Showing papers on "Titration published in 1992"

Effects of protein charge heterogeneity in protein-polyelectrolyte complexation

Abstract: The interaction between globular proteins and synthetic polyelectrolytes was studied in 0.10 M NaCl solutions. pH titrations were used to induce the binding of three monomeric proteins of substantially different isoelectric points (BSA, chicken egg lysozyme and bovine pancreas ribonuclease) to polyelectrolytes with a range of charge densities. THe formation of soluble polyion-protein complexes is followed, upon progressive pH change, by phase separation (complex coacervation). Using known protein titration curves, the net charge on the protein at critical conditions could be calculated

On the chemistry of the Keggin Al13 polymer

Abstract: The acid-base properties of the polynuclear Al13 complex have been studied by alkalimetric titration using three different experimental methods. While the positive charge of 7+ of the Al13 polymer is preserved at pH values below 6, it is lost almost completely between pH 6 and 7, i.e., within 1 pH unit. During the titrations, the Al13 complex tends to aggregate, especially at high concentrations and after long experimental duration. It was found that the titration curve of the aggregated complex is significantly less steep than that of the dissolved form. The acid-base properties indicate that the aggregated Al13 represents an intermediate state between the dissolved polymer and the solid aluminum hydroxides.

Formation of fibrous molecular assemblies by amino acid surfactants in water

Abstract: Gel-like solutions are obtained at medium pH and low temperature for aqueous solutions of N-acyl-L-aspartic acids (C n Asp, n=12-18). Electron microscopic observation reveals that helical, fibrous molecular assemblies are formed in the gel-like solutions. The rheological behavior reflects the formation of swarms or bundles of fibers. The hydrogen bonding of amide groups and the moderate degree of ionization of carboxyl groups are confirmed from the infrared absorption bands, consistent with the pH titration result

Redox properties of the iron‐sulfur clusters in activated Fe‐hydrogenase from Desulfovibrio vulgaris (Hildenborough)

Abstract: The periplasmic Fe-hydrogenase from Desulfovibrio vulgaris (Hildenborough) contains three iron-sulfur prosthetic groups: two putative electron transferring [4Fe-4S] ferredoxin-like cubanes (two F-clusters), and one putative Fe/S supercluster redox catalyst (one H-cluster). Combined elemental analysis by proton-induced X-ray emission, inductively coupled plasma mass spectrometry, instrumental neutron activation analysis, atomic absorption spectroscopy and colorimetry establishes that elements with Z > 21 (except for 12–15 Fe) are present in 0.001–0.1 mol/mol quantities, not correlating with activity. Isoelectric focussing reveals the existence of multiple charge conformers with pI in the range 5.7–6.4. Repeated re-chromatography results in small amounts of enzyme of very high H2-production activity determined under standardized conditions (∼ 7000 U/mg). The enzyme exists in two different catalytic forms: as isolated the protein is ‘resting’ and O2-insensitive; upon reduction the protein becomes active and O2-sensitive. EPR-monitored redox titrations have been carried out of both the resting and the activated enzyme. In the course of a reductive titration, the resting protein becomes activated and begins to produce molecular hydrogen at the expense of reduced titrant. Therefore, equilibrium potentials are undefined, and previously reported apparent Em and n values [Patil, D. S., Moura, J. J. G., He, S. H., Teixeira, M, Prickril, B. C., DerVartanian, D. V., Peck, H. D. Jr, LeGall, J. & Huynh, B.-H. (1988) J. Biol. Chem. 263, 18732–18738] are not thermodynamic quantities. In the activated enzyme an S= 1/2 signal (g= 2.11, 2.05, 2.00; 0.4 spin/ protein molecule), attributed to the oxidized H cluster, exhibits a single reduction potential, Em, 7=–307 mV, just above the onset potential of H2 production. The midpoint potential of the two F clusters (2.0 spins/protein molecule) has been determined either by titrating active enzyme with the H2/H+ couple (E'm=–330 mV) or by dithionite-titrating a recombinant protein that lacks the Hcluster active site (Em7.5=–340 mV). There is no significant redox interaction between the two F clusters (n ∼ 1).

Influence of pH on the equilibrium association constants for oligodeoxyribonucleotide-directed triple helix formation at single DNA sites.

