Fractions of thermodynamic functions for native lysozyme adsorption onto moderately hydrophobic surface

Calorimetric measurement of adsorption enthalpies of native lysozyme(Lyz) on a moderately hydrophobic surface at 25°C, pH 7.0 and various salt concentrations was performed. Based on the thermodynamics of stoichiometric displacement theory (SDT), we calculated the fractions of thermodynamic functions involving four subprocesses during a displacement adsorption process from the directly determined enthalpies in combination with adsorption isotherm measurements. The thermodynamic fractions reveal the relative degree of the four subprocesses for contributions to enthalpy, entropy and free energy. The results show that native Lyz adsorption on a moderately hydrophobic surface is an entropy driven process contributed mainly by conformational loss of adsorbed Lyz.     

Thermodynamic investigation of effect of salt concentrations on denatured α-Amylase adsorbed onto a moderately hydrophobic surface

The displacement adsorption enthalpies (ΔH) of denatured α-Amylase (by 1.8 mol L⁻¹ GuHCl) adsorbed onto a moderately hydrophobic surface (PEG-600, the end-group of polyethylene glycol) from solutions (x mol L⁻¹ (NH₄)₂SO₄, 0.05 mol L⁻¹ KH₂PO₄, pH 7.0) at 298 K are determined by microcalorimeter. Further, entropies (ΔS), Gibbs free energies (ΔG) and the fractions of ΔH, ΔS, and ΔG for net adsorption of protein and net desorption of water are calculated in combination with adsorption isotherms of α-Amylase based on the stoichiometric displacement theory for adsorption (SDT-A) and its thermodynamics. It is found that the displacement adsorptions of denatured α-Amylase onto PEG-600 surface are exothermic and enthalpy driven processes, and the processes of protein adsorption are accompanied with the hydration by which hydrogen bond form between the adsorbed protein molecules favor formation of β-sheet and β-turn structures. The Fourier transformation infrared spectroscopy (FTIR) analysis shows that the contents of ordered secondary structures of adsorbed α-Amylase increase with surface coverages and salt concentrations increment.     

Microcalorimetric determination of displacement adsorption enthalpies of protein refolding on a moderately hydrophobic surface at 308 K

Both microcalorimetric determination of displacement adsorption enthalpies ΔH and measurement of adsorbed amounts of guanidine – denatured lysozyme (Lys) refolding on the surface of hydrophobic interaction chromatography (HIC) packings at 308 K were carried out and compared with that at 298 K. Study shows that both temperature and concentration of guanidine hydrochloride (GuHCl) affect the molecular mechanism of hydrophobic interaction of protein with adsorbent based on the analysis of dividing ΔH values into three kinds of enthalpy fractions. The adsorption in higher concentrations of GuHCl (>1.3 mol L^–1) at 308 K is an enthalpy-driving process, and the adsorption under other GuHCl concentrations is an entropy-driving process. The fact that the Lys denatured by 1.8 mol L^–1 GuHCl forms a relatively stable intermediate state under the studied conditions will not be changed by temperature.     

Studies of Monolayer and Mixed Micelle Formation of Anionic and Nonionic Surfactants in the Presence of Adenosine-5-monophosphate

The interactions between the anionic surfactant di-(2-ethylhexyl) phosphate sodium salt (DEP) and two nonionic surfactants, dimethyldecyl phosphineoxide (DDPO) and dimethyltetradecyl phosphineoxide (DTPO), at the interface and in the micellar phases were investigated in the absence and presence of adenosine-5-monophosphoric acid disodium salt (AMP). The mixed systems were DEP–DDPO, DEP–DDPO/AMP (0.001 mol⋅L⁻¹), DEP–DTPO, and DEP–DTPO/AMP (0.001 mol⋅L⁻¹) at different bulk mole fractions of the anionic component (α ₁=0.9,0.8,0.6,0.4,0.2). The mixed systems studied were investigated based on the theoretical models of Rubingh and Clint. The results showed surface tension reduction efficiency. The adsorbed mixed monolayer demonstrated stronger interactions than the mixed micelles, whereas AMP increased the interfacial interactions more than those in the micellar phase. The Gibbs energy of mixing suggests that the stability of the mixed micellar phase is greater than that of the micellar phases of the individual components. The synergism that occurred in the different mixed phases is discussed.     

