Cross-interaction Coefficients Research Articles

Modulating Weak Protein-Protein Cross-Interactions by the Addition of Free Amino Acids at Millimolar Concentrations.

In this paper, we quantify weak protein-protein interactions in solution using cross-interaction chromatography (CIC) and surface plasmon resonance (SPR) and demonstrate that they can be modulated by the addition of millimolar concentrations of free amino acids. With CIC, we determined the second osmotic virial cross-interaction coefficient (B23) as a proxy for the interaction strength between two different proteins. We perform SPR experiments to establish the binding affinity between the same proteins. With CIC, we show that the amino acids proline, glutamine, and arginine render the protein cross-interactions more repulsive or equivalently less attractive. Specifically, we measured B23 between lysozyme (Lys) and bovine serum albumin (BSA) and between Lys and protein isolates (whey and canola). We find that B23 increases when amino acids are added to the solution even at millimolar concentrations, corresponding to protein/ligand stoichiometric ratios as low as 1:1. With SPR, we show that the binding affinity between proteins can change by 1 order of magnitude when 10 mM glutamine is added. In the case of Lys and one whey protein isolate (WPI), it changes from the mM to the M range, thus by 3 orders of magnitude. Interestingly, this efficient modulation of the protein cross-interactions does not alter the protein's secondary structure. The capacity of amino acids to modulate protein cross-interactions at mM concentrations is remarkable and may have an impact across fields in particular for specific applications in the food or pharmaceutical industries.

Binding and stability properties of PEG2000 to globular proteins: The case of lysozyme

• Lysozyme-PEG interaction was studied using calorimetry and spectroscopic techniques. • A PEG/lysozyme interaction in the low polymer concentration regime occurs. • The interaction process is weak, and it is entropically driven. • At low concentration, PEG induces an increase of the denaturation temperature. Poly(ethylene glycol) is a linear polymer widely used in many biotechnological and industrial applications. It is thought that PEG is an inert polymer and its effect on protein stability and dynamics is essentially due to the excluded volume effect which depends on its concentration. However, increasing evidence suggests that direct interactions can be established between PEG and proteins. With the aim to shed more light on PEG-protein interactions, this work evaluated the enthalpy pair interaction coefficient and the enthalpy cross-interaction coefficient for the system PEG2000-lysozyme. In addition, using calorimetry and spectroscopy, the binding, the stability and the conformational behaviour of lysozyme were evaluated in the low polymer concentration regime. We found that a weak interaction between lysozyme and PEG occurs with a binding constant of 1600 ± 230 M −1 . The interaction process is mainly entropically driven: the release of water molecules associated to protein surface and polymer chains may account for this phenomenon. In addition, we found that the general lysozyme structure was only marginally affected by PEG interaction. A small but significant increase in the denaturation temperature was observed most likely due to weak interactions with the polymer and to the dehydration of protein surface.

New findings for the old problem: Exact solutions for domain walls in coupled real Ginzburg-Landau equations

This work reports new exact solutions for domain-wall (DW) states produced by a system of coupled real Ginzburg-Landau (GL) equations which model patterns in thermal convection, optics, and Bose-Einstein condensates (BECs). An exact solution for symmetric DW was known for a single value of the cross-interaction coefficient, G=3 (defined so that its self-interaction counterpart is 1). Here an exact asymmetric DW is obtained for the system in which the diffusion term is absent in one component. It exists for all G>1. Also produced is an exact solution for DW in the symmetric real-GL system which includes linear coupling. In addition, an effect of a trapping potential on the DW is considered, which is relevant to the case of BEC. In a system of three GL equations, an exact solution is obtained for a composite state including a two-component DW and a localized state in the third component. Bifurcations which create two lowest composite states are identified too. Lastly, exact solutions are found for the system of real GL equations for counterpropagating waves, which represent a sink or source of the waves, as well as for a system of three equations which includes a standing localized component.

Interaction-induced mode switching in steady-state microlasers.

