Influence of maltodextrins with different dextrose equivalent on the thermodynamic properties of legumin in a bulk and at the air–water interface

This paper presents the influence of the potato maltodextrins with different dextrose equivalent (DE 2, 6 and 10) on the legumin thermodynamic properties in the bulk aqueous medium and at the air–water interface both in the simple mixed solutions and under the covalent complex (conjugate) formation (by the Maillard reaction), at pH 7.0 and ionic strength of 0.05 mol dm⁻³. The weak net attractive interaction between legumin and maltodextrin has been found in an aqueous medium by both the light scattering and the mixing calorimetry methods. On the basis of both the mixing and differential scanning calorimetry data a hydrogen bonding is supposed to be fundamental for this interaction. It was found that these attractive interactions produced an increase in the protein hydrophilicity and consequently a decrease in the protein surface activity. The effect was more pronounced for the maltodextrin with the largest dextrose equivalent (DE 10). The covalent complexation between legumin and maltodextrin induced the change of the fine hydrophobic–hydrophilic balance in the protein globule due to both addition of the hydrophilicity of the covalently attached polysaccharide and the partial protein unfolding as a result of the such kind of attachment. The combined data of tensiometry, light scattering, mixing and differential scanning calorimetry demonstrated the importance of the maltodextrin polymerization (DE) in controlling both the protein hydrophilicity (thermodynamic affinity for the aqueous phase) and surface activity.     

Thermodynamic and functional properties of legumin (11S globulin from Vicia faba) in the presence of small-molecule surfactants: effect of temperature and pH

We report on the effect of a set of water-dispersible small-molecule surfactants (the main and the longest-hydrocarbon components of which are a citric acid ester of monostearate, a sodium salt of stearol-lactoyl lactic acid, and a polyglycerol ester of stearic acid) on molecular, thermodynamic, and functional properties of the major storage protein of broad beans (Vicia faba) legumin in different molecular states (native, heated, and acid-denatured). The interaction between legumin and the surfactants has been characterized by a combination of thermodynamic methods, namely, mixing calorimetry and multiangle laser static and dynamic light scattering. It was found that hydrogen bonds, electrostatic interactions, and hydrophobic contacts provided a basis for the interactions between the surfactants and both the native and the denatured protein in aqueous medium. Intensive association of the protein molecules in a bulk aqueous medium in the presence of the surfactants was revealed by static and dynamic laser light scattering. In consequence of this, both the surface activity and the gel-forming ability of legumin increased markedly, which has been shown by tensiometry, estimation of protein foaming capacity, and steady-state viscometry. A likely molecular mechanism underlying the effects of small-molecule surfactants on legumin structure-forming properties at the interface and in a bulk aqueous medium is discussed.     

Surface activity at the planar interface in relation to the thermodynamics of intermolecular interactions in the ternary system: maltodextrin-small-molecule surfactant-legumin

We report on the effect of potato maltodextrins with variable dextrose equivalent (Paselli SA-2, SA-6 and SA-10) on the surface behavior at the air-water interface of the mixture: legumin+small-molecule surfactant. Distinct in nature small-molecule surfactants (model: sodium salt of capric acid, Na-caprate; and commercially important: a citric acid ester of monoglyceride, CITREM) have been under our consideration. The role of the structure of both of the maltodextrins and the small-molecule surfactants in the effect studied has been elucidated by measurements in a bulk aqueous medium of the enthalpy of their interaction from mixing calorimetry, value of weight average molecular weight of the maltodextrins and the thermodynamics of the pair maltodextrin-solvent and maltodextrin-protein interactions from laser static light scattering. The combined data of mixing calorimetry and light scattering suggest some complex formation between the small-molecule surfactants and the maltodextrins. Predominantly hydrophobic interactions along with hydrogen bonding form the basis of the complexes. The effect of the maltodextrins on the thermodynamics of the protein heat denaturation and thereby on the protein conformational stability in the presence of the small-molecule surfactants has been studied by differential scanning calorimetry. The interrelation between the thermodynamics of intermolecular interactions in a bulk and the surface behavior at the planar air-water interface of the ternary systems (maltodextrin+legumin+small-molecule surfactant) has been elucidated by tensiometry. The effect of the maltodextrins on the surface activity of mixtures of legumin with the small-molecule surfactants is governed by the competitive in relation to the protein interactions with the small-molecule surfactants and a subsequent change in the thermodynamic properties of the both biopolymers, which are favorable to the ternary complex formation.     

