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At least 1,225 records · Page 68Linked to original sources
PubMed2005

A colorimetric titration method for quantification of millimolar glucose in a pH 7.4 aqueous phosphate buffer.

3-Pyridinylboronic acid is identified as a key sensing element for reversible sugar complexation in an aqueous solution at the physiological pH. The utility of a sensing element has been demonstrated through a simple colorimetric titration of glucose using absorption spectroscopy in the visible region. The mechanism of the high diol/triol binding affinity of pyridine boronic acid in the neutral pH is discussed based on the 1H and 11B NMR spectroscopic studies.

Boronic Acids↗
PubMed2004

Calorimetric vs. van't Hoff binding enthalpies from isothermal titration calorimetry: Ba2+-crown ether complexation.

The 1:1 complexation reaction between Ba(2+) and 18-crown-6 ether is re-examined using isothermal titration calorimetry (ITC), with the goal of clarifying previously reported discrepancies between reaction enthalpies estimated directly (calorimetric) and indirectly, from the temperature dependence of the reaction equilibrium constant K (van't Hoff). The ITC thermograms are analyzed using three different non-linear fit models based on different assumptions about the data error: constant, proportional to the heat and proportional but correlated. The statistics of the fitting indicate a preference for the proportional error model, in agreement with expectations for the conditions of the experiment, where uncertainties in the delivered titrant volume should dominate. With attention to proper procedures for propagating statistical error in the van't Hoff analysis, the differences between Delta H(cal) and Delta H(vH) are deemed statistically significant. In addition, statistically significant differences are observed for the Delta H(cal) estimates obtained for two different sources of Ba(2+), BaCl(2) and Ba(NO(3))(2). The effects are tentatively attributed to deficiencies in the standard procedure in ITC of subtracting a blank obtained for pure titrant from the thermogram obtained for the sample.

Barium Compounds↗
PubMed2005

Direct observation of the enthalpy change accompanying the native to molten-globule transition of cytochrome c by using isothermal acid-titration calorimetry.

The enthalpy change accompanying the reversible acid-induced transition from the native (N) to the molten-globule (MG) state of bovine cytochrome c was directly evaluated by isothermal acid-titration calorimetry (IATC), a new method for evaluating the pH dependence of protein enthalpy. The enthalpy change was 30 kJ/mol at 30 degrees C, pH 3.54, with 500 mM KCl. The results of the global analysis of the temperature dependence of the excess enthalpy from 20 to 35 degrees C demonstrated that the N to MG transition is a two-state transition with a small heat capacity change of 1.1 kJ K(-1) mol(-1). The present findings were also indicative of the pH dependence of the enthalpy and the heat capacity of the MG state, -13 kJ mol(-1) pH(-1) and -1.0 kJ K(-1) mol(-1) pH(-1), respectively, at 30 degrees C within a pH range from 2 to 3.

Animals↗
PubMed2005

Apparent pKa shifts of titratable residues at high denaturant concentration and the impact on protein stability.

Urea and guanidine-hydrochloride (GdnHCl) are frequently used for protein denaturation in order to determine the Gibbs free energy of folding and kinetic folding/unfolding parameters. Constant pH value is applied in the folding/unfolding experiments at different denaturant concentrations and steady protonation state of titratable groups is assumed in the folded and unfolded protein, respectively. The apparent side-chain pKa values of Asp, Glu, His and Lys in the absence and presence of 6 M urea and GdnHCl, respectively, have been determined by 1H-NMR. pKa values of all four residues are up-shifted by 0.3-0.5 pH units in presence of 6 M urea by comparison with pKa values of the residues dissolved in water. In the presence of 6 M GdnHCl, pKa values are down-shifted by 0.2-0.3 pH units in the case of acidic and up-shifted by 0.3-0.5 pH units in the case of basic residues. Shifted pKa values in the presence of denaturant may have a pronounced effect on the outcome of the protein stability obtained from denaturant unfolding experiments.

Aspartic Acid↗
PubMed2005

Zonisamide for West syndrome: a comparison of clinical responses among different titration rate.

