Ionization Of Carboxyl Groups Research Articles

Phase behavior and properties of polyvinyl alcohol/gelatin blends with novel pH-dependence

A novel change of phase behavior and properties of polyvinyl alcohol (PVA)/gelatin blends as a function of pH was reported. The PVA/gelatin blends were found to be completely miscible in acidic condition (pH 8), and immiscible in neutral condition (pH was ca. 6). As a result, the membranes cast from acidic condition showed the highest tensile strength and the lowest alcohol vapor permeation (AVP) rate; those obtained from neutral condition showed the lowest tensile strength and highest AVP rate; the properties of membranes cast from basic condition lay in between. The interaction between PVA and gelatin was investigated via Fourier transform infrared spectrum (FTIR), differential scanning calorimetry (DSC), and Zetasizer measurement. The novel pH-dependence of the blends was ascribed to the protonation of amino groups of gelatin in acidic condition, which resulted in a strong electrostatic attraction between NH of gelatin and OH of PVA. The partial miscibility in basic condition was due to the ionization of carboxyl groups of gelatin, which caused a stretching of gelatin via electrostatic repulsive force and a breakage of the H-bonding among the molecular chains, leading to a limited interaction between PVA and gelatin and forming a partially miscible blend. In neutral conditions, there were almost no charges (very limited protonation and ionization) at the weak polyampholyte gelatin, and the strong H-bonding among gelatin molecules themselves or PVA molecules themselves caused the phase separation between gelatin and PVA. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 239–247, 2009

Polyelectrolyte polymer properties in relation to male contraceptive RISUG ® action

RISUG ® a polyelectrolytic hydrogel (styrene maleic anhydride and dimethyl sulfoxide) has proven to be efficacious as a contraceptive for a long term when injected into the lumen of vas deferens. Currently it is in advanced phase III clinical trials in India. Present investigation analyzes the swelling characteristics of RISUG ® hydrogel in different pH buffers and various biological fluids to understand its retention in the vas deferens as reported in previous studies. Significant variation in degree of swelling and equilibrium swelling ratio with transformation of Fickian to non-Fickian mode of diffusion was observed with increased pH. This might be due to ionization of carboxylic groups at high pH resulting in increased electrostatic repulsive force and high osmotic pressure inside the hydrogel network affecting its physical cross-linking and increases the free volume. Conversely, at low pH the dissociation of carboxylic group is limited making the hydrogel more stable. Interaction with various biomolecules present in various biological fluids was also studied. SEM, AFM and FTIR were used to analyze the topological and structural parameters of the polymer in different mediums. Loosening of structure and increasing porosity with significant adsorption of various biomolecules was observed. AFM revealed a significant change in overall roughness of polymer surface on interaction with different biological fluids. These observations suggest that the swelling and increased roughness will lead to increased resistance to sperm movement in the vas deferens.

Radiation preparation and characterization of pH-sensitive hydrogel of acrylic acid/cyclodextrin based copolymer

A β-cyclodextrin (β-CD) based monomer (MAH-β-CD) containing vinyl and carboxyl functional groups was synthesized by reaction of β-CD with maleic anhydride (MAH). A novel hydrogel, poly(AAc-co-MAH-β-CD) with pH and ionic strength sensitivities, was prepared by irradiating the aqueous solution mixture of acrylic acid (AAc) and MAH-β-CD with electron beam. The effect of the feed ratio of the components and irradiation dose on the swelling and deswelling properties of the hydrogel was studied, respectively, the effect of pH and ionic strength on the swelling ratio was determined. Experimental results showed that these copolymer hydrogels did not show any noticeable change in swelling ratio at lower pH range (pH 1–3). However they showed an abrupt increase in swelling ratio at the range of pH 3–6, due to the ionization of carboxyl groups. Fourier transform infrared (FT-IR) spectrometer was applied in the attenuated total reflectance (ATR) mode for analyzing the structure change of the hydrogels after the treatment of different pH buffer solutions.

