Self-association Equilibrium Constants Research Articles

Experimental data and modeling for excess enthalpies of 2-Pentanol with n-alkanes (C7–C9) at T = (293.15, 298.15 and 303.15) K

Excess molar enthalpies, HE, for the binary mixtures of 2-pentanol with n-alkanes(n-heptane, n-octane, and n-nonane) have been determined at three different temperatures T = (293.15, 298.15 and 303.15) K and normal atmospheric pressure over the entire composition range using a Calvet microcalorimeter. All mixtures show endothermic mixing with the maximum values of the excess enthalpies occurring in the n-alkane-rich region. The HE data are smoothed using Redlich–Kister equation. The applicability of the Treszczanowicz–Benson, ERAS,Renon–Prausnitz and Chen–Bagley models to correlate HE of studied mixtures is tested, and the agreement between experimental and theoretical results is satisfactory. Each model includes a self-association equilibrium constant that represents hydrogen bonding and an adjustable parameter that reflects physical interactions.

Using Sedimentation Velocity to Investigate the Nucleotide-Linked Assembly of E. coli ClpA

The AAA+ (ATPases Associated with various cellular Activities) group of proteins is a large superfamily whose members are present in single-celled and multi-celled organisms. This family is subdivided into two classes; Class I ATPases contain two ATPase domains per monomer while Class II ATPases contain one ATPase domain per monomer. Both classes utilize ATP binding and hydrolysis to undergo conformational changes that allow them to perform mechanical work. Further, the biologically active form for most members of the AAA+ family is a hexamer. Quantitatively examining the biologically-relevant activities for AAA+ proteins requires a precise determination of the concentration of hexamers in solution. The difficulty in determining the hexamer concentration is that the oligomeric state of interest often resides in a dynamic equilibrium. This leads to a distribution of oligomeric states in solution. Therefore, insight into nucleotide-linked assembly is required to be able to understand how any of the AAA+ motors function. ClpA is a Class I AAA+ protein. Apparent self-association equilibrium constants were obtained using sedimentation velocity and were subsequently analyzed using binding polynomials that incorporate either zero cooperativity or infinite cooperativity into the nucleotide binding process to yield apparent nucleotide binding constants. Here we show that the population of ClpA dimers, tetramers, and hexamers is dependent upon nucleotide concentration. Further, apparent nucleotide binding constants were found for each oligomeric state. The above-stated information was used to generate a prediction of ClpA hexamer population that takes into account nucleotide-linked assembly. We anticipate the results of this study will set the stage for being able to predict the concentration of hexamers at any given nucleotide concentration, therefore allowing us to probe the kinetic and energetic properties of ClpA as well as other members of the AAA+ superfamily.

Temperature dependence of associative equilibria of DMSO according to raman scattering spectra

Raman scattering spectra of dimethyl sulfoxide (DMSO) are studied in the area of the line corresponding to symmetric CSC stretching vibrations of the molecule. It is established that this line is composed of a low-frequency component that corresponds to the vibrations of monomeric molecules and a high-frequency component that corresponds to the vibrations of DMSO dimers. Values of self-association equilibrium constants Ka varying in range from 0.20 (23°C) to 0.081 (100°C) are obtained. Since the intensities of the respective components of the line contour are proportional to the compound’s concentrations, the enthalpy of DMSO self-association (ΔH = −11.7 ± 0.9 kJ/mol) is determined from the temperature dependences.

On Modulating the Phase Behavior of Block Copolymer/Homopolymer Blends via Hydrogen Bonding