We administered zonisamide (ZNS) to patients with West syndrome in different titration protocols and compared their short-term therapeutic effects. We designed three protocols to raise the serum ZNS concentration (SZC): (1) increase the dose in three steps, from 3 to 10 mg/kg every 3 days, (2) increase the dose from 5 to 10 mg/kg over 3-7 days, and (3) start with 10 mg/kg and maintain this dosage for 2 weeks. The subjects were 23 infants with West syndrome, 8 of whom comprised the 1st group, 5 the 2nd group, and the remaining 10, the 3rd group. As a result, excellent and good effects were obtained in a total of seven patients (30.4%) and one patient, respectively (1/8 in the 1st step-up group, 3/5 in the 2nd step-up group, and 4/10 in the 3rd group). The maximum SZC was higher in the excellent and good effect groups (n=8; 32.0+/-8.0 microg/ml) than in the ineffective group (n=15; 22.4+/-8.2 microg/ml) (P<0.05). The period of time required for cessation of spasms appeared shorter in the 3rd group (n=4; mean=5.7 days) than in the 1st and 2nd groups (n=4; mean=10.3 days). There were few side effects except for transient hyperthermia and gastrointestinal symptoms. Our new protocol of starting with 10 mg/kg of ZNS can be introduced safely and make a therapeutic judgment feasible within 2 weeks.

Anticonvulsants↗
PubMed2005

Tendency for oxidation of annelid hemoglobin at alkaline pH and dissociated states probed by redox titration.

The redox titration of extracellular hemoglobin of Glossoscolex paulistus (Annelidea) was investigated in different pH conditions and after dissociation induced by pressure. Oxidation increased with increasing pH, as shown by the reduced amount of ferricyanide necessary for the oxidation of hemoglobin. This behavior was the opposite of that of vertebrate hemoglobins. The potential of half oxidation (E1/2) changed from -65.3 to +146.8 mV when the pH increased from 4.50 to 8.75. The functional properties indicated a reduction in the log P50 from 1.28 to 0.28 in this pH range. The dissociation at alkaline pH or induced by high pressure, confirmed by HPLC gel filtration, suggested that disassembly of the hemoglobin could be involved in the increased potential for oxidation. These results suggest that the high stability and prolonged lifetime common to invertebrate hemoglobins is related to their low tendency to oxidize at acidic pH, in contrast to vertebrate hemoglobins.

Animals↗
PubMed2006

The demicellization of alkyltrimethylammonium bromides in 0.1 M sodium chloride solution studied by isothermal titration calorimetry.

The demicellization of the cationic detergents dodecyltrimethylammonium bromide, tetradecyltrimetylammonium bromide, and cetyltrimethylammonium bromide was studied at temperatures between 20 and 60 degrees C in 0.1 M NaCl (pH 6.4) using isothermal titration calorimetry (ITC). We determined the critical micellization concentration (cmc) of the cationic detergents which show a minimum at temperatures between 20 and 34 degrees C. In accordance with the lengthening of the hydrophobic tail of the detergents the cmc decreases with increasing alkyl chain length. The thermodynamic parameters describing the changes of enthalpy (DeltaH(demic)), the changes of entropy (DeltaS(demic)) and the Gibbs free energy change (DeltaG(demic)) for demicellization were first obtained using the pseudophase-separation model. The aggregation number n at the cmc as well as the demicellization enthalpy, entropy and Gibbs free energy change were also calculated using a simulation based on the mass-action model. Furthermore, we investigated the demicellization of CTAB in deionized water in comparison to demicellization in sodium chloride solution to determine the influence of counter ion binding on the demicellization.

Calorimetry↗
PubMed2005

Isothermal titration calorimetry method for determination of cyclodextrin complexation thermodynamics between artemisinin and naproxen under varying environmental conditions.

A novel isothermal titration calorimetry method was used to determine the complexation thermodynamics for hydroxypropyl-beta-cyclodextrin with artemisinin and naproxen at varying temperature and pH. The new method is very useful for studying complexation reactions between cyclodextrin and drugs with poor solubility and all the thermodynamic parameters of the cyclodextrin complexation were determined. The analysis of the thermodynamic data reveals involvement of hydrophobic bonding in the cyclodextrin complexes studied. The data also reveals the presence of enthalpy-entropy compensation in the system and provide information as to the orientation of the drug molecule inside the cyclodextrin cavity. From the thermodynamic parameters for dissociation of HPBCD complexes of artemisinin and naproxen at pH 2 it is concluded that the complexation is primarily driven by enthalpy with entropic assistance at all temperatures studied. From the dissociation studies of HPBCD complexes of naproxen at pH 10 it is concluded that the complexation is predominantly driven by entropy and moderately by enthalpy at lower temperatures and by enthalpy with entropic assistance at higher temperatures.