(3R,5S)-5(3)-Carboxy-3,4,5,6-tetrahydro-2H-1,4-thiazin-4-ium-3(5)-carboxylate

The molecule of the zwitterionic title compound, C6H9NO4S, which lies on a mirror plane, shows a puckered chair conformation of the six-membered ring with the S and N atoms out of the mean plane of the other four C atoms by 0.929 (2) and 0.647 (2) Å, respectively. The ionized carboxyl group is equatorially oriented. The hydrogen-bonding network includes very short O—H⋯O [2.470 (2) Å] and N—H⋯S [3.471 (2) and 3.416 (2) Å] inter­molecular contacts.

PH‐responsive semi‐interpenetrating networks hydrogels of poly(acrylic acid‐acrylamide‐methacrylate) and amylose. I. Synthesis and characterization

AbstractThe semi‐interpenetrating networks gels of hydrophobically‐modified poly(acrylic acid‐acrylamide‐methacrylate) and amylose were prepared using 4,4′‐bis(methacryloylamino) azobenzene as a crosslinker. Investigation of mechanical strength of gels showed that the equilibrium elastic modulus G changed from 12.4 to 32.9 g/cm2 with increasing the contents of amylose in the case of low content of crosslinker. The swelling behavior of gels indicated that the aggregation of the hydrophobic groups of hydrogels could be destroyed by expanded amylose and the electrostatic repulsion between the charged COO− groups because of the ionization of the carboxylic groups in pH 7.4 buffer solutions. The hydrophobicity and flexibility of the side chain, the entanglement extent of the amylose, and the ionization of carboxyl groups in the gels greatly affected the swelling rates and swelling ratios. Investigations of swelling/deswelling periodically in pH 2.2 and 7.4 media indicated that the semi‐IPN gel was more pH‐sensitive than the nonsemi‐IPN gel. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Steady surface modification of polydimethylsiloxane microchannel and its application in simultaneous analysis of homocysteine and glutathione in human serum

A novel polydimethylsiloxane (PDMS) surface modification method for microchip electrophoresis has been developed to make a stable and sufficient electroosmotic flow (EOF). Poly(l-glutamic acid) (PGA) which had ionizable carboxyl groups at a high pH-range was immobilized on the surface of microchannel fabricated with PDMS. The surface modification involved surface oxidation by plasma, the silanization of 3-aminopropyldimethylethoxysilane (APDMES) and immobilization of PGA via amide bond. The modified channel was extremely stable against consecutive electric power supply over 5h, and its long-term stability was demonstrated by the efficient separation of four amino acid derivatives reproducibly after a week. Additionally, homocysteine (Hcy), important risk factor of cardiovascular disease, osteoporosis and problems in pregnancy, was successfully measured in human serum in modified PDMS channel with the other thio amino acid simultaneously.

Study of the swelling dynamics with overshooting effect of hydrogels based on sodium alginate- g-acrylic acid

A series of hydrogels were synthesized by graft cross-link copolymerization of sodium alginate (SA) and acrylic acid (AA) using N, N-methylene-bis-(acrylamide) as a cross-linker. By study of the swelling kinetics of the hydrogels in different buffer solutions, the overshooting effect was observed in acidic medium, namely the gels firstly swelled to a maximum value following by a gradual deswelling until the equilibrium. The phenomenon is interpreted as a cooperative physical cross-linking caused by the hydrogen bond formation between the carboxyl groups of the hydrogels in a hydrophobic environment. The hydrogen bond formation was further confirmed by FT-IR spectra. The dependence of overshooting effect on the pH of buffer solution was more noticeable in comparison with the composition of hydrogels, demonstrating that the cooperative physical cross-linking caused by the hydrogen bond formation is dominant. Whether or not the overshooting effect appears is not only relative to the pH of buffer solution, but also depends on the p K a of carboxyl groups on the network. The overshoot processes of the hydrogels under acidic medium at pH below the p K a follow a quantitative model proposed by Díez-Peńa et al., and the theoretical curves are in very good agreement with the experimental data. While in pH > p K a buffer solutions, the overshoot phenomenon does not appear arising from the repulsive interaction between the ionized carboxyl groups, the swelling processes follow Schott second-order rate equation.