We have investigated the phase behavior of poly(4-vinylphenol-b-styrene) (PVPh-b-PS) when respectively blended with poly(4-vinylpyridine) (P4VP), poly(methyl methacrylate) (PMMA), and PVPh homopolymers by mediated hydrogen bonding strengths with the PVPh block of the copolymer. The Fourier transform infrared spectroscopic result indicates that the PVPh-b-PS/P4VP blend has a much higher fraction (fH) of hydrogen-bonded PVPh blocks for a significantly higher miscibility compared with the blends with PMMA and PVPh homopolymers. Consequently, the PVPh-b-PS/P4VP blend, behaving as a neat diblock copolymer, exhibited a series of order−order phase transitions from the lamellar, gyroid, hexagonally packed cylinder to body-centered cubic structures when the P4VP content increases from 6 to 71% (volume fraction), as evidenced consistently by transmission electron microscopy and small-angle X-ray scattering. In contrast, both the PVPh-b-PS/PMMA and PVPh-b-PS/PVPh blends maintained essentially the lamellar structure upon a similar volume fraction increase in the homopolymers; the lamellar structure, however, was distorted to different extents at higher volume fractions of the additives, depending on the hydrogen bonding strength. On the basis of the results, the ratio of interassociation equilibrium constant (KA) over self-association equilibrium constant (KB), KA/KB, is introduced as a convenient guide in estimating the phase behavior of similar polymer blends featuring hydrogen bonding interactions between the homopolymer additive and copolymer: with a KA/KB ratio much larger than unity, the blend system tends to behave as a neat copolymer; with a KA/KB ratio significantly smaller than unity, phase separation instead of order−order phase transitions can be expected for the blend above certain volume fraction of homopolymer additive.

Profiles of equilibrium constants for self-association of aromatic molecules

Analysis of the noncovalent, noncooperative self-association of identical aromatic molecules assumes that the equilibrium self-association constants are either independent of the number of molecules (the EK-model) or change progressively with increasing aggregation (the AK-model). The dependence of the self-association constant on the number of molecules in the aggregate (i.e., the profile of the equilibrium constant) was empirically derived in the AK-model but, in order to provide some physical understanding of the profile, it is proposed that the sources for attenuation of the equilibrium constant are the loss of translational and rotational degrees of freedom, the ordering of molecules in the aggregates and the electrostatic contribution (for charged units). Expressions are derived for the profiles of the equilibrium constants for both neutral and charged molecules. Although the EK-model has been widely used in the analysis of experimental data, it is shown in this work that the derived equilibrium constant, K(EK), depends on the concentration range used and hence, on the experimental method employed. The relationship has also been demonstrated between the equilibrium constant K(EK) and the real dimerization constant, K(D), which shows that the value of K(EK) is always lower than K(D).

Opposing Effects of Inositol Hexakisphosphate on Rod Arrestin and Arrestin2 Self-Association

The robust cooperative formation of rod arrestin tetramers has been well-established, whereas the ability of other members of the arrestin family to self-associate remains controversial. Here, we used purified arrestins and multi-angle light scattering to quantitatively compare the propensity of the four mammalian arrestin subtypes to self-associate. Both non-visual and cone arrestins only form oligomers at very high non-physiological concentrations. However, inositol hexakisphosphate (IP6), a fairly abundant form of inositol in the cytoplasm, greatly facilitates self-association of arrestin2. Arrestin2 self-association equilibrium constants in the presence of 100 microM IP6 suggest that an appreciable proportion could exist in an oligomeric state but only in intracellular compartments where its concentration is 5-10-fold higher than average. In contrast to arrestin2, IP6 inhibits self-association of rod arrestin, indicating that the structure of these two tetramers in solution is likely different.

Application of SIMSTEX to Oligomerization of Insulin Analogs and Mutants

The propensity of various insulins and their analogs to oligomerize was investigated by mass spectrometric methods including measurement of the relative abundances of oligomers in the gas phase and the kinetics of H/D amide exchange. The kinetics of deuterium uptake show a good fit when the exchanging amides are placed in three kinetic groups: fast, intermediate, and slow. r-Human insulin, of the insulins investigated, has fewer amides that exchange at intermediate rates and more that exchange at slow rates, in accord with its higher extent of association in solution. We adapted PLIMSTEX (protein ligand interactions by mass spectrometry, titration, and H/D exchange) to determine protein/ligand affinities in solution, to determine self-association equilibrium constants for proteins, and to apply them to various insulin analogs. We term this adaptation SIMSTEX (self-association interactions using mass spectrometry, self-titration and H/D exchange); it gives affinity constants that compare well with the literature results. The results from SIMSTEX show that some mutants (e.g., GlnB13) have an increased tendency to self-associate, possibly slowing down their action in vivo. Other mutants (e.g., lispro and AspB9) have lower propensities for self-association, thus providing potentially faster-acting analogs for use in controlling diabetes.