Artemisinins↗
PubMed2006

Isothermal titration calorimetry and 1H NMR studies on host-guest interaction of paeonol and two of its isomers with beta-cyclodextrin.

Thermodynamic parameters of inclusion complex of beta-cyclodextrin (beta-CD) with paeonol and two of its isomers in aqueous solution have been determined with nano-watt-order isothermal titration calorimetry (ITC) and the host-guest inclusion structure has been investigated by using 1H NMR spectra at 298.2 K. The analysis of thermodynamic data reveals that stoichiometry of beta-CD complex with paeonol (Pae) or acetovanillone (Ace) is 1:1 whereas the inclusion complex of beta-CD with 2-hydroxyl-5-methoxyacetophone (Hma) is in 1:1 coexistence with 2:1 stoichiometry. Further analysis indicates that formation of all the complexes is simultaneously driven by enthalpy and entropy, the inclusion complexation of Pae.beta-CD, Ace.beta-CD and Ham.beta-CD2 is predominantly driven by entropy while Ham.beta-CD by enthalpy. The 1H NMR spectra data provide clear evidence of the inclusion phenomena, which shows that the aromatic ring of the guest molecule insert itself into the torus from the narrow side of the cavity.

Acetophenones↗
PubMed2006

Cell mediated immunity (CMI) and post transplant viral infections--role of a functional immune assay to titrate immunosuppression.

Cell mediated immunity (CMI) was assessed by the ImmuKnow assay in 12 patients after kidney transplantation, who presented with viral infection. Treatment included lowering of immunosuppression in all cases and antiviral treatment if indicated. The assay was repeated during the follow up. The ImmuKnow assay at time of presentation of viral infections was 56.8+/-58.2 (range 3-178; median 22) ATP ng/ml. With the clearance of viral infection and lowering of immunosuppression, the assay showed an increase in the level of CMI at 194.5+/-118.9 (range 53-409; median 150) ATP ng/ml. There was viral clearance or stabilization in all cases and there was no incidence of allograft rejection. The ImmuKnow assay of CMI can be used to titrate initial immunosuppression reduction and its subsequent increase, in patients with viral infection after transplantation.

Adenosine Triphosphate↗
PubMed2006

Enzymatic activity assay of D-hydantoinase by isothermal titration calorimetry. Determination of the thermodynamic activation parameters for the hydrolysis of several substrates.

Isothermal titration calorimetry (ITC) has been applied to the determination of the activity of D-hydantoinase (EC 3.5.2.2) with several substrates by monitoring the heat released during the reaction. The method is based on the proportionality between the reaction rate and the thermal power (heat/time) generated. Microcalorimetric assays carried out at different temperatures provided the dependence of the catalytic rate constant on temperature. We show that ITC assay is a nondestructive method that allows the determination of the catalytic rate constant (kcat), Michaelis constant (KM), activation energy and activation Gibbs energy, enthalpy and entropy of this reaction.

Amidohydrolases↗
PubMed2004

Ionization of short polymethacrylic acid: titration, DLS, and model calculations.

In this work the charging of polymethacrylic acid in excess electrolyte solution is investigated experimentally by titration and dynamic light scattering. The results are analyzed by a penetrable sphere model, which employs the Poisson-Boltzmann equation for the description of electrostatic interactions and takes into account specific binding of H+ and Na+. The evaluation of the DLS data yields two relaxation modes. The slow mode is present only at finite degrees of charging and is therefore caused by collective diffusion. The fast mode, which corresponds to diffusion coefficients in the range from (1.1 to 1.5) x 10(-10) m2 s(-1), is present over the whole pH range. This reflects the diffusional dynamics of the polyion itself and allows the calculation of hydrodynamic radii for equivalent spheres (RH). These increase from 1.5 nm at pH 2.14 up to 1.8 nm for a degree of deprotonation alpha=0.47 at pH 5.86. With a further increase of pH the radii slightly decrease to 1.6 nm. Setting the radius of the penetrable sphere equal to RH, we can successfully model the overall charging curve with logK0H=4.85 and logK0Na=-0.6. This means that weak complexes of the type COO---Na are formed, which reduce the effective charge inside the polyelectrolyte coil.