Electrostatic Layer-by-Layer Self-Assembly of Poly(carboxybetaine)s: Role of Zwitterions in Film Growth

We report on self-assembly of a weak zwitterionic polymerpoly(carboxybetaine), PCB5within a polyelectrolyte multilayer film. Multilayer growth and ionization of carboxylic groups are monitored in situ using ATR-FTIR. Deposition of PCB5 within the film is contrasted for two pH regimes: (1) at slightly acidic pH values where PCB5 carries overall cationic charge, but a significant number of dipoles provided by ionized carboxylic groups also exists within the polymer chain, and (2) at neutral and basic pH values where these polymers become zwitterionic. In the latter case, no multilayers could be produced with either polyanions or polycations. In the former case, PCB5 can be deposited within the polymer film via alternating adsorption with a variety of polyanions, such as heparin, DNA, poly(carboxylic acid)s, or poly(styrenesulfonate). The produced films can be erased from the solid surface by exposure to solutions with higher pH values, when the fraction of charged carboxylic groups in PCB5 is increased. The presence of zwitterions in PCB5 chains allowed building hybrid layers which contained PCB5 bound with both a polyanion and a polycation. In contrast to PCB5/polyanion films, hybrid films showed a stability region extended to neutral pH values and dissolved at both pH > 7 and pH < 4. The PCB5-containing films show promise for bioseparation and biotechnology applications when used as sacrificial layers or for controlled release of incorporated small molecules.

Polystyrene-b-Poly(tert-butyl acrylate) and Polystyrene-b-Poly(acrylic acid) Dendrimer-Like Copolymers: Two-Dimensional Self-Assembly at the Air−Water Interface

The two-dimensional self-assembly at the air/water (A/W) interface of two dendrimer-like copolymers based on polystyrene and poly(tert-butyl acrylate) (PS-b-PtBA) or poly(acrylic acid) (PS-b-PAA) was investigated through surface pressure measurements (isotherms, isochores, and compression-expansion hysteresis experiments) and atomic force microscopy (AFM) imaging. The two dendrimer-like block copolymers have an 8-arm PS core (Mn = 10 000 g/mol, approximately 12 styrene repeat units per arm) with a 16-arm PtBA (Mn = 230 000 g/mol, approximately 112 tert-butyl acrylate repeat units per arm) or PAA (Mn = 129 000 g/mol, approximately 112 acrylic acid repeat units per arm) corona. The PS-b-PtBA sample forms stable Langmuir monolayers and aggregates into circular surface micelles up to a plateau observed in the corresponding isotherm around 24 mN/m. Beyond this threshold, the monolayers collapse above the interface, resulting in the formation of large and irregular desorbed aggregates. The PS-b-PAA sample has ionizable carboxylic acid groups, and its A/W interfacial self-assembly was therefore investigated for various subphase pH values. Under basic conditions (pH = 11), the carboxylic acid groups are deprotonated, and the PS-b-PAA sample is therefore highly water-soluble and does not form stable monolayers, instead irreversibly dissolving in the aqueous subphase. Under acidic conditions (pH = 2.5), the PS-b-PAA sample is less water-soluble and becomes surface-active. The pseudoplateau observed in the isotherm around 5 mN/m corresponds to a pancake-to-brush transition with the PAA chains dissolving in the water subphase and stretching underneath the anchoring PS cores. AFM imaging revealed the presence of circular surface micelles for low surface pressures, whereas the biphasic nature of the pseudoplateau region was confirmed with the gradual aggregation of the micellar PS cores above the PAA chains. The aggregation numbers for both samples were estimated around 3-5 dendrimer-like copolymers per circular surface micelle. These rather low values confirmed the tremendous influence of molecular architecture on the two-dimensional self-assembly of block copolymers.