Determination of the self-association and inter-association equilibrium constants of a carboxylic acid and its mixtures with pyridine derivates

Infrared spectroscopy has been employed to determine absorptivity coefficients and the self-association equilibrium constant of 3,4,5-tris(dodecyloxy)benzoic acid in cyclohexane. The obtained value is consistent with data reported in bibliography for other similar compounds. The equilibrium constant describing ethylpyridine/carbonyl hydrogen bonds has been determined for mixtures of the above-mentioned acid and two ethylpyridine isomers. The effect of steric hindrance is discussed.

Infrared spectroscopic studies of the self-association of aromatic urethanes

The self-association equilibrium constants for aromatic urethanes have been determined by infrared spectroscopy from solutions in inert solvents. N H and carbonyl stretching vibration regions have been analysed for two low molecular weight compounds. In some cases bands attributable to cis and trans conformers in aromatic urethanes have been detected.

Excess enthalpies of five pentanol isomer + n-heptane systems measured at 303.15 K and a model describing the behavior

Experimental excess enthalpy data are reported for mixtures of n-heptane with five pentanol isomers: 1-pentanol, 2-pentanol, 3-methyl-1-butanol, 3-methyl-2-butanol and 2-methyl-2-butanol. The data were obtained using isothermal flow calorimetry and are compared with previous studies. These new data are represented by a model that combines the physical interactions of the Scatchard–Hildebrand regular solutions model with classical hydrogen bonding of the Kretschmer–Wiebe model. The model includes an adjustable parameter that reflects physical interactions and a self-association equilibrium constant that represents hydrogen bonding.

NMR determination of the hetero-association of phenanthridines with daunomycin and their competitive binding to a DNA oligomer.

500 MHz (1)H NMR spectroscopy has been used to determine thermodynamic and structural information on the hetero-association of daunomycin (DAU) with the phenanthridine mutagenic dyes ethidium bromide (EB) and propidium iodide (PI). The NMR complexation data have been analysed by a statistical-thermodynamic model which takes into account indefinite association for both the self-association of the drugs and their hetero-association. The results have been used to estimate the effect of the side chains of the phenanthridines on the competitive binding between DAU and the mutagens with DNA. Knowledge of the equilibrium constants for self-association of the phenanthridines and DAU, their hetero-association and their complexation with a DNA fragment, the deoxytetranucleotide 5'-d(TpGpCpA), enabled the relative content of each of the EB-DAU, PI-DAU, EB-DAU-d(TGCA) and PI-DAU-d(TGCA) complexes to be calculated as a function of drug concentration in mixed solutions. The results provide some insight into the molecular basis of the action of combinations of biologically-active molecules. When intercalating drugs are used in combination, it is found that the decrease in binding of drug or mutagen with DNA is due both to formation of drug-mutagen hetero-association complexes in the mixed solution and to competition for the binding sites by the aromatic molecules; the relative importance of each process depends on the molecular properties of the drug or mutagen molecules being considered. Thus, the longer branched side chain of PI and the electrostatic contribution of the extra positive charge of the molecule compared with the ethyl group of EB results in lower affinity for self-association of PI molecules and their hetero-association with DAU, but increases the degree of binding of PI with DNA.

Hetero-association of caffeine and aromatic drugs and their competitive binding with a DNA oligomer.