Thermodynamic characteristics of the acid dissociation of dopamine hydrochloride in water-ethanol solutions

Enthalpies of the interaction of protonated dopamine with a hydroxide ion in water-ethanol mixtures in the concentration range of 0–0.8 EtOH mole fractions are measured calorimetrically. The neutralization process of dopamine hydrochloride is shown to occur endothermally in solvents with an ethanol concentration of ≥0.5 mole fractions. Standard thermodynamic characteristics (Δ_r H ^○, Δ_r G ^○, and Δ_r S ^○) of the first-step acid dissociation of dopamine hydrochloride in solutions are calculated with regard to the autoprotolysis enthalpy of binary solvents. It is found that dissociation enthalpies vary within 9.1–64.8 kJ/mol, depending on the water-ethanol solvent composition.     

Bile salt structural effect on the thermodynamic properties of a catanionic mixed adsorbed monolayer

The interfacial effects of two bile salts (sodium deoxycholate (NaDC) and sodium dehydrocholate (NaDHC)) in a catanionic mixed adsorbed monolayer have been investigated at 25 °C. The surfactant interfacial composition, the interfacial orientation of the molecules and the energy changes are analysed to show a thermodynamic evidence of the hydrophobic BSs effect during its intercalation into interfacial adsorbed didodecyldimethyl ammonium bromide (DDAB) molecules. Both mixed systems (NaDC–DDAB and NaDHC–DDAB) have analogous adsorption efficiencies, which are similar from a pure DDAB monolayer and superior to that obtained for both bile salts molecules. Nevertheless, their adsorption effectiveness is different: NaDC causes an increment of Γ while NaDHC produces the opposite effect. The adsorption efficiency in surface tension reduction is due to the existence of interfacial synergistic interactions (confirmed by the analysis of β ^γ and ΔG _ad ⁰ values). Maximum synergistic interaction is seen for α _BSs = 0.4. The hydrophobic steroid backbone of NaDHC molecule presents a deep interfacial penetration than NaDC. This fact causes a great disturbance of DDAB hydrocarbon tails and conduces to a large separation of molecules (high A _m values) which explains the reduction of adsorption effectiveness (low Γ _m values).     

The thermodynamic functions of formation and transfer of copper(II) and nickel(II) ethylenediaminetetraacetate complexes with ammonia in aqueous-ethanolic solutions

The formation of mixed-ligand NiEdtaNH₃²⁻ and CuEdtaNH₃²⁻ complexes in an aqueous-ethanolic solvent with the mole fractions of ethanol 0.1 and 0.2 at 298.15 K and ionic strength I = 0.4 (KNO₃) was studied calorimetrically. The thermodynamic characteristics of formation and transfer parameters of these complexes were calculated. The influence of the mixed solvent on the thermodynamic characteristics of these reactions was studied.     

Studies on Thermodynamics of Micellization and Solvophobic Interactions of Novel Surfactant–Cr(III) Complexes in Non-aqueous Solvents

The critical micelle concentration (CMC) of surfactant–Cr(III)–dodecylamine complexes of the type cis-α-[Cr(trien)(C₁₂H₂₅NH₂)X]²⁺ (where trien = triethylenetetramine; X = F⁻, Cl⁻, Br⁻) has been studied in n-alcohol and in formamide at different temperatures, by electrical conductivity measurements. From the CMC values as a function of temperature, various thermodynamic properties have been evaluated: standard Gibbs energy changes (Δ_mic G ⁰), standard enthalpy changes (Δ_mic H ⁰) and standard entropy changes (Δ_mic S ⁰) for micellization. Critical micelle concentrations have also been measured as a function of the percentage composition of alcohol added. The solvent composition dependences of these thermodynamic parameters were determined. It is suggested that alcohol addition leads to an increase in formamide penetration into the micellar interface that depends on the alcohol’s chain length. The results are discussed in terms of an increased hydrophobic effect, dielectric constant of the medium, the chain length of the alcohols, and the surfactant in the solvent mixture.     