On relationships between molecular structure, interaction and surface behavior in mixture: small-molecule surfactant+protein

We report on the effect of distinct in nature small-molecule surfactants (model, a sodium salt of capric acid, Na-caprate; and commercially important, a citric acid ester of monoglyceride, CITREM; a sodium salt of stearol-lactoyl lactic acid, SSL (Na(+)); polyglycerol ester, PGE (080)) on molecular properties in a bulk and at the air-water interface of globular legumin and random-coiled micellar sodium caseinate. The role of the structure of both proteins and small-molecule surfactants in the effect studied has been elucidated by measurements in a bulk aqueous medium of the enthalpy of their interaction from mixing calorimetry, the change in value of weight average molecular weight of the proteins and the thermodynamics of the pair protein-protein interactions from laser static light scattering as well as, in addition, by measurements of the change in hydrodynamic radius for micellar sodium caseinate from laser dynamic light scattering. The effect of the small-molecule surfactants on the thermodynamics of the protein heat denaturation and thereby on the protein conformational stability has been studied by differential scanning calorimetry in the case of globular legumin. The interrelation between the effects of the small-molecule surfactants on the properties of the proteins in a bulk and at the planar air-water interface has been elucidated by tensiometry. The combined data of mixing calorimetry, differential scanning calorimetry and laser light scattering suggest some complex formation between the small-molecule surfactants and the proteins in a bulk aqueous medium. Predominantly hydrophobic interaction along with electrostatic and hydrogen bonding form the basis of the complex formation. The found effect of the small-molecule surfactants on the surface activity of their mixtures with proteins is governed primarily by both the extent of the protein association, resulting in specific hydrophobicity/hydrophilicity of the surface of the protein associates, and the specific protein conformational stability, for the globular protein, produced by the interaction between the proteins and the small-molecule surfactants.     

The effect of sucrose on the thermodynamic properties of ovalbumin and sodium caseinate in bulk solution and at air–water interface

This paper presents a study of the effect of sucrose on the molecular parameters and thermodynamic properties in a bulk aqueous medium and at the air–water interface for two proteins differing both in nature and structure, that is Na-caseinate and ovalbumin. To get more insight into the molecular nature of the effect of sucrose, mixing calorimetry, light scattering and tensiometry measurements have been made under different pHs (7.0 and 5.5) and temperatures (20–55°C) at an ionic strength of 0.005 mol dm⁻³. Combined temperature dependencies of light scattering and mixing calorimetry testify to hydrogen bonding (sucrose-protein and/or sucrose-water) as being the primary basis of the effect of sucrose on the molecular and thermodynamic properties of the proteins in the bulk and at interface of an aqueous medium. At pH 7.0, in the case of ovalbumin, the interaction with sucrose causes an increase in the protein hydrophilicity in the bulk aqueous medium followed by a decrease in the protein surface activity, whilst for Na-caseinate, there is an increase in the protein hydrophobicity due to Na-caseinate micelle dissociation and, consequently, to an increase in the protein surface activity. Lowering the pH to 5.5, accompanied by a strengthening of the competition between less charged proteins and sucrose for water molecules, induces a rise in the protein hydrophobic aggregation in the bulk. The special features of the latter process are probably mainly responsible for the changes in the surface activity of the proteins under influence of sucrose at pH 5.5.     

Effect of maltodextrins on the surface activity of small-molecule surfactants

We report on the effect of commercially important polysaccharides (maltodextrins with variable dextrose equivalent (Paselli SA-2, MD-6 and MD-10) on the surface activity at the air–water interface of small-molecule surfactants (sms), possessing different hydrophobic–lipophilic balance ((SSL (Na⁺), the main component is a sodium salt of stearol–lactoyl lactic acid, and PGE (080), polyglycerol ester of C18 fatty acid), and widely used in food products. A marked change of the surface activity of sms was found in the presence of maltodextrins by tensiometry. The combined data of laser multiangle light scattering and mixing calorimetry have suggested that this result is governed by specific complex formation between maltodextrins and sms in aqueous medium. Measurements have been made of the molar mass, the second virial coefficient and the enthalpy of intermolecular interactions in aqueous solutions. The implication of a degree of polymerization of maltodextrins in this phenomenon was shown. The interrelation between the molecular parameters of the formed complexes and their surface activity at the air–water interface has been revealed and discussed.     

