Cellulose in 0,1 M NaNO₃ sind ziemlich hoch im pH-Bereich 2 bis 8 (D 5·10³ ml/g). Sie sind erheblich niedriger in Gegenwart von NaCl oder NaCN. Die Verteilungskoeffizienten von Ag⁺ sind verhältnismäßig niedrig in 0,1 M NaNO₃, erheblich höher in Gegenwart von NH₄NO₃ (D 10³ mol/g in schwach saurem Medium) und niedrig in Gegenwart von NaCl, Na₂S₂O₃ oder NaCN. Die Verteilungskoeffizienten von Hg²⁺ sind niedrig in 0,1 M NaNO₃ sowie in Gegenwart von NaCN, aber höher in 0,5 NaCl (D 2·10² ml/g). Verantwortlich für die Sorption sind in erster Linie die speziellen Wechselwirkungen der vorherrschenden Spezies mit dem Sorbens unter den gegebenen Bedingungen, weniger die Komplexbildungsgleichgewichte in Lösung.

     

Organic anion recognition of naphthalenesulfonates by steroid-modified beta-cyclodextrins: enhanced molecular binding ability and molecular selectivity

Two beta-cyclodextrin (beta-CD) derivatives bearing steroid groups (1 and 2) were synthesized by the condensation of mono(6-aminoethylamino-6-deoxy)-beta-CD with cholic acid and deoxycholic acid, respectively, and their original conformations and binding behavior to the organic anion of naphthalenesulfonate derivatives were investigated by using 1H NMR spectroscopy and spectrofluorometric titration in combination with computational methods. The 2D NMR experiments reveal that the steroid groups attached to the beta-CD rim could be deeply embedded in the beta-CD cavity to form the intramolecular (for 1) or intermolecular (for 2) inclusion complexes in aqueous solution. Upon complexation with naphthalenesulfonate derivatives, modified beta-CDs display two obviously different binding modes, that is, the competitive inclusion mode and the induced-fit inclusion mode, which is consistent with the results of molecular modeling study. The two modes and the strict size/shape fitting relationship between the hosts and guests reasonably explain the different binding behaviors and molecular selectivity of host beta-CDs 1 and 2 toward the naphthalenesulfonate guests. Therefore, the cholic acid- or deoxycholic acid-modified beta-CDs could effectively recognize the size/shape of guest molecules as compared with the parent beta-CD, giving good molecular selectivity up to 24.9 for the disodium 2,6-naphthalenedisulfonate/disodium 1,5-naphthalenedisulfonate pair by the host 1.     

Sorption of phenol on cellulose from binary aqueous-organic mixtures

Sorption of phenol on cellulose from water-dimethyl sulfoxide and water-acetonitrile mixtures was studied. Different shape of the isotherms of phenol sorption form these mixtures is due to different character of phenol solvation in different concentration regions of water-dimethyl sulfoxide and water-acetonitrile mixtures.     

Sorption and permeation behaviour of metal thiocyanate complexes on cellulose acetate polymers

Various metal thiocyanate complexes in aqueous solution were sorbed on solid cellulose acetate polymers. The sorption selectivity increased in the order Zn(2+) > Fe(3+) > Cu(2+) > Co(2+) > Ni(2+). The sorption behaviour followed a Langmuir-type adsorption isotherm, and the maximum adsorption capacity was 6.1 x 10(-5) mole of complex per g of polymer under optimum conditions. The zinc species sorbed appear to be NH(4)Zn(H(2)O)(SCN)(3) or (NH(4))(2)Zn(SCN)(4) according to analysis of the sorption equilibrium. The ion-association species formed by the complex zinc anion and the ammonium ion was supposed to be sorbed (or "extracted") onto the polymer matrix. As an application of sorption of metal complexes, a new hyperfiltration process was proposed for selective separation of metal ions. Thus, a mixture of metal thiocyanate complexes was hyperfiltered through cellulose acetate membranes. Permeation of certain metal complexes was preferred, and the selectivity was found to be similar to the sorption selectivity. These findings lead to a generalized idea that hyperfiltration separation of ionic species, particularly anionic metal complexes, can be attained by using polymer membranes which selectively adsorb or extract such ionic species as ion-association complexes onto the polymer matrix.     

Peptidolipid as binding site of acetylcholinesterase: molecular recognition of paraoxon in Langmuir films

