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At least 955 records · Page 53Linked to original sources
PubMed2004

Calculation of the specific rate of catabolic activity (Ac) from the heat flow rate of soil microbial reactions measured by calorimetry: significance and applications.

The calculation of parameters involved in the kinetics of the microbial soil reactions linked to the carbon cycle is strongly limited by the methodologies employed. Hence, a mathematical model is proposed to quantify easily the specific rate of catabolic activity A(c) by microcalorimetry based on Belaich's model. It permits to quantify A(c) from the plots of the heat flow rate vs. time recorded from soil samples amended with glucose. It was applied for several soil samples collected in the Amazon. The results obtained were compared, and statistical and graphical analyses were used to provide the biophysical significance of A(c) in soils. Results suggest that A(c) could be used as an empirical measure of stress. It correlates positively with the heat yield, Y(Q/X), of the soil microbial growth reactions, indicating that higher specific rates of catabolic activity cause higher dissipation of energy per unit of cell, yielding less-efficient metabolic reactions, which could affect negatively the soil quality. It is strongly affected by the initial microbial population and by the percentage of nitrogen in the samples. The statistical analysis also demonstrated that A(c) is more sensitive to changing environmental conditions than Y(Q/X), yielding more-accurate information about the soil metabolic processes.

Calorimetry↗
PubMed2005

Analysis of photopolymerization behavior of UDMA/TEGDMA resin mixture and its composite by differential scanning calorimetry.

The aim of this study was to investigate the extent of polymerization (Ep) in terms of polymerization rate of UDMA/TEGDMA resin mixtures and its composite resin, by using a differential scanning calorimeter (DSC) technique employing a photopolymerization apparatus. The resin mixtures used in this study consisted of urethane dimethacrylate (UDMA) as a base monomer and triethyleneglycol dimethacrylate (TEGDMA) as a low viscosity monomer. The concentration of TEGDMA in the mixed monomer was varied to 0, 20, 40, 60, 80, and 100 mol %. Additionally, using a base monomer consisting of 60 mol % UDMA and 40 mol % TEGDMA, four kinds of composites with silica filler of 0, 20, 40, 60, and 70 wt %, were prepared in this study. The general reaction profile and Ep values were obtained from the DSC curves. Increasing the concentration of TEGDMA resulted in a decrease in the viscosity of the UDMA/TEGDMA mixture, a delay in the time to maximum polymerization rate, and an increase in the Ep values of the resin mixtures. Furthermore, Ep values decreased with increasing filler content between 0 and 60 wt % but did not decrease further between 60 and 70 wt %.

Calorimetry, Differential Scanning↗
PubMed2006

Characterization of temperature-induced phase transitions in five polymorphic forms of sulfathiazole by terahertz pulsed spectroscopy and differential scanning calorimetry.

The far-infrared properties of all five known polymorphic forms of the drug sulfathiazole have been studied by terahertz pulsed spectroscopy and low-frequency Raman spectroscopy. The observed spectra of the different polymorphs are distinctly different. Terahertz pulsed spectroscopy proves to be a rapid and complementary alternative to other physical characterization techniques reported in the literature for distinguishing between the five forms. Variable-temperature measurements (293-473 K) of all polymorphic forms have been performed. The phase transitions observed have been related to thermal analysis data. Form I is the form stable at high temperature of sulfathiazole with a melting point of about 475 K. Form II melts at around 470 K and recrystallizes at higher temperatures to form I. Forms III, IV, and V all convert to form I via a solid-solid phase transition at temperatures below 450 K. The phase transitions can be monitored by terahertz pulsed spectroscopy. Polymorphic impurities of the samples can be detected in the room temperature spectra and their effect on the phase transition behavior can be studied.

Calorimetry, Differential Scanning↗
PubMed1978

Nonisothermal aqueous calorimetry: computation of process-dependent temperature change and aspects of calorimeter design.

A general method for determining the process-dependent (intrinsic) temperature change in a nonisothermal calorimeter is presented. The nonisothermal approach to calorimetric investigations requires an estimate of the magnitude of the process independent (extrinsic) temperature change during the reaction period. The proposed method can be applied to any calorimeter whose output is a discrete or continuous temperature--time profile. It is based on a first-derivative transformation of the temperature--time profile and the partitioning of the observed temperature variation into two components: pure extrinsic variation, which occurs outside the reaction period, and the combined extrinsic and intrinsic effects during the reaction period. Close examination of the pure extrinsic variation was considered essential, since it provided the basis for identifying the form of a descriptive mathematical function consistent with the observed extrinsic behavior. Once a suitable function was selected, parameters for the equation were determined through a linear regression procedure. The resulting equation was used to predict the extrinsic variation within the reaction period. Subtraction of predicted extrinsic variation from the observed total variation and integration over the time course of the experiment provide an estimate of the process-dependent temperature change. The differential approach was examined for processes performed in a calorimeter of simple design. Aspects of calorimeter design and advantages of the proposed method of data analysis are discussed.

Calorimetry↗
PubMed1978

Application of immersional calorimetry to investigation of solid-liquid interactions: microcrystalline cellulose-water system.