Incorporation conditions guiding the aggregation of a glycosylphosphatidyl inositol (GPI)-anchored protein in Langmuir monolayers

This work investigates the process of incorporation of a glycosylphosphatidyl inositol (GPI)-anchored alkaline phosphatase into Langmuir monolayers of dimyristoyl phosphatidic acid (DMPA). Three different methods of protein incorporation were assayed. When the protein solution was injected below the air–water interface after formation of the lipid monolayer a micro-heterogeneous distribution of alkaline phosphatase throughout the interface was observed. Adsorption kinetics studied by fluorescence microscopy, associated with surface pressure measurements, led to the proposition of a model in which the protein penetration is modulated by the surface packing of the monolayer and intermolecular interactions occurring between the phospholipid and the protein. At initial surface pressures higher than 20 mN m⁻¹, the protein is quickly adsorbed on the interface and the lateral diffusion drives the alkyl chains to turn towards the air phase while the polypeptide moiety faces the aqueous subphase.     

Interaction of monolayers of calix[4]resorcinarene derivatives with copper ions in the aqueous subphase

Stable monolayers of novel amphiphilic calix[4]resorcinarene derivates at the air–water interface were prepared. Their interactions with copper ions from the aqueous subphase were investigated by measuring surface pressure–area and surface potential–area isotherms, as well as by Brewster angle microscopy. Theoretical aspects of interpreting the dependence of the surface pressure on the bulk copper ions concentration were discussed. The interaction of copper ions with calix[4]resorcinarene derivates was interpreted in terms of Gibbs–Shishkovsky adsorption equation.     

Effects of denaturation on soy protein-xanthan interactions: comparison of a whipping-rheological and a bubbling method

The effect of xanthan on foam formation and on physical mechanisms of destabilization involved in the breakdown of foams made from native and denatured soy protein at neutral pH was studied by a bubbling and a whipping-rheological method. Parameters describing foam formation and destabilization by liquid drainage and disproportionation obtained by the two methods showed that the addition of xanthan was accompanied by delayed rates of drainage and disproportionation and reduced foam height decay (collapse). Drainage showed the largest reduction, mainly because of the increased bulk viscosity. In the absence of xanthan, protein denaturation enhanced foam formation and stability against drainage and disproportionation, but increased the collapse of foams. In the presence of xanthan, differences in foam formation and drainage/disproportionation stability between native and denatured soy protein were greatly reduced. However, differences in foam collapse were greatly enhanced. The increased stability of foams in the presence of xanthan could not be explained purely in terms of increased aqueous phase viscosity. More specific interactions of xanthan and soy proteins at the air-water interface influencing the surface rheology, and the protein composition and aggregation, are involved.     

Anion Effects on Sodium Ion and Acid Molecule Adduction to Protein Ions in Electrospray Ionization Mass Spectrometry

Gaseous protein–metal ion and protein–molecule complexes can be readily formed by electrospray ionization (ESI) from aqueous solutions containing proteins and millimolar concentrations of sodium salts of various anions. The extent of sodium and acid molecule adduction to multiply charged protein ions is inversely related and depends strongly on the proton affinity (PA) of the anion, with extensive sodium adduction occurring for anions with PA values greater than ~300 kcal·mol^–1 and extensive acid molecule adduction occurring for anions with PA values less than 315 kcal·mol^–1. The role of the anion on the extent of sodium and acid molecule adduction does not directly follow the Hofmeister series, suggesting that direct protein–ion interactions may not play a significant role in the observed effect of anions on protein structure in solution. These results indicate that salts with anions that have low PA values may be useful solution-phase additives to minimize nonspecific metal ion adduction in ESI experiments designed to identify specific protein-metal ion interactions.     

Influence of specific anions on the orientational ordering of thermotropic liquid crystals at aqueous interfaces

We report that specific anions (of sodium salts) added to aqueous phases at molar concentrations can trigger rapid, orientational ordering transitions in water-immiscible, thermotropic liquid crystals (LCs; e.g., nematic phase of 4'-pentyl-4-cyanobiphenyl, 5CB) contacting the aqueous phases. Anions classified as chaotropic, specifically iodide, perchlorate, and thiocyanate, cause 5CB to undergo continuous, concentration-dependent transitions from planar to homeotropic (perpendicular) orientations at LC-aqueous interfaces within 20 s of addition of the anions. In contrast, anions classified as relatively more kosmotropic in nature (fluoride, sulfate, phosphate, acetate, chloride, nitrate, bromide, and chlorate) do not perturb the LC orientation from that observed without added salts (i.e., planar orientation). Surface pressure-area isotherms of Langmuir films of 5CB supported on aqueous salt solutions reveal ion-specific effects ranking in a manner similar to the LC ordering transitions. Specifically, chaotropic salts stabilized monolayers of 5CB to higher surface pressures and areal densities (12.6 mN/m at 27 ?(2)/molecule for NaClO(4)) and thus smaller molecular tilt angles (30° from the surface normal for NaClO(4)) than kosmotropic salts (5.0 mN/m at 38 ?(2)/molecule with a corresponding tilt angle of 53° for NaCl). These results and others reported herein suggest that anion-specific interactions with 5CB monolayers lead to bulk LC ordering transitions. Support for the proposition that these ion-specific interactions involve the nitrile group was obtained by using a second LC with nitrile groups (E7; ion-specific effects similar to 5CB were observed) and a third LC with fluorine-substituted aromatic groups (TL205; weak dipole and no ion-specific effects were measured). Finally, we also establish that anion-induced orientational transitions in micrometer-thick LC films involve a change in the easy axis of the LC. Overall, these results provide new insights into ionic phenomena occurring at LC-aqueous interfaces, and reveal that the long-range ordering of LC oils can amplify ion-specific interactions at these interfaces into macroscopic ordering transitions.     