Dynamics of short as compared with long poly(acrylic acid) chains hydrophobically modified with pyrene, as followed by fluorescence techniques

New low and high molecular weight poly(acrylic acid), PAA, 2000 g mol(-1) and 450,000 g mol(-1), respectively, were tagged with pyrene (low and high contents of probe) and its behaviour in solution was investigated using absorption and fluorescence (steady-state and time-resolved) techniques. Fluorescence data shows that the degree and level of intramolecular association strongly depends on the molecular weight. With the short pyrene-labeled PAA chains in aqueous solution, the excimer-to-monomer fluorescence ratio I(E)/I(M) decreases with the increase of pH, oppositely to the increase in the I(E)/I(M) ratio with the increase in pH previously observed with the long chain PAA. Time-resolved data suggest that excimer formation with the short pyrene-labeled PAA polymers (ca. 28 acrylic acid monomers per chain) in water is largely due to excitation of Ground State Dimers, GSD. The increment of pH, and the consequent gradual ionization of the carboxylic groups in the chain, initially increases the fraction of GSD, possibly due to the occurrence of special micelle-like chain conformations, inside which the pyrene units are accommodated. A further increase of the pH above the pK(a) values, resulting in the full ionization of carboxylic groups, apparently destabilizes such chain conformations, which leads to a pH effect on the photophysical properties identical to that of the long chain polymers. In water, the dynamic data shows the existence of two excimers coexisting with two monomer classes. In methanol and dioxane (good solvents for the pyrene probe) at room temperature, where one excimer and two monomers are present, all rate constants could be obtained, as well as the fractions of ground-state species. It is thus shown that different types of interactions are produced with small- and long-sized PAA polymers, i.e., the size of the polymer matters.

The Effect of Valence on the Ion-Exchange Process: Theoretical and Experimental Aspects on Compound Binding/Release

The effect of valence of mobile counter-ions (extracting electrolytes), mobile co-ions, and drug-like compounds was evaluated on drug binding/release in ion-exchange fibers. The experimental results support the Donnan theory and suggest that incorporation of monovalent salicylic acid (SA) and divalent 5-hydroxyisophthalic acid (di-COOH) into the anion-exchange fibers was attained mainly as a result of electrostatic (ionic) interaction, with additional contribution of non-electrostatic interactions. Increasing the capacity of ion-exchanger increased the molar amount of compound loading. More efficient release of model anions was observed at increasing valence or concentration of the extracting counter-ion. Potency to release the compounds decreased in the order of citrate (-3) > sulfate (-2) > chloride (-1). The valence of co-ions (sodium (+1) vs. calcium (+2)) in the external solution had only a slight effect on the release. Due to dual site binding (two ionized carboxylate groups), the amount of di-COOH bound into the fibers was half of that of monovalent SA. Also the release was significantly reduced, as the electrostatic interaction was stronger in the case of divalent compound. Simulations on the effect of valence on the Donnan potential and theoretical modeling of the release efficiencies by the external ions supported successfully the conclusions above.

PH-responsive self-assembly of carboxyl-terminated hyperbranched polymers

This work has presented a typical example to reveal the great influence of the terminal groups on the self-assembly of hyperbranched polymers. The hyperbranched polymers with hydroxyl terminal groups (HBPO-OH) were hydrophobic and precipitated in water, however, they displayed a pH-responsive self-assembly behavior when the terminal groups were replaced by carboxyl groups. The obtained carboxyl-terminated hyperbranched polymers (HBPO-COOH) existed as unimolecular micelles at high pH (12.21) due to the ionization of carboxyl groups, while the polymers aggregated into multimolecular micelles from 10 to 500 nm with the decrease of pH as a result of the partial protonation of the carboxyl groups. The size of the obtained micelles depended strongly on the solution pH - the lower the pH, the bigger the micelles. TEM, DLS, ATR-FT-IR, (1)H NMR and AFM measurements substantiated that the multimolecular micelles were formed by the secondary aggregation of unimolecular micelles driven by the hydrogen bonding interaction depending on the solution pH.