Peptidolipid C18H35O (stearoyl)-Phe-Trp-Ser-His-Glu (peptidolipid A) was synthesized and spread at the air-water interface to study the interaction with an organophosphorus compound. Paraoxon, sodium dihydrogen phosphate, or 4-nitrophenyl phosphate disodium was added to the subphase, but only paraoxon changed the surface pressure-area (pi-A) isotherm of peptidolipid A. This indicated a specific interaction between paraoxon and peptidolipid A. To clarify which amino acid residue of peptidolipid A was responsible for the interaction, peptidolipid B, namely, C18H35O-Gly-His-Ser-Glu-Glu, was synthesized and studied as a Langmuir film. The difference between the pi-A isotherms of peptidolipid B in the absence and presence of paraoxon in the subphase was minimal; consequently, the presence of amino acids phenylalanine (Phe) and tryptophan (Trp) in peptidolipid A may explain the interaction between peptidolipid A and paraoxon. The compression-decompression cycles and kinetic studies of peptidolipid A showed that the Langmuir film was stable. The in situ optical properties of the peptidolipid A Langmuir film such as UV-vis and fluorescence spectroscopies were examined to elucidate the interaction between peptidolipid A and paraoxon. UV-vis absorption of peptidolipid A was investigated in the presence and absence of paraoxon in the subphase. The emission maximum of fluorescence of Trp in peptidolipid A was observed at 351 nm on pure water, and the band intensity decreased when the concentration of paraoxon increased in the subphase. This suggested that the Trp was involved in the molecular recognition process. Epifluorescence micrographs showed domains of peptidolipid A on the pure water subphase. In the presence of paraoxon in the subphase, the Langmuir film of peptidolipid A showed a homogeneity, which was another indication of the recognition between paraoxon and peptidolipid A.     

Sorption of dyes on cellulose II: effect of alkali treatment of fibre and dye structure

Cellulose is a linear 1,4-β-glucan polymer where the units are able to form highly ordered structures, as a result of extensive interaction through intra- and intermolecular hydrogen bonding of the three hydroxyl groups in each cellulose unit. Alkali has a substantial influence on morphological, molecular and supramolecular properties of cellulose II polymer fibres causing changes in crystallinity. Lyocell fibres pre-treated with 0.0, 2.0, and 4.0 mol dm⁻³ aqueous NaOH solution were dyed with hydrolyzed reactive dyes that had different molecular shapes and sizes. Overall exhaustion (q _e), value of K, and −ΔG increased for lyocell samples pre-treated with aqueous NaOH solution in the following order: 2.0 > 4.0 > 0.0 mol dm⁻³ NaOH. The same trends were observed for colour strength (K/S) values of the dyeings. Pre-treatment of lyocell with 2.0 mol dm⁻³ NaOH creates the substrate that achieves the most thermodynamically favourable system for sorption of hydrolyzed reactive dyes, as at this concentration crystallinity decreases (with respect to 0.0 mol dm⁻³ NaOH treated lyocell) to afford higher sorption; however, at higher alkali concentrations the macro-sorbent forms a compacted unit that limits diffusion within the sorbent interior. Molecular size of the sorbate dye has a significant effect on the sorption process: for the largest dye structure the sorption isotherm is most closely correlated to a Langmuir isotherm; as the size of the dye decreases correlation to a Langmuir isotherm is observed, but with good correlation to the Freundlich isotherm; as the size of the dye is decreased further sorption is more typical of a Freundlich isotherm.     

Photochemically amplified detection of molecular recognition events: an ultra-sensitive fluorescence turn-off binding assay

Amplified fluorescence quenching methodology based on massive autocatalytic photo-unmasking of a dual function sensitizer-quencher is developed and adopted for photoassisted ultra-sensitive detection of molecular recognition events. The resulting binding assay, based on a molecular recognition-triggered photo-amplified cascade with concomitant decrease of fluorescence is validated with the biotin-avidin pair, achieving attomolar detection.     

Single crystals of cellulose IVII: Influence of the cellulose molecular weight

The influence of the degree of polymerization (DP) of cellulose was evaluated in the preparation of micron-sized cellulose IV_II lamellar crystals in order to ascertain whether a regular chain-folded morphology could develop during their growth. For this purpose, sharp fractions of cellulose acetate were collected by preparative gel permeation chromatography. Aliquots of these fractions were deacetylated and crystallized in dilute solutions containing water, methylamine, and DMSO, and held at 150°C under pressure. Well-developed cellulose IV_II lamellar crystals were obtained with fractions of DP 22–24 whereas higher-DP material gave polycrystalline aggregates. This behavior indicates that large lamellar crystals of cellulose IV_II can be obtained only with unfolded short cellulose chains. The occurrence of chain-folded crystals with high–DP cellulose samples cannot be demonstrated.     

Sorption of water by cellulose from binary water-organic solutions

Composition change isotherms are obtained experimentally for cellulose-water-organic solution systems, and individual isotherms of water sorption on cellulose are calculated. We measure the thermal effects of cellulose wetting with water-organic mixtures. It is shown that the features of water sorption on cellulose in the presence of organic component are defined by changes in the nature of clustering in the investigated binary solutions.     

光谱-分子模拟法分析杨梅素与人血清白蛋白的分子作用机制

在模拟生理条件下,用多种光谱法结合分子对接法测定了杨梅素(MY)与人血清白蛋白(HSA)的相互作用.研究结果表明,MY能够明显猝灭HSA的荧光,MY与HSA的相互作用为复合式静态结合过程,结合强度较强.热力学和分子对接结果表明,MY与HSA是自发结合的,维持MY与HSA的相互作用力主要是氢键和范德华力.能量转移结果表明...     