Salt influence in the polymer-surfactant interaction in solution. A fluorescence probe investigation of the EHEC/SDS/ water system

 The effect of small amounts of salt on the interaction between two fractions of ethyl(hydroxy)ethyl cellulose, EHEC, and sodium dodecyl sulfate, SDS, has been investigated by means of steady-state fluorescence measurements. The two polymer fractions display different properties in hydrophobicity expressed as different cloud points. The results are discussed in relation to hydrodynamic (viscosity) and thermodynamic (equilibrium dialysis) properties. The micropolarity as sensed by the probe pyrene shows that the polymers begin to interact with SDS at a lower concentration in the presence of salt. The average aggregation numbers of polymer-bound clusters, N _p, were obtained by fluorescence-quenching data in combination with equilibrium dialysis experiments. N _p was found to increase in the presence of salt for the EHEC fraction with a high cloud point (CP). The polymer with a low CP displays higher N _p in the presence of salt at low SDS concentrations, but exhibit lower N _p at higher SDS concentrations than in the salt-free system. The microviscosity index as determined by intramolecular excimer formation of 1, 3-di(1-pyrenyl)propane (P3P) is highest for the lowest N _p and there is a corre-lation with N _p in the presence as well as absence of salt for both EHEC fractions. It is found that when the same fractional amount of SDS is bound to the polymers, 10–20% of the value of saturation, the increase in macroviscosity occurs and the microviscosity shows high rigidity. Received: 3 March 1997 Accepted: 23 May 1997     

Surfactant assisted preparation and characterization of carboxymethyl cellulose Sn(IV) phosphate composite nano-rod like cation exchanger

Carboxymethyl cellulose Sn(IV) phosphate composite nano-rod like cation exchanger with diameter in the range of 20–40 nm, length in the range of 100–150 μm and particle size in the range of 21–38 nm have been successfully prepared by surfactant assisted sol–gel method. Scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, fourier transform infra red spectroscopy and thermogravimetric analysis-differential thermal analysis studies were carried out to study the structure and morphology of this composite nano-rod like cation exchanger. Freundlich adsorption isotherm is well fitted for the adsorption of pyridine on the surface of this composite nano-rod like cation exchanger. The thermodynamic parameters such as Freundlich constant, thermodynamic equilibrium constant (K ₀), standard free energy changes (ΔG ⁰), standard enthalpy changes (ΔH ⁰) and standard entropy changes (ΔS ⁰) have been evaluated. These parameters indicated that the adsorption of pyridine on the surface of composite nano-rod like cation exchanger was feasible, spontaneous and exothermic in nature which suggests for the potential application of pyridine removal from water.     

Some characteristics of a protein-coated ODS column and its use for the determination of drugs by the direct injection analysis of plasma samples

Summary It was found that an ODS column of small pore (120?) which was coated with denatured plasma proteins (protein-coated ODS) no longer adsorbed plasma proteins from aqueous solution but retained the characteristics of native ODS for small hydrophobic molecules. Elemental analysis and nitrogen desorption (BET) analysis showed that the protein-coated ODS contained ca 25 mg proteing⁻¹ dry resin and that the pore diameter or pore volume was similar to that of native ODS. The coated denatured proteins, which seemed to be adsorbed on the external surface of the porous resin, were not eluted under usual reverse-phase elution conditions. Operating as either an analytical column or a pre-column, this protein-coated ODS column was used to analyse spiked-drugs in plasma. The recovery of all the spiked-drugs (such as doxorubicin, methotrexate) was almost quantitative (98–102%) with good reproducibilities (c.v., less than 2%). The present method was useful for the determination of total, that is, free + bound-to-plasma-proteins, hydrophobic drugs in plasma in view of its accuracy and simplicity. Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984     