Abstract: The energetics of oligodeoxyribonucleotide-directed triple helix formation for the pyrimidine.purine.pyrimidine structural motif were determined over the pH range 5.8-7.6 at 22 degrees C (100 mM Na^+ and 1 mM spermine) using quantitative affinity cleavage titration. The equilibrium binding constants for 5'-TTTTTCTCTCTCTCT-3' (1) and 5'-TTTTTm^5CTm^5CTm^5CTm^5CTm^5CT-3' (2, m^5C is 2'-deoxy-5-methylcytidine) increased by 10- and 20-fold, respectively, from pH 7.6 to 5.8, indicating that the corresponding triple-helical complexes are stabilized by 1.4 and 1.7 kcal.mol-1, respectively, at the lower pH. Replacement of the five cytosine residues in 1 with 5-methylcytosine residues to yield 2 affords a stabilization of the triple helix by 0.1-0.4 kcal.mol^(-1) over the pH range 5.8-7.6. An analysis of these data in terms of a quantitative model for a general pH-dependent equilibrium transition revealed that pyrimidine oligonucleotides with cytidine and 5-methylcytidine form local triple-helical structures with apparent pKa's of 5.5 (C+GC triplets) and 5.7 (m^5C+GC triplets), respectively, and that the oligonucleotides should bind to single sites on large DNA with apparent affinity constants of ~10^6 M^(-1) even above neutral pH.

Comparison of methods to assess surface acidic groups on activated carbons

Abstract: The effect of oxidation with nitric acid on activated carbons from different origins has been studied by inverse gas chromatography at infinite dilution, Boehm titration, and mass titration to determine the point of zero charge (PZC) of carbons. The effect of oxidation is to generate acidic groups, and this becomes more pronounced as the temperature of oxidation is increased. We find that the Boehm titration results and the carbons' PZCs correlate with the severity of the oxidation treatment

Application of mass titration to the point of zero charge determination

Abstract: The mass titration method for determination of the zero point of charge (PZC) was proposed by Noh and Schwarz (J. Colloid Interface Sci. 130, 157 (1989)). This method was developed for (hydr)oxides and is limited to pure samples. It is based on the fact that pH approaches the limiting value (pH∞) by addition of solid powder to an aqueous medium. The limiting pH∞ value, at a high solid content, is then equal to pHpzc. This article is an extension of the original work and concerns the samples contaminated with an acid or base which are not irreversibly bound

Equilibria and reaction mechanism of the complexation of methylboronic acid with polyols

Abstract: Methylboronic acid, CH 3 B(OH) 2 , forms 1:1 anionic complexes with polyols according to the reaction CH 3 B(OH) 2 +H 2 L⇔Cx - +H 3 O + . The polyols which were studied are 1,2-ethanediol, 1,2-propanediol, 1,3-propanediol, and 1,2,3-propanetriol. Complexes with both five-membered and six-membered chelate rings are formed. Equilibrium constants in the four systems vary between 10 -9 and 10 -10 , and they were determined by both pH titration and 11 B NMR spectroscopy

Thermodynamics of oligodeoxyribonucleotide-directed triple helix formation: an analysis using quantitative affinity cleavage titration

Abstract: The free energy for oligodeoxyribonucleotide-directed triple helix formation at a single site on a DNA plasmid fragment has been analyzed using quantitative affinity cleavage titration Measurement of the amount of site-specific cleavage of a 339-bp radiolabeled DNA duplex produced at 24-degrees-C (100 mM Na+, 1 mM spermine-4HCl, 50 mM Tris-acetate, pH 70) over a concentration change of four orders of magnitude for oligodeoxyribonucleotide-EDTAFe 1Fe, (5'-T*TTTTCTCTCTCTCT-3') yields an equilibrium binding constant, K(T) = 37 +/- 11 x 10(6) M-1 (DELTA-G(T) = -90 +/- 02 kcalmol-1) Quantitative affinity cleavage titration affords association constants that are identical within experimental uncertainty with those obtained from quantitative DNase I footprint titration of the same oligonucleotide with and without EDTAIn (1In and 6, respectively) Removal of one thymidine and one cytidine residue from the 3' end of 1Fe reduces the free energy of binding by 05 kcalmol-1, and removal of two thymidine and two cytidine residues from its 3' terminus decreases the binding free energy by 11 kcalmol-1 at pH 70 Single internal base triplet mismatches result in a destabilization of the local triple-helical structure by 25-30 kcalmol-1 Quantitative affinity cleavage titration is a general method which should allow for the measurement of equilibrium constants for the association of many DNA-binding molecules to single sites on relatively large DNA under a broad range of solution conditions

Ambivalent metal ion binding properties of cytidine in aqueous solution

Abstract: The acidity constant of H(cytidine) + and the stability constants of the 1:1 complexes formed between Mg 2+ , Ca 2+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , or Cd 2+ and cytidine (Cyd) were determined by potentiometric pH titration in aqueous solution (I=0.5 M (NaNO 3 ); 25 o C); some of the equilibrium constants were also independently measured by UV spectrophotometry.