Electrochemical and EQCM investigation of l-selenomethionine in adsorbed state at gold electrodes

Abstract Selenomethionine (SeMet) adsorbs irreversibly from an aqueous solution to the gold surface. The resulting surface layer has a composite character and consists of SeMet and selenohomocysteine (SeHcy), which forms by the hydrolytic cleavage of the first one. The occurrence of these compounds was demonstrated by cyclic voltammetry and EQCM investigations. An anodic scan results in two poorly separated peaks corresponding to the oxidative desorption processes (E1/2 about 0.70 V and Ep about 0.92 V vs. the Ag|AgCl (3 M KCl) electrode, respectively). The first one corresponds to the oxidation of SeHcy to RSeO2H (where R stands for HO2CCH(NH2)(CH2)2−) whereas the second results from the anodic conversion of SeMet to RSeO3H with RSeO2H as a possible intermediate. SeMet oxidation may continue in the region of gold oxide formation if not completed in the pre-oxide region. Partial decomposition of RSeO2H gives rise to H2SeO3 which was evidenced by its reduction reaction to Se(0) at about 0.25 V. In turn, Se(0) can be oxidized back to H2SeO3 at about 0.83 V. Surface Se(0) inhibits the electron transfer reaction of the Fe ( CN ) 6 3 - / 4 - redox couple. Ionization of the amino acid functions in the adsorbed state was investigated by capacitance vs. pH determinations. It was thus found that the carboxyl ionization constant (pKa1) shifts from 2.19 in the aqueous solution to 5.1 in the adsorbed state whereas the adsorption has almost no effect on the ionization constant of the amino group (pKa2 = 9.05 in the adsorbed state vs. 9.15 in the solution form). Adsorption effect on the ionization constants was accounted for by the interplay of the effect of the surface charge on the local activity of the hydrogen ion, on one had, and intramolecular hydrogen bonding, on the other hand. A SeMet modified electrode can bind reversibly the copper(II) ion in the same way as cysteine and homocysteine proving that the amino acid functions are not involved in surface interactions. Selenium head group forms therefore the link between the adsorbate and the gold surface.

Structure−DPPH• Scavenging Activity Relationships: Parallel Study of Catechol and Guaiacol Acid Derivatives

The scavenging behavior of a series of catechol and guaiacol acid derivatives toward DPPH(*) was examined having as a starting point the order of activity derived on the basis of theoretically calculated BDE values. The studied compounds were protocatechuic, homoprotocatechuic, dihydrocaffeic, and caffeic acids and also vanillic, homovanillic, dihydroferulic, and ferulic acids. Catechol and guaiacol were used as reference compounds. Observations from the parallel study were made with regard to structural features (number and position of hydroxyl groups and the side-carbon chain characteristics) that regulated the behavior of the compounds experimentally. The exceptional DPPH(*) scavenging behavior observed for homoprotocatechuic acid in ethanol and for caffeic acid in acetonitrile could not be supported by the respective BDE values. Ferulic was the most active among guaiacolic acids, whereas dihydroferulic exhibited the highest stoichiometry. Ionizable carboxylic groups seem to affect considerably the relative order of activity as was also evidenced using the ORAC assay. Questions raised about the validity of widely accepted views on criteria for SARs are discussed with regard to literature findings.