Effect of Buffer Species on the Complexation of Basic Drug Terfenadine with β-Cyclodextrin

The complexation of terfenadine (Terf) with β-cyclodextrin (β-CD) in solution and solid state has been investigated by phase solubility diagram (PSD), differential scanning calorimetry (DSC), powder X-ray diffractometry (PXD) and proton nuclear magnetic resonance (¹H-NMR). The PSD results indicated that the salt saturation with the buffer counter ion (citrate⁻², H₂PO₄⁻¹ and Cl⁻¹ ions) of Terf (pK _a = 9.5) and the hydrophobic effect play in tandem to increase the value of the complex formation constant (K₁₁) measured at different conditions of pH, ionic strength, buffer type and buffer concentration. The correlation of the free energy of complex formation (ΔG₁₁) with the free energy of inherent solubility of Terf (ΔG_So) obtained by changing the pH, ionic strength and buffer concentration was used to measure the contribution of the hydrophobic effect (desolvation) to complex formation. The hydrophobic effect was found to constitute 57.8% of the driving force for complex stability, while other factors including specific interactions contribute −13.4 kJ/mol. ¹H-NMR spectra of Terf–citrate and Terf–HCl salts gave identical chemical shift displacements (ΔΔ) upon complexation, thus indicating that the counter anions are positioned somewhere outside of the β-CD cavity. DSC, XRPD and ¹H-NMR proved the formation of solid Terf/acid/β-CD ternary complexes.     

Nonaqueous Solution Studies on the Protonation Equilibria of some Phenolic Acids

The protonation equilibria for some phenolic acids in nonaqueous solutions have been studied by pH-potentiometry. The dissociation constants, pK _a, of these phenolic acids and the thermodynamic functions, ΔG ^o,ΔH ^o and ΔS ^o, for the successive and overall protonation processes of these phenolic acids have been derived at different temperatures in three different mixtures of water and dioxane (mole fractions of dioxane were 0.083, 0.174 and 0.33). Titrations were also carried out in (water + dioxane) with ionic strengths of 0.15, 0.20 and 0.25 mol⋅dm⁻³ NaNO₃, and the resulting dissociation constants are reported. A detailed thermodynamic analysis of the effects of organic solvent, dioxane, temperature and ionic strength on the protonation processes of phenolic acids is presented and discussed to determine the factors which control these processes. Ahmed E. Fazary; previous address: Egyptian Organization for Biological Products and Vaccines, 51 Wezaret El-Zeraa Street, Agouza, Giza, Egypt. Tel. +2010-3017357.     

Effect of bovine serum albumin on the micellization of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers in aqueous solutions by fluorescence spectroscopy

Effect of bovine serum albumin (BSA) on the temperature-dependent association behavior of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers was investigated using pyrene fluorescence spectroscopy. The critical micellization temperature (CMT) of pluronics in aqueous solution was increased by the addition of BSA. A closed association model was used to obtain the standard free energies (△G0), enthalpies (△H 0), and entropies (△S 0) of micellization. The standard enthalpy and entropy of micellization for pluronic polymers in water were decreased with an increase of the BSA content. The more PPO component in the pluronic polymer, the higher the changed values of micellization enthalpy and entropy. The hydrophobic part of the pluronics, PPO, was responsible for the interaction between pluronics and BSA. Hydrophobic interaction between PPO and BSA was correlated to the alternation of the PPO-PPO interaction by the addition of BSA, which would shift the CMT toward higher temperature and alter the thermodynamic parameters of micellization for pluronics in aqueous solutions.     