We demonstrate that due to strong modal interactions through cross-gain saturation, the onset of a new lasing mode can switch off an existing mode via a negative power slope. In this process of interaction-induced mode switching (IMS) the two involved modes maintain their identities, i.e. they do not change their spatial field patterns or lasing frequencies. For a fixed pump profile, a simple analytic criterion for the occurrence of IMS is given in terms of their self- and cross-interaction coefficients and non-interacting thresholds, which is verified for the example of a two-dimensional microdisk laser. When the spatial pump profile is varied as the pump power is increased, IMS can be induced even when it would not occur with a fixed pump profile, as we show for two coupled laser cavities. Our findings apply to steady-state lasing and are hence different from dynamical mode switching or hopping. IMS may have potential applications in robust and flexible all-optical switching.

Enthalpies of Dilution and Enthalpies of Mixing of Amino Acids with Sucrose in Aqueous Solutions at 298.15 K

The enthalpies of mixing of aqueous solutions have been determined for sucrose with six different amino acids (glycine, l-alanine, l-serine, l-valine, l-proline and l-threonine) at 298.15 K, by using a LKB-2277 flow microcalorimetric system. These results, along with the enthalpies of dilution of these solutes for the initial solutions, were used to determine the enthalpic interaction coefficients (hxy, hxyy, hxxy) of the McMillan–Mayer Theory. The pair-wise cross interaction coefficients of amino acids and sucrose are discussed from the viewpoint of solute–solute interactions.

The base-catalysed cyclisation of phenyl N-(2-hydroxybenzyl)-N-methylcarbamates is concerted.

The kinetics of the cyclisation in aqueous solution of phenyl-(2-hydroxybenzyl)-N-methylcarbamates to 3-methyl- 3,4-dihydrobenzo[e][1,3]oxazin-2-ones and phenolate ions fit the rate law: kobs = kc/(1 + [H3O+]/Ka) The values of kc and pKa fit Brønsted equations against the pKa's of the corresponding free phenols but the system does not conform to the reactivity-selectivity hypothesis. The values of the Brønsted parameters beta Y and beta X vary as a function of Y and X according to the equations: beta X = -0.179pKaHY + 0.87 beta Y = -0.179pKaHX + 2.30 The magnitude and sign of the Cordes-Thornton cross-interaction coefficient pXY (-0.179) rule out a stepwise mechanism involving a tetrahedral intermediate and is consistent with a concerted displacement mechanism. A similar concerted mechanism is proposed for the base-catalysed cyclisation of phenyl-N-(2-hydroxyphenyl)-N-methylcarbamate esters to benzoxazol-2-ones.

The decomposition of methyl hemiacetals of benzaldehyde in aqueous solution: a study of the effect of aromatic substitution

The acid-base catalysed decomposition of hydrates and hemiacetals of carbonyl compounds are classical examples of reactions where (slow) proton transfer is coupled with heavy atom reorganization, i.e., C—O bond breaking and solvent reorganization. We have studied the influence of m- and p-substitution in the carbonyl electrophile on the kinetics of the acid and base catalysis of the decomposition of methyl hemiacetals of benzaldehyde. The experimental data are well described by three-dimensional More O'Ferrall - Jencks energy contour diagrams according to principles developed by Jencks (the BEMA HAPOTHLE). Thus, for acid catalysis, a Cordes cross-interaction coefficient pxy' = δρ/δpKa = 0.15 indicates the coupled nature of the rate-limiting step in a class e mechanism, similar to conclusions reached from systematic substitution in the nucleophile. Our more extensive set of data for base catalysis permits a more rigorous analysis according to the BEMA HAPOTHLE. The data are consistent with a class n mechanism as also suggested earlier on the basis of substitution in the nucleophile. A slight upward curvature observed in the Hammett plots for the various catalysts is described by the direct correlation parameter py = -δρ/δσ = -0.11. This second derivative demonstrates the concerted nature of the C—O bond cleavage and O-H formation in the transition state, which changes with changing substituent. A class n mechanism for base catalysis is also supported by the observation of a Cordes cross-interaction parameter pxy = -δρ/δpKa = -δβ/δσ = 0.03, which describes the experimentally observed decrease in Hammett ρ with increasing pKa of the catalyst. This change may be rationalized by the movement of a saddle point on a diagonal reaction coordinate in the energy contour diagram, as a resultant of shifts parallel and perpendicular to the coordinate, when the energy along one side of the diagram is changed. It is concluded that observed rate changes as a result of substitution in the electrophile are consistent with and present further confirmation of earlier suggested mechanisms of hemiacetal decomposition reactions.Key words: methyl hemiacetals of benzaldehydes, acid-base catalysed breakdown, Hammett plots, More O'Ferrall - Jencks diagrams.