Self-aggregation of ionic C(10) surfactants having different headgroups with special reference to the behavior of decyltrimethylammonium bromide in different salt environments: a calorimetric study with energetic analysis

Self-aggregation of C10 ionic surfactants with different head groups, viz., decylpyridinium chloride, sodium decylsulfate, decylammonium bromide, decyldimethylammonium bromide, and decyltrimethylammonium bromide, was studied in the aqueous medium by microcalorimetric and conductometric methods. The effects of temperature and different salts (NaF, NaCl, NaBr, NaI, Na2SO4, Na2S2O7, Na-benzoate, and Na-salicylate) were also studied on decyltrimethylammonium bromide representatives. The cmc, counterion binding, and energetics of micellization were evaluated and discussed. The energetic parameters, enthalpy, entropy, and specific heat of micellization obtained from direct calorimetry and the indirect van't Hoff method were compared and discussed.     

Citric acid adsorption on TiO2 nanoparticles in aqueous suspensions at acidic and circumneutral pH: surface coverage, surface speciation, and its impact on nanoparticle-nanoparticle interactions

Citric acid plays an important role as a stabilizer in several nanomaterial syntheses and is a common organic acid found in nature. Here, the adsorption of citric acid onto TiO(2) anatase nanoparticles with a particle diameter of ca. 4 nm is investigated at circumneutral and acidic pHs. This study focuses on both the details of the surface chemistry of citric acid on TiO(2), including measurements of surface coverage and speciation, and its impact on nanoparticle behavior. Using macroscopic and molecular-based probes, citric acid adsorption and nanoparticle interactions are measured with quantitative solution phase adsorption measurements, attenuated total reflection-FTIR spectroscopy, dynamic light scattering techniques, and zeta-potential measurements as a function of solution pH. The results show that surface coverage is a function of pH and decreases with increasing pH. Surface speciation differs from the bulk solution and is time dependent. After equilibration, the fully deprotonated citrate ion is present on the surface regardless of the highly acidic solution pH indicating pK(a) values of surface adsorbed species are lower than those in solution. Nanoparticle interactions are also probed through measurements of aggregation and the data show that these interactions are complex and depend on the detailed interplay between bulk solution pH and surface chemistry.     

Copper-metallomesogen structures obtained by ionic self-assembly (ISA): molecular electromechanical switching driven by cooperativity

In a stepwise noncovalent multiple-interaction strategy, copper(II) salts were complexed with the sodium salts of bathophenanthrolinedisulfonic acid (BPS) and bathocuproinedisulfonic acid (BCS), and organized into nanostructured materials by the addition of ammonium surfactants by means of the ionic self-assembly (ISA) route. In the case of the methyl-substituted BCS complexes, a slow color change from green to brick red was observed. UV and EPR investigations showed that the color change was due to a change in oxidation state, the resulting brick red color is typical for Cu(I) species. It is concluded that steric interactions and mechanical packing into a supramolecular structure drive this electronic transition at the metal center. When complexation is performed with double-tail ammonium surfactants, these metallomesogenic materials exhibit thermotropic liquid-crystalline phase behavior, as investigated by polarized light microscopy, differential scanning calorimetry (DSC), and temperature-dependent wide-angle and small-angle X-ray analyses. The complexity of the observed phases increased with increasing tail length of the surfactants. Complexation with double-tail C(18) surfactants yielded highly organized materials for both the BPS and BCS ligands.     