Enzymatic electrodes nanostructured with functionalized carbon nanotubes for biofuel cell applications

Nanostructured bioelectrodes were designed and assembled into a biofuel cell with no separating membrane. The glassy carbon electrodes were modified with mediator-functionalized carbon nanotubes. Ferrocene (Fc) and 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonate) diammonium salt (ABTS) bound chemically to the carbon nanotubes were found useful as mediators of the enzyme catalyzed electrode processes. Glucose oxidase from Aspergillus niger AM-11 and laccase from Cerrena unicolor C-139 were incorporated in a liquid-crystalline matrix-monoolein cubic phase. The carbon nanotubes–nanostructured electrode surface was covered with the cubic phase film containing the enzyme and acted as the catalytic surface for the oxidation of glucose and reduction of oxygen. Thanks to the mediating role of derivatized nanotubes the catalysis was almost ten times more efficient than on the GCE electrodes: catalytic current of glucose oxidation was 1 mA cm⁻² and oxygen reduction current exceeded 0.6 mA cm⁻². The open circuit voltage of the biofuel cell was 0.43 V. Application of carbon nanotubes increased the maximum power output of the constructed biofuel cell to 100 μW cm⁻² without stirring of the solution which was ca. 100 times more efficient than using the same bioelectrodes without nanotubes on the electrode surface.     

Hydrophobic and Physical Properties of the Two Step Processed Ambient Pressure Dried Silica Aerogels with Various Exchanging Solvents

The experimental results by using various exchanging solvents in the preparation of two step (acid and base) processed ambient pressure dried hydrophobic silica aerogels, are reported. Silica alcogels were prepared by hydrolysis with oxalic acid and condensation with NH₄OH of ethanol diluted tetraethylorthosilicate (TEOS) precursor and hexamethyldisilazane(HMDZ) methylating agent. The exchanging solvents used were: hexane, cyclohexane, heptane, benzene, toluene and xylene. The physical properties such as % of volume shrinkage, density, pore volume, % of porosity, thermal conductivity, % of optical transmission, surface area, pore size distribution and contact angle (θ) of the silica aerogels with water, were measured as a function of EtOH/TEOS molar ratios (R) for all the exchanging solvents. It was found that the physical and hydrophobic properties of the silica aerogels strongly depend on the nature of the solvent and R. Heptane solvent resulted in highly transparent (≈90% optical transmission at 700 nm for 1 cm thick sample), low density (≈0.060 g/cm³), low thermal conductive (≈0.070 W/m·K), high % of porosity (97%), high surface area (750 m²/g), uniform porosity and hydrophobic (θ ≈ 160°) aerogels compared to other solvents. On the otherhand, xylene resulted in aerogels with higher hydrophobicity (θ ≈ 172°) among other solvents.     

Experimental Analysis of Complex Formation of Niflumic Acid with β-Cyclodextrins

Complex formation of niflumic acid with β-, hydroxypropyl-β- and methyl-β-cyclodextrins in aqueous solution (pH 7.4) were studied by calorimetry of solution, ¹H NMR spectroscopy and solubility method. The enhancement of niflumic acid solubility in the presence of hydroxypropyl-β-cyclodextrin was detected. This effect is explained on the basis of ¹H NMR data confirming the inclusion of hydrophobic trifluoromethylphenyl residue of niflumic acid molecule into the macrocyclic cavity. The thermodynamic parameters of 1:1 binding were derived from the data of␣calorimetry and solubility measurements. It was obtained, that complex formation of niflumic acid with β-cyclodextrin and both its derivatives is enthalpy driven. Substitutes surrounding the macrocyclic cavity slightly influence the thermodynamics of complex formation resulting in decrease of stability of the complexes formed.     