Sorption properties of cellulose filters with covalently immobilized thiosemicarbazide

A novel thin-layer cellulose adsorbent bearing chemically immobilized thiosemicarbaside is synthesized. The adsorbent is used for the preconcentration of cobalt(II), mercury(II), and cadmium(II) followed by their X-ray fluorescence quantification in the concentrate. The physicochemical parameters of metal sorption are determined.     

Sorption of vapors of organic solvents with cyanoethyl hydroxyethyl cellulose

The interaction of cyanoethyl hydroxyethyl cellulose with acetone, methylene chloride, and trifluoroacetic acid in the temperature range 10–50°C was studied by the solvent vapor sorption technique. The temperature and concentration dependences of the Flory-Huggins parameter for polymer-solvent interaction Z₁ and the mechanism of the structural rearrangement in the systems were determined.     

Diastereomeric molecular recognition and binding behavior of bile acids by L/D-tryptophan-modified beta-cyclodextrins

[reaction: see text] Binding behavior of L- and D-tryptophan-modified beta-cyclodextrins (L/D-Trp-beta-CD) (1 and 2) with four bile acids, i.e., cholate (CA), deoxycholate (DCA), glycocholate (GCA), and taurocholate (TCA), has been investigated by fluorescence, circular dichroism, and 2D-NMR spectroscopies and fluorescence lifetime measurement, as well as isothermal titration microcalorimetry. From the induced circular dichroism (ICD) and 2D NMR spectra, it is deduced that the D-Trp moiety of 2 attached to beta-CD is more deeply self-included in the cavity than that of the antipodal L-Trp moiety of 1, indicating appreciably enantioselective binding of the chiral sidearm by beta-CD. Interestingly, the original difference in conformation between 1 and 2 led to quite a large difference in affinity toward DCA, giving 3.3 times higher binding ability for 2 than for 1. Thermodynamically, the inclusion complexation of 1 and 2 with bile acids was entirely driven by favorable enthalpy change (DeltaH degrees) with accompanying negative entropy change (DeltaS degrees). The stronger binding of bile acids by L/D-Trp-beta-CD is attributable to higher enthalpic gains. The combined use of the calorimetric and NMR ROESY spectral examinations revealed the correlation between the thermodynamic parameters and the role of sidearm conformation in modified beta-cyclodextrins.     

Dynamic mechanism of E2020 binding to acetylcholinesterase: a steered molecular dynamics simulation

The unbinding process of E2020 ((R,S)-1-benzyl-4-[(5,6-dimethoxy-1-indanon)-2-yl]-methylpiperidine) leaving from the long active site gorge of Torpedo californica acetylcholinesterase (TcAChE) was studied by using steered molecular dynamics (SMD) simulations on a nanosecond scale with different velocities, and unbinding force profiles were obtained. Different from the unbinding of other AChE inhibitors, such as Huperzine A that undergoes the greatest barrier located at the bottleneck of the gorge, the major resistance preventing E2020 from leaving the gorge is from the peripheral anionic site where E2020 interacts intensively with several aromatic residues (e.g., Tyr70, Tyr121, and Trp279) through its benzene ring and forms a strong direct hydrogen bond and a water bridge with Ser286 via its O24. These interactions cause the largest rupture force, approximately 550 pN. It was found that the rotatable bonds of the piperidine ring to the benzene ring and dimethoxyindanone facilitate E2020 to pass the bottleneck through continuous conformation change by rotating those bonds to avoid serious conflict with Tyr121 and Phe330. The aromatic residues lining the gorge wall are the major components contributing to hydrophobic interactions between E2020 and TcAChE. Remarkably, these aromatic residues, acting in three groups as "sender" and "receiver", compose a "conveyer belt" for E2020 entering and leaving the TcAChE gorge.     

Exploring the binding mechanism of dihydropyrimidinones to human serum albumin: spectroscopic and molecular modeling techniques

The binding mechanism of molecular interaction between 5-(ethoxycarbonyl)-6-methyl-4-phenyl-3,4-dihydropyrimidin-2(1H)-one (a dihydropyrimidinones derivative, EMPD) and human serum albumin (HSA) was studied using spectroscopic methods and modeling technique. The quenching mechanism was investigated in terms of the binding constants and the basic thermodynamic parameters. The results of spectroscopic measurements suggested that EMPD have a strong ability to quench the intrinsic fluorescence of HSA through static quenching procedure. The drug-protein complex was stabilized by hydrophobic forces and hydrogen bonding as indicated from the thermodynamic parameters and synchronous fluorescence spectra, which was consistent with the results of molecular docking and accessible surface area calculation. Competitive experiments indicated that a displacement of warfarin by EMPD, which revealed that the binding site of EMPD to HSA was located at the subdomains IIA. The distance between the donor and the acceptor was 4.85nm as estimated according to F?rster's theory of non-radiation energy transfer. The effect of metal ions on the binding constants was also investigated. The results indicated that the binding constants between EMPD and HSA increased in the presence of common metal ions.