New insight into the surface denaturation of proteins: electronic sum frequency generation study of cytochrome c at water interfaces

In situ characterization of surface denaturation of a protein was realized by newly developed interface-selective multiplex electronic sum frequency generation spectroscopy. The observed electronic spectra of cytochrome c at the air/water interface exhibited a broad feature, which demonstrated coexistence of the nativelike and denatured protein at the interface. This situation of the mixed conformation at the air/water interface did not change in the acidic condition of pH=2 where the protein was completely denatured in the bulk water. In sharp contrast, only native spectrum was observed at the silica/water interface.     

Influence of non-covalent interactions on the thermodynamic stereoselectivity of the protonation of some dipeptides

ΔG⁰, ΔH⁰ and ΔS⁰ protonation values of some pairs of diastereoisomeric dipeptides have been determined by potentiometry and calorimetry in aqueous solution at 25°C and I = 0.1 mol dm⁻³ (KNO₃). On the basis of the results obtained it has been possible to assess the role played by two different non-covalent interactions, namely the electrostatic interaction and the solvophobic interaction, on the thermodynamic stereoselectivity in the proton complex formation, shown by the systems investigated.     

Estimation of critical micelle concentrations of bile acids by reversed-phase high performance liquid chromatography

The critical micelle concentration (CMC) for bile salts or other surfactants is defined as that solute concentration at which appreciable changes in such phenomena as light scattering, surface tension, or solubilization of other organic molecules occur, these changes indicating appearance of surfactant aggregates. The CMC thus reflects hydrophobic interactions of the surfactant with itself. The self-association of hydrophobic molecules resembles the partition of a solute into the lipophilic phase in reversed-phase high performance liquid chromatography (RPLC): Both processes can be considered as transfers of a molecule from an aqueous to a lipophilic medium. The critical micelle concentration of a particular bile salt, being a measure of its hydrophobic self-association, should therefore be correlated with its Chromatographic mobility since they are fundamentally related phenomena. Experimentally, significant correlations between these quantities are obtained, both for bile salts andn-alky1 sulfonates, and only microgram amounts of sample are required for RPLC measurements. Among three homologous series of bile salt surfactants, CMC values predicted from RPLC measurements agree, within a standard error of 7%, with CMC values determined directly. This suggests the applicability of reversed-phase liquid chromatography to the micro-scale determination of critical micelle concentrations of bile salts,n-alkyl sulfonates, and other homologous series of surfactants.This work was supported in Part by NIH Grants HL-07878 (W.H.E.) and AI-21873 (B.G.B.) and by a Fulbright Senior Fellowship (B.G.B.). This is paper LXXX in the series Bile Acids by W.H.E.Deceased March 29, 1986     

Quantitative approach to conformations of proteins

A quantitative conformational theory of proteins is developed that enables one to predict the native structure of a protein from its amino acid sequence. The theory is based on the following principles: (1) the spatial structure and conformational properties of a protein are predetermined by its amino acid sequence; (2) the native conformation of a protein corresponds to the free energy minimum; (3) all interactions within a protein molecule are specified as short-, mediumy-, and long-range types, interactions of different types being consistent with each other. The role of the short-, medium-, and long-range interactions in the spatial organization of a protein globule is discussed, and a step-by-step analysis of amino acid sequences with gradually increasing lengths is presented. The proposed theory is based on a semiempirical computational method that involves quantitative evaluation of all pairwise atomic interactions within a protein molecule in an aqueous medium. Examples illustrating the suggested approach are presented.     

Hydrophobic and electrostatic interactions in adsorption of surface-active substances. Tetraalkylammonium salts

A relationship between the standard free energies of adsorption from aqueous solution at the oil/water interface and the radii of organic cations as exemplified by symmetric tetraakylammonium salts has been studied. Hydrophobic effects are shown to be major contributors to the interaction of surfactants with the interface. An adsorption coefficient to quantitate the hydrophobic effects and to specify the changes of standard adsorption energy depending upon the cavity surface area of the detergent hydrocarbon radical in aqueous solution has been proposed. A new formulation of the Traube rule, taking into account the hydrophobic effects concomitant with a transfer of surfactants from the water bulk onto the interface, has also been given.Standard free energies for the adsorption of organic and inorganic ions from aqueous solution at the interface of immiscible liquids have been found. The proposed method is based on an extrapolation of the relationship between the standard adsorption energy of tetraalkylammonium salts and the square of cationic radius to zero ionic radius. The standard free energy of adsorption for an inorganic counter-ion is derived from an intercept on the y-axis cut off by a straight line. The experimental adsorption data on inorganic salts have been used to calculate the standard free energies of adsorption for a variety of ions.A method of estimating the difference in potential at the oil/water interface between the adsorption plane and the aqueous solution has been proposed. The sign of potential provides a clue to the orientation of water molecules at the interface between immiscible liquids.