Spectrophotometric determination of pH and its application to determination of thermodynamic equilibrium constants

Abstract: Highly accurate pH estimations are conveniently made by evaluation of the spectra of an acid-base indicator suitable for the pH range of interest, if the pK a,ind is accurately known. Stoichiometric pH values were to determine pK a,ind to an accuracy of ±0.008 or better, utilizing a neutralization titration of a reference standard week acid or base

Thermodynamics of ion binding to phosphatidic acid bilayers. Titration calorimetry of the heat of dissociation of DMPA.

Abstract: The heat of dissociation of the second proton of 1,2-dimyristoylphosphatidic acid (DMPA) was studied as a function of temperature using titration calorimetry. The dissociation of the second proton of DMPA was induced by addition of NaOH. From the calorimetric titration experiment, the intrinsic pK0 for the dissociation reaction could be determined by applying the Gouy-Chapman theory. pK0 decreases with temperature from ca. 6.2 at 11 degrees C to 5.4 at 54 degrees C. From the total heat of reaction, the dissociation enthalpy, delta Hdiss, was determined by subtracting the heat of neutralization of water and the heat of dilution of NaOH. In the temperature range between 2 and 23 degrees C, delta Hdiss is endothermic with an average value of ca. 2.5 kcal.mol-1 and shows no clear-cut temperature dependence. In the temperature range between 23 and 52 degrees C, delta Hdiss calculated after subtraction of the heat of neutralization and dilution is not the true dissociation enthalpy but includes contributions from the phase transition enthalpy, delta Htrans, as the pH jump induces a transition from the gel to the liquid-crystalline phase. The delta Cp for the reaction enthalpy observed in this temperature range is positive. Above 53 degrees C, the pH jump induces again only the dissociation of the second proton, and the bilayers stay in the liquid-crystalline phase. In this temperature range, delta Hdiss seems to decrease with temperature. The thermodynamic data from titration calorimetry and differential scanning calorimetry as a function of pH can be combined to construct a complete enthalpy-temperature diagram of DMPA in its two ionization states.

Ca2+ binding environments on natural and synthetic polymeric DNA's.

Abstract: Here are reported 43Ca nmr chemical shift and line width measurements obtained during 43CaCl04 titrations of two natural and two synthetic polymeric DNA's. Titrations of the natural DNA's demonstrate the existence of at least two classes of bound 43Ca2+. The 43Ca2+ nmr relaxation and chemical shift behavior observed during titration of C. perfringens DNA (31%GC) is dominated by a delocalized, non-specific interaction. In contrast, titration of M.lysodeikticus DNA (72%GC) indicates that a small fraction of the 43Ca2+ experiences significant motional retardation and/or an increase in the electric field gradient when associated to the DNA and thus appears to be locally bound to discrete sites on the DNA. These results, and previous results for calf thymus DNA(39% GC) demonstrate that higher GC content correlates with an increase in favorable Ca2+ binding environments. Titrations of synthetic DNA demonstrate that Ca2+ binding is remarkably sensitive to local DNA structure.

Preparation and thermal analysis of synthetic malachite CuCO3·Cu(OH)2

Abstract: The synthesis of malachite CuCO3·Cu(OH)2 or Cu2CO3(OH)2 was studied through titrations of copper(II) salt solutions with a solution of sodium carbonate at different temperatures. The precipitates were characterized by TG, IR and chemical analysis. The composition varies depending on thepH of the solution and the temperature. Purer malachite was synthesized by simple mixing of a solution of copper(II) nitrate or sulfate with a solution of sodium carbonate at 50°C.

The pH range of the Mohr titration for chloride ion can be usefully extended to 4-10.5

Abstract: The pH ranges found in textbooks for this titration are misleading because of inadequate explanations for their basis.