Journal Article↗
PubMed2005

Isothermal titration calorimetry and dynamic light scattering studies of interactions between gemini surfactants of different structure and Pluronic block copolymers.

The interactions between triblock copolymers of poly(ethylene oxide) and poly(propylene oxide), P103 and F108, EO(n)PO(m)EO(n), m=56 and n=17 and 132, respectively, and m-s-m type gemini surfactants, m=8, 10, 12, and 18, and s = 3, 6, 12, and 16, have been studied in aqueous solution using isothermal titration calorimetry and dynamic light scattering techniques. The enthalpograms of F108 as a function of surfactant concentration show one broad peak at polymer concentrations C(p) < or = 0.50 wt%, below the cmc of the copolymer at 25 degrees C. It is attributed to interactions between the surfactant and the triblock copolymer monomer. DLS results show hydrodynamic radii (R(h)) initially consistent with copolymer monomers that change to values consistent with gemini surfactant micelles as the surfactant concentration is increased. In P103 solutions at C(p) > or = 0.05 wt%, two peaks appear in the enthalpograms, and they are attributed to the interactions between the gemini surfactant and the micelle or monomer forms of the copolymer. An origin-based nonlinear fitting program was employed to deconvolute the two peaks and to obtain estimates of peak properties. An estimate of the fraction of copolymer in aggregated form was also obtained. The enthalpy change due to interactions between the surfactants and P103 aggregates is very large compared to values obtained for traditional surfactants. This suggests that extensive reorganization of copolymer aggregates and surrounding solvent occurs during the interaction. DLS results for the P103 systems containing C(p) > or = 0.05% show evidence of very large aggregates in solution, likely P103 micelle clusters. The transitions observed in the hydrodynamic radii are consistent with a breakdown of micelle clusters with addition of gemini surfactant, followed by mixed micelle formation and/or deaggregation into monomer P103. This is followed by interactions similar to those typically observed in surfactant-nonionic polymer systems. Mechanisms for the interaction and the observed structural changes are discussed.

Biophysics↗
PubMed2005

Laser light scattering and isothermal titration calorimetric studies of poly(ethylene oxide) aqueous solution in presence of sodium dodecyl sulfate.

The aqueous solution of poly(ethylene oxide) (PEO) in the presence of different concentrations of sodium dodecyl sulfate (SDS) was examined by laser light scattering and isothermal titration calorimetric techniques. A small fraction of PEO aggregates were found to coexist with unimeric PEO chains in dilute solution. The presence of monovalent salt does not alter the hydrodynamic properties of PEO in aqueous solution. Addition of a monovalent anionic surfactant, such as SDS, induces cooperative binding of surfactant monomers to PEO backbones at SDS concentrations ranging from 4.0 mM (critical aggregation concentration) to 16.5 mM (saturation concentration). The hydrodynamic radius of PEO unimers decreases initially and then increases with SDS concentration, resulting from the structural reorganization of the PEO/SDS complex. Beyond the saturation concentration, the hydrodynamic radii of PEO/SDS complex are independent of SDS concentration.

Lasers↗
PubMed2005

Lead biosorption study with Rhizopus arrhizus using a metal-based titration technique.

Acid-base and metal-based potentiometric titration methods were used to analyze sorption mechanisms of lead by Rhizopus arrhizus fungal biomass. Biosorption was not considered globally but as the result of successive sorption reactions on various binding sites with different selectivities. Precipitation occurred rapidly when lead concentration increased. Lead was sorbed essentially by carboxylic groups and by phosphates and sulfonates (less abundant) of the organic matter. The lead affinity to carboxylic, sulfonate and phosphate binding sites depended on the association coefficient with proton or counter-ion and on the spatial distribution of the surface sites promoting the formation of mono- or bi-dentate complexes. Chemical bonds and binding sites were confirmed using microscopic and spectroscopic techniques (IR, MET-EDAX). It appeared that although the total organic acidity was reached, number of ionized and free carboxylic groups were not involved in lead sorption reactions. In spite of lead speciation in the solution, surface micro-precipitation was observed and the two processes, surface adsorption and micro-precipitation, are sequential and possibly overlapping. At low concentrations (<10(-6) M) adsorption is the dominant phenomenon and beyond (>10(-5) M) surface clusters appeared before the predicted solution precipitation phenomenon.