Compressive Elastic Moduli of Poly(N‐Isopropylacrylamide) Hydrogels Crosslinked with Poly(Dimethyl Siloxane)

Hydrophobically modified and thermally reversible neutral and ionic copolymer hydrogels were prepared from N‐isopropylacrylamide (NIPAAm), vinyl terminated poly (dimethylsiloxane) (VTPDMS) and itaconic acid (IA) by free radical solution polymerization, and their properties such as swelling ratio and compression modulus were studied at the 25°C. The incorporation of VTPDMS as a hydrophobic macrocrosslinker into the structures of neutral NIPAAm hydrogels increased their mechanical strength around 10 times than those of the ones crosslinked with conventional tetra functional monomer, i.e., N,N′‐methylene bisacrylamide (BIS). Compression modulus decreased with an increase in IA content for ionic samples and increased with increasing molecular weight and content of VTPDMS for neutral samples. It was assumed that in the first case, electrostatic repulsive forces resulting from the ionized carboxyl groups of IA were responsible for decreasing mechanical strength, while in the second case, hydrophobic interactions between dimethylsiloxane units of VTPDMS chains enhanced the compression moduli. According to the results presented in this work, it can be said that the right balance of hydrophobic and hydrophilic constituents and adjustment of the number of ionized groups, as well as crosslinking degree, change the structure and physical properties of NIPPAAm hydrogels.

Influence of Stereoregularity of the Polymer Chain on Interactions with Surfactants: Binding of Cetylpyridinium Chloride by Isotactic and Atactic Poly(methacrylic acid)

Association of a cationic surfactant cetylpyridinium chloride, CPC, with isotactic and atactic poly(methacrylic acid), i-PMA and a-PMA, respectively, in aqueous 0.01 M NaCl solutions was studied by pH and fluorescence measurements in conjunction with potentiometric studies using a surfactant-sensitive membrane electrode. pH measurements have demonstrated that the presence of an oppositely charged surfactant increases ionization of carboxyl groups on PMA at low degrees of neutralization. The increase is more pronounced in the case of i-PMA. The isotactic form of PMA is not soluble in water at zero degrees of neutralization but can be rendered soluble by the addition of CPC at the surfactant to a polyion molar ratio of around 0.4. In the solubilized complex, the positive charge of the CPC molecule is facing the polar solvent, whereas surfactant tails are oriented toward the i-PMA compact coil. Binding isotherms and cooperativity parameters show that chain tacticity has an important influence on the interaction of cetylpyridinium cation with polymethacrylate anion. At the onset of cooperative binding, the association is stronger with i-PMA than with the atactic form, as demonstrated by lower CAC values and higher values of the cooperativity parameters. In contrast, more surfactant is bound by a-PMA in the region where polyion becomes saturated with surfactant ions. Results are interpreted by taking into account local chain conformations as obtained from quantum mechanical semiempirical molecular orbital calculations. Greater hydrophobicity and possibly higher charge density of i-PMA on one hand and more flexibility of the a-PMA chain on the other are held responsible for these observations.

Conformational analyses of 2,3‐dihydroxypropanoic acid as a function of solvent and ionization state as determined by NMR spectroscopy

Vicinal (1)H--(1)H coupling constants were used to determine the conformational preferences of 2,3-dihydroxypropanoic acid (1) (DL-glyceric acid) in various solvents and its different carboxyl ionization states. The stereospecific assignments of J(12) and J(13) were confirmed through the point-group substitution of the C-3 hydrogen with deuterium, yielding rac-(2SR,3RS)-[3-(2)H]-1, and the observation of only J(13) in the (1)H NMR spectra. While hydrogen bonding and steric strain may be expected to drive the conformational equilibrium, their role is overshadowed by a profound gauche effect between the vicinal hydroxyl groups that mimics other substituted ethanes, such as 1,2-ethanediol and 1,2-difluoroethane. At low pH, the conformational equilibrium is heavily weighted toward the gauche-hydroxyl rotamers with a range of 81% in DMSO-d(6) to 92% in tert-butyl alcohol-d(10). At high pH, the equilibrium exhibits a larger dependence upon the polarity and solvating capability of the medium, although the gauche effect still dominates in D(2)O, 1,4-dioxane-d(8), methanol-d(4), and ethanol-d(6) (96, 89, 85, and 83% gauche-hydroxyls respectively). The observed preference for the gauche-hydroxyl rotamers is believed to stem primarily from hyperconjugative sigma(C--H) --> sigma*(C--OH) interactions.