Effect of pH and ionic strength on the interaction of humic acid with aluminium oxide

The effect of pH and neutral electrolyte on the interaction between humic acid/humate and γ-AlOOH (boehmite) was investigated. The quantitative characterization of surface charging for both partners was performed by means of potentiometric acid–base titration. The intrinsic equilibrium constants for surface charge formation were logK _a,1 ^int=6.7±0.2 and logK _a,2 ^int = 10.6±0.2 and the point of zero charge was 8.7±0.1 for aluminium oxide. The pH-dependent solubility and the speciation of dissolved aluminium was calculated (MINTEQA2). The fitted (FITEQL) pK values for dissociation of acidic groups of humic acid were pK ₁ = 3.7±0.1 and pK ₂ = 6.6±0.1 and the total acidity was 4.56 mmol g⁻¹. The pH range for the adsorption study was limited to between pH 5 and 10, where the amount of the aluminium species in the aqueous phase is negligible (less than 10⁻⁵ mol dm⁻³) and the complicating side equilibria can be neglected. Adsorption isotherms were determined at pH ∼ 5.5, ∼8.5 and ∼9.5, where the surface of adsorbent is positive, neutral and negative, respectively, and at 0.001, 0.1, 0.25 and 0.50 mol dm⁻³ NaNO₃. The isotherms are of the Langmuir type, except that measured at pH ∼ 5.5 in the presence of 0.25 and 0.5 mol dm⁻³ salt. The interaction between humic acid/humate and aluminium oxide is mainly a ligand-exchange reaction with humic macroions with changing conformation under the influence of the charged interface. With increasing ionic strength the surface complexation takes place with more and more compressed humic macroions. The contribution of Coulombic interaction of oppositely charged partners is significant at acidic pH. We suppose heterocoagulation of humic acid and aluminium oxide particles at pH ∼ 5.5 and higher salt content to explain the unusual increase in the apparent amount of humic acid adsorbed. Received: 20 July 1999 /Accepted in revised form: 20 October 1999     

Thermodynamic study on inclusion complex formation of riboflavin with hydroxypropyl-β-cyclodextrin in water

The inclusion complex formation of riboflavin (RF) with hydroxypropyl-β-cyclodextrin (HP-β-CD) in water was investigated by ¹H NMR, UV-vis spectroscopy, and solubility methods. A 1:1 stoichiometry and thermodynamic parameters of complex formation (K, Δ_c G ⁰, Δ_c H ⁰, and Δ_c S ⁰) were determined. Complexation was characterized by negative enthalpy and entropy changes due to prevalence of van der Waals interactions and hydrogen bonding between polar groups of the solutes. A partial insertion of RF into macrocyclic cavity was revealed on the basis of ¹H NMR data and molecular mechanics calculation. Location of benzene ring of RF molecule inside the hydrophobic cavity of HP-β-CD results in an increase of aqueous solubility of the former.     

The Gibbs Energies of Activation of Lysozyme for Viscous Flow in Water + Dimethyl Sulfoxide Solutions at 298.15 K

Gibbs energies of activation for viscous flow of binary water (1) + dimethyl sulfoxide (2) mixtures, Δμ ₁₂^○, and of lysozyme (3) in corresponding ternary mixtures, Δμ ₃^○, were determined at 298.15 K. The binary mixtures have a maximum in the value of the excess quantity for Δμ ₁₂^○ at a dimethyl sulfoxide mole fraction of x ₂≈0.31. The values of Δμ ₃^○ are larger than Δμ ₁₂^○ at all values of x ₂, even when normalized by their molar volumes, suggesting that the solvents interact more strongly with lysozyme than with themselves. The values of Δμ ₃^○ significantly increased in the range of x ₂=0.3 to 0.4 because of an increase in solvent-lysozyme interactions, which resulted from an increase in the accessible surface area of lysozyme that was exposed by its unfolding. The mean value obtained for Δμ ₃^○ per amino acid of lysozyme at x ₂=0.2 is greater than that for hydrophobic amino acids, indicating that the solvent interacts with hydrophilic amino acids more strongly than with hydrophobic ones.