A comprehensive characterization of the specific solid-liquid interaction for microcrystalline cellulose and water is presented. The procedure consisted of a conjoint vapor adsorption and immersional wetting experiment. The following information was obtained with respect to the solid. Estimates of the total surface are (138 m2/g) and the external surface (9.2 m2/g) were calculated from the adsorption and immersion data, respectively. Existence of an energetically homogeneous surface was verified by a linear decrease in the heat of immersion of samples containing adsorbed moisture approximately up to monolayer capacity. Integral and differential free energy, enthalpy, and entropy changes accompanying the adsorption process were calculated, and a lack of swelling was substantiated by comparison with a similar study of cellulose fibers. Immersional hysteresis was observed, and its magnitude suggested that sorption hysteresis was of enthalpic as well as entropic origin. The experimental method is potentially valuable for routine characterization of hydrophilic powders.

Adsorption↗
PubMed1991

Heat of fusion measurement of a low melting polymorph of carbamazepine that undergoes multiple-phase changes during differential scanning calorimetry analysis.

Two polymorphs of carbamazepine that melt at 176 and 189 degrees C and are known to be enantiotropic have been characterized more fully than in previous reports. For the first time, a value for the heat of fusion (29.3 kJ/mol) of the lower melting form is estimated, even though single melting DSC endotherms were not obtained. A method is described for summing the heat flows of intermediate transitions that occur when low melting carbamazepine is heated to a temperature where it will remain melted. The method should have general applicability in studies of polymorphs. Other characteristics of the carbamazepine enantiotropic pair are reported, including the transition temperature (71 degrees C).

Calorimetry, Differential Scanning↗
PubMed2006

Criteria for downhill protein folding: calorimetry, chevron plot, kinetic relaxation, and single-molecule radius of gyration in chain models with subdued degrees of cooperativity.

Recent investigations of possible downhill folding of small proteins such as BBL have focused on the thermodynamics of non-two-state, "barrierless" folding/denaturation transitions. Downhill folding is noncooperative and thermodynamically "one-state," a phenomenon underpinned by a unimodal conformational distribution over chain properties such as enthalpy, hydrophobic exposure, and conformational dimension. In contrast, corresponding distributions for cooperative two-state folding are bimodal with well-separated population peaks. Using simplified atomic modeling of a three-helix bundle-in a scheme that accounts for hydrophobic interactions and hydrogen bonding-and coarse-grained C(alpha) models of four real proteins with various degrees of cooperativity, we evaluate the effectiveness of several observables at defining the underlying distribution. Bimodal distributions generally lead to sharper transitions, with a higher heat capacity peak at the transition midpoint, compared with unimodal distributions. However, the observation of a sigmoidal transition is not a reliable criterion for two-state behavior, and the heat capacity baselines, used to determine the van't Hoff and calorimetric enthalpies of the transition, can introduce ambiguity. Interestingly we find that, if the distribution of the single-molecule radius of gyration were available, it would permit discrimination between unimodal and bimodal underlying distributions. We investigate kinetic implications of thermodynamic noncooperativity using Langevin dynamics. Despite substantial chevron rollovers, the relaxation of the models considered is essentially single-exponential over an extended range of native stabilities. Consistent with experiments, significant deviations from single-exponential behavior occur only under strongly folding conditions.

Calorimetry↗
PubMed1994

Estimation of the folding/unfolding energetics of marginally stable proteins using differential scanning calorimetry.

We demonstrate a method for obtaining accurate estimates of the thermodynamic parameter values characteristic of a two-state folding/unfolding transition under conditions in which the onset of cold denaturation prevents the native state from being fully populated at any temperature. This situation occurs for proteins exhibiting low thermal stability, which may be intrinsic, the result of amino acid substitution, or the consequence of protein-solvent interactions (e.g., extremes of pH, the presence of denaturants, extremes of ionic strength). Conventional analysis of calorimetric scans obtained under such conditions yields erroneous values for the enthalpy, entropy, and heat capacity changes characteristic of the folding/unfolding transition of the protein. This paper describes for the first time the weighted average enthalpy function. In contrast to the van't Hoff and calorimetric enthalpies, the weighted average enthalpy yields thermodynamic parameter values which differ by less than 5% form the true value, even for situations in which the population of molecules in the native state at the start of the transition is half of the total.

Calorimetry, Differential Scanning↗
PubMed2000

Exact analysis of competition ligand binding by displacement isothermal titration calorimetry.

A rigorous method for the least-squares nonlinear regression analysis of displacement isothermal titration calorimetric data is presented. The method can fit the binding isotherm of a ligand which is competitively inhibited in its binding by another bound ligand to a molecule with n identical and independent binding sites. There are no other assumptions for the method and no approximations. Analysis of previously published data of the strong binding of acarbose to glucoamylase is presented as an example. The regression equations have been programmed for the Origin software supplied with the widely used titration calorimeters from Microcal, Inc., and an Origin Function Definition File with instructions is freely available from the author upon e-mail request.

Animals↗