The decomposition of methyl hemiacetals of benzaldehyde in aqueous solution: a study of the effect of aromatic substitution

The acid-base catalysed decomposition of hydrates and hemiacetals of carbonyl compounds are classical examples of reactions where (slow) proton transfer is coupled with heavy atom reorganization, i.e., C—O bond breaking and solvent reorganization. We have studied the influence of m- and p-substitution in the carbonyl electrophile on the kinetics of the acid and base catalysis of the decomposition of methyl hemiacetals of benzaldehyde. The experimental data are well described by three-dimensional More O'Ferrall - Jencks energy contour diagrams according to principles developed by Jencks (the BEMA HAPOTHLE). Thus, for acid catalysis, a Cordes cross-interaction coefficient pxy' = δρ/δpKa = 0.15 indicates the coupled nature of the rate-limiting step in a class e mechanism, similar to conclusions reached from systematic substitution in the nucleophile. Our more extensive set of data for base catalysis permits a more rigorous analysis according to the BEMA HAPOTHLE. The data are consistent with a class n mechanism as also suggested earlier on the basis of substitution in the nucleophile. A slight upward curvature observed in the Hammett plots for the various catalysts is described by the direct correlation parameter py = -δρ/δσ = -0.11. This second derivative demonstrates the concerted nature of the C—O bond cleavage and O-H formation in the transition state, which changes with changing substituent. A class n mechanism for base catalysis is also supported by the observation of a Cordes cross-interaction parameter pxy = -δρ/δpKa = -δβ/δσ = 0.03, which describes the experimentally observed decrease in Hammett ρ with increasing pKa of the catalyst. This change may be rationalized by the movement of a saddle point on a diagonal reaction coordinate in the energy contour diagram, as a resultant of shifts parallel and perpendicular to the coordinate, when the energy along one side of the diagram is changed. It is concluded that observed rate changes as a result of substitution in the electrophile are consistent with and present further confirmation of earlier suggested mechanisms of hemiacetal decomposition reactions.Key words: methyl hemiacetals of benzaldehydes, acid-base catalysed breakdown, Hammett plots, More O'Ferrall - Jencks diagrams.

The role of preferential interactions between similar domains in determining the behavior of aqueous solutions of aminoalkanols. A microcalorimetric study

Enthalpies of dilution of binary aqueous solutions containing 1 mol kg −1 NaOH as cosolvent and the homologous series of α,ω-aminoalkanols and 2-amino-1-alkanols as solutes have been determined by flow microcalorimetry at 298 K. Enthalpic self and cross interaction coefficients of the virial expansion of the excess enthalpies were evaluated. These coefficients are positive and increase at increasing alkyl chain length. The difference between the coefficients of positional isomers of aminoalkanols resembles that shown by the corresponding diols in water. The cross interaction coefficient relative to the ternary system containing hexane-1,6-diamine and hexane-1,6-diol is smaller than those relative to the respective binary solutions. The coefficients were interpreted using the “preferential configuration” model: it is inferred that interactions between like hydrophilic groups are preferred when compared to the like–unlike ones.

A model for the interaction between hydrophilic and hydrophobic solutes. A calorimetric study of the aqueous solutions containing alkylureas and urea at 298.15 K

The heats of dilution of ternary aqueous solutions of urea and alkylureas and of ternary aqueous solution of mono, di and tetra alkylureas have been determined calorimetrically at 298.15 K. Pairwise cross interaction coefficients of the virial expansion of the excess enthalpies were determined. The behaviour of these systems was interpreted in terms of preferential interactions between hydrophilic domains and of repulsive interactions between hydrophobic and hydrophilic domains.