Adsorption↗
PubMed2006

Theoretical study of the acid-base properties of the montmorillonite/electrolyte interface: influence of the surface heterogeneity and ionic strength on the potentiometric titration curves.

The parallel shape of the potentiometric titration curves for montmorillonite suspension is explained using the surface complexation model and taking into account the surface heterogeneity. The homogeneous models give accurate predictions only if they assume unphysically large values of the equilibrium constants for the exchange process occurring on the basal plane. However, the assumption that the basal plane is energetically heterogeneous allows to fit the experimental data (reported by Avena and De Pauli [M. Avena, C.P. De Pauli, J. Colloid Interface Sci. 202 (1998) 195-204]) for reasonable values of exchange equilibrium constant equal to 1.26 (suggested by Fletcher and Sposito [P. Fletcher, G. Sposito, Clay Miner. 24 (1989) 375-391]). Moreover, we observed the typical behavior of point of zero net proton charge (pznpc) as a function of logarithm of the electrolyte concentration (log[C]). We showed that the slope of the linear dependence, pznpc=f(log[C]), is proportional to the number of isomorphic substitutions in the crystal phase, which was also observed in the experimental studies.

Journal Article↗
PubMed2006

Similar recovery from bispectral index-titrated isoflurane and sevoflurane anesthesia after outpatient gynecological surgery.

STUDY OBJECTIVE: To test the hypothesis that the recovery of gynecological day-case patients is equally fast after isoflurane and sevoflurane anesthesia, when administration of the inhaled agent is adjusted by monitoring the bispectral index (BIS). DESIGN: Prospective, randomized, controlled, single-blinded clinical study. SETTING: University-affiliated women's hospital. PATIENTS: 120 adult female patients, ASA physical status I or II, scheduled for ambulatory surgery under general anesthesia. INTERVENTIONS: Patients were randomized to receive either isoflurane or sevoflurane as the maintenance anesthetic. BIS values were titrated to remain between 50 and 60 during the maintenance of anesthesia by adjusting the inspired concentration of the inhaled agent. Administration of the inhaled agent was discontinued abruptly at the end of the procedure. MEASUREMENTS: The times to achieving several recovery end points were recorded. The main outcome parameter was the time to home-readiness. In the postoperative care unit, sedation was evaluated with the digit-symbol substitution test. The degree of pain and nausea was evaluated on the visual analog scale. MAIN RESULTS: There were no statistically significant differences in the times to home-readiness, or in any other parameters of early or intermediate recovery between the 2 groups. The degrees of sedation, pain, and nausea in the postoperative care unit were similar in the 2 groups. CONCLUSIONS: Isoflurane and sevoflurane are equally acceptable maintenance anesthetics in terms of the speed and quality of recovery in gynecological ambulatory surgery patients when the dose of the inhaled agent is adjusted to achieve a BIS between 50 and 60.

Adult↗
PubMed2006

31P NMR and isothermal titration calorimetry studies on polyoxomolybdates-catalyzed hydrolysis of ATP.

ATP hydrolysis in the presence of polyoxomolybdates at pH levels of 6, 4, and 2 has been investigated with a help of high pressure liquid chromatography (HPLC) analyses, 31P- and 1H NMR measurements, and isothermal titration calorimetry (ITC). The polyoxomolybdates-induced ATP-hydrolysis proceeded satisfactorily in pH < 6 media at 20 degrees C with an optimum pH level of 4, while it was significantly depressed at low temperature of < or = 5 degrees C. At pH levels of 6 and 4, ADP was a main product, and the involvement of [(PO4)2Mo5O15](6-)-like ATP-molybdate complex as an intermediate was implied. At pH 2 ATP was decomposed to AMP with small generation of ADP through the formation of the ATP-molybdate complex isostructural with [(O3POPO3)Mo6O18(H2O)4]4- as an intermediate. The ITC result at pH 4 showed an occurrence of two types of the exothermic binding reactions between molybdate and ATP with binding constants (K) of 6.61x10(4) and 9.40x10(3) M(-1) and molar enthalpy values (deltaH) of -6.32x10(4) and -4.73x10(3) J mol(-1), respectively. Together with the results of 1H NMR measurements, it is deduced that the molybdates interact with not only phosphate sites in the ATP side-chain, but also adenine-ring with an accompanying aggregation of molybdates at pH 4.

Adenosine Triphosphate↗