NMR spectroscopy has been used to elucidate the molecular basis of the action of caffeine (CAF) on the complexation with DNA of mutagens such as ethidium bromide, propidium iodide, proflavine and acridine orange, and anticancer drugs such as actinomycin D and daunomycin. The hetero-association of CAF and each of the aromatic ligands in 0.1 mol L(-1) phosphate buffer (pD=7.1) has been investigated as a function of concentration and temperature by 500 MHz 1H NMR spectroscopy and analysed in terms of a statistical-thermodynamic model, in which molecules form indefinite aggregates for both self-association and hetero-association. The analysis leads to determination of the equilibrium constants of hetero-association and to the values of the limiting chemical shifts of the heteroassociation of CAF with each of the aromatic molecules. The hetero-association constants between CAF and each of the aromatic drugs/dyes are found to be intermediate in magnitude between those for self-association of CAF and the corresponding drug/dye. The most probable structures of the 1:1 CAF + ligand hetero-association complexes have been determined from the calculated values of the induced limiting chemical shifts of the drug protons. Knowledge of the equilibrium constants for self-association of CAF and the aromatic ligands, for their hetero-association and their complexation with a DNA fragment, the deoxytetranucleotide 5'-d(TpGpCpA), enabled the relative content of each of the CAF-ligand and CAF-ligand-d(TGCA) complexes to be calculated as a function of CAF concentration in mixed solutions. It is concluded that, on addition of CAF to the solution, the decrease in binding of drug or mutagen with DNA is due both to competition for the binding sites by CAF and the aromatic molecules, and to formation of CAF-ligand hetero-association complexes in the mixed solution; the relative importance of each process depends on the drug or mutagen being considered.

Infrared spectroscopic studies of the self-association of ethyl urethane

Infrared spectroscopy has been used to determine self-association equilibrium constants for ethyl urethane (EU) from solutions in inert solvents. These constants have been obtained from both NH and carbonyl stretching regions and the results have been compared. The effect of the solvent nature on the obtained equilibrium constants has been studied.

Infrared Characterization and Determination of Self-Association Equilibrium Constants for Methacrylic Acid Copolymers

The synthesis of essentially random methacrylic acid (MAA) copolymers is reported. Precursor copolymers of styrene (ST) or 2,3-dimethylbutadiene (DMB) and a tert-butyldimethylsilylmethacrylate comonomer were synthesized initially by free-radical solution polymerization, followed by silyl deprotection to yield the desired amorphous MAA copolymers. The infrared characterization of these copolymers is discussed with particular emphasis placed on the effect of temperature and composition on hydrogen bonding, the formation of anhydrides at elevated temperatures, and quantitative measurements, leading to an estimation of the dimensionless equilibrium constant describing the self-association of MAA moieties. Comparisons are made to low molecular weight analogues and previous studies on ethylene-co-methacrylic acid copolymers. An important experimental finding is that the number of hydrogen-bonded carboxylic acid dimers that are formed at equilibrium is a strong function of chain stiffness.

Determination of Self-Association Equilibrium Constants of Ethanol by FTIR Spectroscopy

The self-association of ethanol diluted in CCl4 has been studied by IR spectroscopy. Three OH stretching bands were assigned to monomers, linear dimers, and linear multimers. The self-association was described with two equilibrium constants, one for the formation of dimers, K2, the other for the formation of multimers, KA. These equilibrium constants were determined for ethanol concentration ranging from 0.19 to 0.76 mol L-1 by using a method developed by Coggeshall and Saier. At T = 25 °C, K2 was found to be equal to 1.08 L mol-1 in terms of molar concentration or 18.4 in terms of molar fraction; KA was found to be equal to 3.52 L mol-1 in terms of molar concentration or 60.0 in terms of molar fraction. The enthalpy of formation for K2 was found to be equal to −4.61 kcal mol-1 and to −4.15 kcal mol-1 for KA.

Thermochemical investigations of hydrogen-bonded solutions: development of a predictive equation for the solubility of anthracene in binary hydrocarbon

Acree W.E., Jr., Zvaigzne A.I. and Tucker S.A., 1994. Thermochemical investigations of hydrogen-bonded solutions: development of a predictive equation for the solubility of anthracene in binary hydrocarbon + alcohol mixtures based upon Mobile Order theory. Fluid Phase Equilibria, 92: 233-253. Experimental solubilities are reported for anthracene dissolved in twelve binary solvent mixtures containing either 2-propanol or 1-octanol with n-hexane, n -heptane, n -octane, cyclohexane, methylcyclohexane and 2,2,4-trimethylpentane at 25.0°C. Results of these measurements, along with previously published solubility data for anthracene in binary 1-propanol+alkane and 1-butanol + alkane mixtures, are used to test the limitations and applications of expressions derived from Mobile Order theory. The best descriptive equation expresses the logarithm of the volume fraction solubility in binary mixtures as a simple volume fraction average of the corresponding logarithm solubilities in the two pure solvents, and contains additional terms to represent contributions from nonspecific physical interactions between dissimilar solvent components, from molecular size disparity, and from the reduction in mobile disorder caused by the self-association by hydrogen bonding of the alcohol solvent. The newly derived expression predicts the observed anthracene solubilities to within an overall average deviation of ca. 4.4% using a single value of K c = 5000 for the self-association equilibrium constant.