A model for the interactions involving hydrophilic structure maker solutes

Enthalpies of dilution of ternary aqueous solutions containing glucose and alkan-1-ols, alkan-1,2-diols and alkan-m,n-diols were determined at 298.15 K by flow microcalorimetry. The pair-wise cross interaction coefficients of the virial expansion of the excess enthalpies were evaluated: they are positive and depend in a complex manner on the length of the alkyl chain of the alkanols. The behaviour of these systems is interpreted in terms of preferential interactions between the hydrophilic groups of the alkanols and the destructured domain present on glucose.

Molecular recognition: A thermodynamic study of non bonded interactions in aqueous solutions of iminoacids. Effects of steric and geometric isomerism

Enthalpies of dilution of binary and ternary aqueous solutions containing L-and D-proline, cis-L- and cis-D-4-hydroxy proline, trans-L-4-hydroxy proline, have been determined by flow calorimetry at 25°C. Pairwise self-and cross-interaction coefficients of the virial expansion of the excess enthalpies were evaluated. They are positive for solutions of unsubstituted prolines. On the contrary, upon the introduction of a hydroxy group, they become negative, reaching the largest value for the cis-L-lcis-D-4-hydroxy proline system. The results are interpreted in terms of preferential configurations, stabilized by interactions between the hydrophilic groups.

Molecular recognition and effects of steric and geometric isomerism. Thermodynamic study of non-bonding interactions in aqueous solutions of cycloalkanols

Enthalpies of dilution of binary and ternary aqueous solutions containing cyclobutanol, cyclopentanol, cyclohexanol, cycloheptanol, cyclooctanol, (1R,2R)-trans-cyclohexane-1,2-diol, (1S,2S)-trans-cyclohexane-1,2-diol, (1R,2S)-cis-cyclohexane-1,2-diol, (1R,2R)-trans-cyclopentane-1,2-diol, (1S,2S)-trans-cyclopentane-1,2-diol, (1R,2S)-cis-pentane-1,2-diol, cis-cyclooctane-1,2-diol, cis-cyclooctane-1,4-diol, cis-cyclooctane-1,5-diol, cis,cis-cyclohexane-1,3,5-triol and myo-inositol were determined by microcalorimetry at 298.15 K. Pairwise self- and cross-interaction coefficients of the virial expansion of the excess enthalpies were evaluated. The values of these coefficients are positive, depending in a very complex manner on the number of the hydroxy groups on the cyclic molecule and on steric and geometric isomerism. They have been interpreted according to an interaction model which indicates a preferential configuration between two hydrated interacting molecules, stabilized by the simultaneous juxtaposition of groups having the same action on water structure.

Preferential configurations in solution. Calorimetric study of ternary aqueous systems containing a destructuring agent and derivatives of α-amino acids at 298 K

Enthalpies of dilution of ternary aqueous solutions containing α-amino acids or their N-acetyl or N-acetylamide derivatives and urea, glycine, N-acetylglycine and N-acetylglycinamide have been determined by flow microcalorimetry at 298.15 K. Pairwise cross-interaction coefficients of the virial expansion of the excess enthalpies were evaluated. The behaviour of these systems seems to be determined by a balance between favourable hydrophilic–hydrophilic and repulsive hydrophilic–hydrophobic interactions. A common behaviour was shown by every studied series of substances, independently of the nature of the destructuring agent. The pairwise cross-interaction coefficients increase with increasing length of the alkyl side chains until a plateau is attained. This seems to be a general rule when homologous series of solutes having hydrophobic domains interact with a structure-breaker solute.

The role of hydrophilic interactions in determining preferential configurations in aqueous solution: alkan-1-ols, alkan-1,2-diols, alkan-α,ω-diols, and α-aminoacids interacting with glycine and its oligomers at 298.15 K

Enthalpies of dilution of ternary aqueous solutions containing glycine, diglycine or triglycine and alkan-1-ols, alkan-1,2-diols, alkan-α,ω-diols, and α-aminoacids have been determined by microcalorimetry at 298.15 K. Pairwise cross interaction coefficients of the virial expansion of the excess enthalpies were evaluated. The values of these coefficients are positive, depending in a very complex manner on the number and position of the hydroxyl groups. The behaviour of these systems seems to be determined by a balance between favourable hydrophilic—hydrophilic and repulsive hydrophilic—hydrophobic interactions. The chaotropic agents employed play a major role: they are more complex than other purely hydrophilic substances, such as urea or biuret, because of the presence of hydrophobic domains on their molecules.