Synthesis of crosslinked oxidized starch and its adsorption behavior for calcium ion

Cornstarch, after crosslinking with epichlorohydrin (4%, v/w, dry basis of the starch), was oxidized with hydrogen peroxide over the catalyst Cu(II). The newly synthesized products were applied to examine the calcium ion removal activity from water, under various conditions. Removal efficiency of calcium ion from aqueous solution increased proportionally with carboxyl content and the dose of crosslinked oxidized starch in the solution. Ionization of carboxyl groups in starch was necessary for the effective calcium removal. When the pH of the solution was adjusted below 4.0, the starch was no longer an effective calcium-sequestering agent. Adsorption isotherm models were developed, wherein the best fit was obtained in the Langmuir model. Thermodynamic study indicated that the adsorption process was exothermic, and the enthalpy change (ΔHθ), the entropy change (ΔSθ), and free energy change (ΔGθ) of the adsorption process were calculated with adsorption isotherm data and basic thermodynamic relations. It was assessed that adsorption occurred by strong electrostatic interactions with the negative adsorption enthalpy (ΔHθ). The regeneration and reusability of oxidized starch were also assessed and were found to retain the adsorption capacity. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1539–1546, 2006

Identification of catalytically important amino acid residues of Streptomyces lividans acetylxylan esterase A from carbohydrate esterase family 4

Multiple sequence alignment of Streptomyces lividans acetylxylan esterase A and other carbohydrate esterase family 4 enzymes revealed the following conserved amino acid residues: Asp-12, Asp-13, His-62, His-66, Asp-130, and His-155. These amino acids were mutated in order to investigate a functional role of these residues in catalysis. Replacement of the conserved histidine residues by alanine caused significant reduction of enzymatic activity. Maintenance of ionizable carboxylic group in side chains of amino acids at positions 12, 13, and 130 seems to be necessary for catalytic efficiency. The absence of conserved serine excludes a possibility that the enzyme is a serine esterase, in contrast to acetylxylan esterases of carbohydrate esterase families 1, 5, and 7. On the contrary, total conservation of Asp-12, Asp-13, Asp-130, and His-155 along with dramatic decrease in enzyme activity of mutants of either of these residues lead us to a suggestion that acetylxylan esterase A from Streptomyces lividans and, by inference, other members of carbohydrate esterase family 4 are aspartic deacetylases. We propose that one component of the aspartate dyad/triad functions as a catalytic nucleophile and the other one(s) as a catalytic acid/base. The ester/amide bond cleavage would proceed via a double displacement mechanism through covalently linked acetyl-enzyme intermediate of mixed anhydride type.

Hydrogen-Bonded Multilayers of Thermoresponsive Polymers

Neutral temperature responsive polymers, such as poly(N-vinylcaprolactam) (PVCL) and poly(vinyl methyl ether) (PVME), were included in ultrathin films using layer-by-layer alternating adsorption of these polymers with poly(methacrylic acid) (PMAA) at low pH. The amounts of polymers adsorbed and ionization of carboxylic groups within a film were quantified using in situ FTIR-ATR (Fourier transform infrared spectroscopy by attenuated total reflection). The strength of interlayer adhesion provided through hydrogen-bonding interactions was inferred from the critical pH value (pHCR) and critical ionization (αCR) for multilayer decomposition which were pHCR 6.2, αCR 2% and pHCR 6.95, αCR 30% for PMAA/PVME and PMAA/PVCL films, respectively. When deposited onto porous support membranes, PMAA/PVME and PMAA/PVCL multilayers exhibited temperature-responsive changes in dye permeability. The transition occurred in a wide temperature range from 25 to 35 °C, reflecting lower cooperativity of the phase separation of PVME...