Hydrogen bonding of N-monosubstituted amides. IR and NMR study of N-butylbenzamides

The self-association through hydrogen bonding of N-butylbenzamides has been studied by infrared spectroscopy in carbon tetrachloride, chloroform and benzene. The spectroscopic characteristics and thermodynamic parameters for the self-association have been determined. Also, the carbon tetrachloride and chloroform solutions of the amides have been studied by 1H NMR spectroscopy. The equilibrium constants for self-association obtained by NMR technique are in good agreement with the corresponding values from infrared measurements.

Structural and functional analysis of spectrin from neonatal erythrocytes

Spectrin was purified by rate zonal sedimentation from low-salt extracts of red cell membranes from neonatal and adult blood. Neonatal and adult spectrin cosedimented in sucrose density gradients, comigrated on SDS gels and displayed identical two-dimensional chymotryptic 123I-labelled peptide maps. Neonatal spectrin and adult spectrin exhibited equivalent affinity for both neonatal and adult ankyrin sites on spectrin-depleted inverted membrane vesicles. Purified spectrin heterodimers from neonatal and adult red cells displayed similar self-association equilibrium constants in a fluid phase dimer-dimer association assay. These results suggest that the unique membrane characteristics of the neonatal erythrocytes are not due to a structural or functional alteration of spectrin. Several alternative hypotheses involving other membrane proteins and their linkages are discussed.

Hydrogen bonding of organic bases with trifluoroacetic acid in chloroform: A method of calculating association constants of multi-equilibria systems

13 C NMR chemical shifts of sixteen organic bases, hydrogen-bonded with trifluoroacetic acid in deuteriochloroform, are used to calculate equilibrium constants for self-association of acid and for hydrogen bonding of base with various acid n-mers. In this treatment each hydrogen bond of the species in equilibrium is assigned a free energy. The equilibrium constants then correspond to changes in these energies. Thermodynamic models are proposed which differ in the extent to which a given hydrogen bond perturbs the free energies of neighboring bonds in the molecular aggregates. Each furnishes a minimum set of independent, freely variable equilibrium constants, the values of which are then determined through a least squares fitting of the experimental data by an iterative procedure.

Covalent cross-linking of the bovine somatotropin dimer. Effects on growth-promoting, receptor-binding and immunological activities and preliminary characterization of the self-association.

Bovine somatotropin, at pH 8.5 in 0.02 M-Bicine [NN-bis-(2-hydroxyethyl)glycine]/0.09M-NaCl, showed by frontal analysis the characteristics of a rapid monomer-dimer equilibrium whose dissociation constant was estimated to be 6.6 X 10(-6)M. Reaction of the hormone with dimethyl suberimidate lead to covalent cross-linking of the dimeric species. Under the conditions chosen (0.4 mg of bifunctional imidate and 1 mg of protein/ml at room temperature for 1 h) the cross-linked dimers accounted for 26% of the total protein, and these were isolated by molecular sieving in 0.29M-NH3/0.12M-NaCl. Covalent stabilization greatly diminished the growth-promoting activity and the ability to interact with somatogenic sites in both rat liver in vivo and rabbit liver microsomal fractions. Evidence indicating a non-critical role for amino groups involved in the covalent cross-linking was provided by a nearly equivalent derivative obtained after reaction with 3,3'-dithiobispropionimidate, which had substantial hormonal activity upon cleavage of the disulphide links. Conversely, immunological reactivity as demonstrated by radioimmunoassay was not affected by cross-linking. Details of the least-squares procedure employed to evaluate the self-association equilibrium constant has been deposited as Supplement SUP 50115 (7 pages) with the British Library Lending Division, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies may be obtained on the terms indicated in Biochem.J. (1981) 193,5.