Mechanism for the General Base Catalyzed Solvolysis of Silyl Ethers

The observed general base catalysis of the solvolysis of dimethylphenylphenoxysilane occurs by a mechanism where the catalyzing base assists by removing a proton from the attacking solvent molecule as the silicon-oxygen bond forms, i.e. true general base catalysis, and not by the kinetically equivalent mechanism of specific base general acid catalysis. This conclusion is based on structure-reactivity relationships with a positive cross interaction coefficient, p xy' =∂β/-∂pK Nuc =∂β Nuc /∂pK Catalyst =0.08, that would be expected for a mechanism involving true general base catalysis

Improved procedures for determining second-derivative cross-interaction coefficients and for establishing a relationship between the value and reaction type

In a Hammett-type study of the second-derivative kinetic influences of dual substitution, one can use internally generated substituent constants, avoiding the need to choose from among the various substituent constant scales prior to statistical analysis; this method leads to a better, often considerably better, analysis than similar methods using external constants. It is shown that the Palm α values generated by this procedure can give a better classification of reaction type than the previously used uncalibrated ρxy values.

The relevance of third-derivative cross-interaction coefficients in Hammett-type treatments of nucleophilic substitution reactions

Six nucleophilic substitution reactions of (Y)-phenylalkyl or (Y)-phenacyl (Z)-arenesulfonates with (X)-anilines or (X)-benzylamines are analysed in terms of a modified Hammett equation incorporating both second- and third-derivative cross-interaction coefficients. For five of the six reported analyses, the standard errors associated with the third-derivative value (ρXYZ) are large and the probabilities that the contribution to the overall analysis is statistically insignificant are very high (0.53–0.92). Although we confirm a previous report that the sixth system appears to have a marginally acceptable ρXYZ value, minor adjustments of six of the 48 rate coefficients (so as better to accommodate trends within the matrix of values) leads to improved correlations, an appreciably reduced ρXYZ(or ρXYZin) value, and a 0.45 probability that the term involving ρXYZ is not statistically significant. Extremely fast methanolyses of the more reactive 1-phenylethyl arenesulfonates are predicted and previously reported studies, by conventional techniques, of their reactions with anilines, in methanol as the solvent, need to be reevaluated.

Role of functional-group position in determining the interaction between structure-making and structure-breaking solutes. A microcalorimetric study of the interactions in aqueous solutions between positional isomers of alkan-m-ols and urea or biuret

Enthalpies of dilution of ternary aqueous solutions containing biuret or urea and positional isomers of alkan-m-ols have been determined by flow microcalorimetry at 298.15 K. Pairwise cross-interaction coefficients of the virial expansion of the excess enthalpies were evaluated. The values of the coefficients are positive, depending in a very complex manner on the length of the alkyl chain and on the position occupied by the hydroxy group along the alkyl chain. The behaviour of these systems seems to be determined by a balance between favourable hydrophilic–hydrophilic and repulsive hydrophilic–hydrophobic interactions. When the hydroxy group occupies more central positions, unfavourable interactions prevail and the cross coefficient for the alkanol bearing the longest alkyl chain is almost zero.

Role of hydrophilic interactions in sustaining preferential orientations: calorimetric and nuclear magnetic resonance studies of ternary aqueous systems containing a destructuring agent and hydroxylated compounds at 298.15 K

Enthalpies of dilution of ternary aqueous solutions containing biuret or urea and alkane-n, m-diols have been determined by flow microcalorimetry at 298.15 K. The pairwise cross-interaction coefficients of the virial expansion of the excess enthalpies were determined. The coefficients are positive and depend in a complex manner on the length of the alkyl chain of the hydroxylated substances. A balance between favourable hydrophilic–hydrophilic and repulsive hydrophilic–hydrophobic interactions is the factor that determines the behaviour of these systems. NMR relaxation time measurements have also been performed: however, this technique does not show the complexity unravelled by calorimetry. For binary solutions of α,ω-diols, a correlation is presented between the relaxation times and the enthalpic self-interaction coefficients.