In order to determine the feasibility of manufacturing wood-gypsum and wood-gypsum-cement particleboards, the hydration reactions of four Eastern Canadian wood species with gypsum and Portland cement ...were studied. The compatibility of hot water extracted and raw wood particles with both gypsum and a gypsum-Portland cement mixture were determined for jack pine, balsam fir, aspen and white birch by isothermal calorimetry. The four species were found to be compatible and suitable for the manufacturing of inorganic-bonded wood composite boards on the basis of the compatibility factor. The hot water extractive content was low in the four species and did not have a consistent impact on the compatibility factor. Nevertheless, hot water soluble extractives did have an impact on the time required to reach the maximum heat flux, delaying it to undesirable levels particularly in the case of balsam fir mixed with neat gypsum. However, the effect of balsam fir on neat gypsum hydration was markedly reduced by the addition of 30% Portland cement. Hot water extraction of the wood particles and the addition of Portland cement to the wood-gypsum mixture significantly reduced the time required to reach the maximum heat flux. The gypsum-cement mixture was less sensitive to the inhibitory effect of wood than neat gypsum.La viabilidad de cuatro especies de madera del este de Canadá para la elaboración de tableros de partículas aglomeradas mezcladas con yeso y con una mezcla de yeso-cemento fue estudiada por calorimetría isotérmica siguiendo las reacciones de hidratación de las mezclas yeso-madera y yeso-cemento-madera. La compatibilidad fue determinada sobre dos tipos de partículas de madera, unas sometidas a un pre-tratamiento de extracción con agua caliente y otras sin pre-tratamiento. Se utilizaron las siguientes especies: Pinus banksiana, Abies balsamea, Populus tremuloides y Betula papyrifera. De acuerdo al factor de compatibilidad, las cuatro especies fueron consideradas compatibles con el yeso y la mezcla yeso-cemento, siendo aptas para la fabricación de tableros compuestos. El contenido de extractivos solubles en agua caliente fue bajo en las cuatro especies y no influyó en la determinación del factor de compatibilidad. Sin embargo, los extractivos afectaron los tiempos de hidratación y en el caso del Abies balsamea retrasaron la hidratación del yeso a un nivel indeseable. Tanto la adición de 30% de cemento al yeso como la extracción de las partículas de madera con agua caliente redujeron visiblemente el tiempo requerido para alcanzar el máximo flujo de calor, además de eliminar el efecto causado por el Abies balsamea en la hidratación del yeso. En general, la mezcla yeso-cemento fue menos sensible a los efectos inhibitorios de la madera que el yeso puro.
Isothermal calorimetry is rapidly becoming an indispensable tool for the quantitative determination of a variety of kinetic and thermodynamic parameters for a wide range of systems. In particular ...calorimetry is finding increased application to the investigation of stability and incompatibility of pharmaceutical materials. In order to draw meaningful conclusions and to predict behaviour in related systems it is necessary to have the means to calculate accurately parameters such as the rate constant and enthalpy. To this end several groups have been developing equations which describe calorimetric output in these terms. This paper will briefly outline some of these equations and discuss some of the limitations that currently exist in their application. A particular emphasis is placed on the recent developments relating to the application of these equations to flow calorimetric data. The main application of these equations is usually found in the pharmaceutical industry. Pharmaceutical formulations are usually extremely complex mixtures consisting of many different excipients as well as the active drug. Because of these large numbers of ingredients it is often observed that multiple chemical and physical process occur over the lifetime of the study. This complexity is then reflected in the calorimetric data rendering the application of the simple equations useless. Dealing with this complexity is a major issue amongst the calorimetric community and some of the recent advances in this field are also discussed.
A modified first-order kinetic law which takes into account defect decay during an ordering process was employed to predict the short-range-order kinetics of a quenched αCu–10at.% Al alloy, in ...conjunction with experiments performed by differential isothermal calorimetry (DIC). The effective activation energy of point defect migration and its temperature dependence strongly suggests the contribution of bound vacancies to the ordering process. An estimate of 87.6 kJ mol
−1 was made for the activation energy of solute–vacancy migration by applying an effective rate constant, a value which is quite reasonable, since it lies between the activation energy of migration of unbound vacancies and the activation energy for complex dissolution. The isothermal curves were utilized to determine the ordering energy: w=−3.66 kJ mol
−1.
Kinetics and energetics of precipitation in solutionized (SOL) aluminum alloy 2124 have been determined by differential scanning calorimetry (DSC) and differential isothermal calorimetry (DIC). DSC ...experiments at several temperature scan rates were analyzed by the Kissinger method to give activation energies and rate constants. From the DIC experiments, we obtained kinetics information using a 2-exponential fit, a rate-averaged time constant, and (for GP formation) an Avrami model. It appears that the 2-exponential fit is applicable when two distinct processes contribute to precipitation, while the rate-averaged time constant is appropriate when one process is dominant. Criteria are established for choosing the proper analysis.
Activation energies and time constants from DSC and DIC agree fairly well for both GP zone formation and precipitation. Kinetics results for GP zone dissolution in SOL 2124 were obtainable only from DSC experiments. Both calorimetric methods indicate that, after GP zones have formed and dissolved, two mechanisms are involved in precipitation. The results are compared to DSC studies of other workers for similar alloys. TEM studies indicate that the two precipitation mechanisms in alloy 2124 involve formation of
S′ (CuMgAl
2) and
θ′ (CuAl
2) phases. Δ
Q, the heat evolved during GP zone formation and precipitation, was measured isothermally over the 30–300°C range. At the temperature of maximum GP zone formation rate (∼70°C), Δ
Q≈−14.7
J/g; at the precipitation maximum (∼270°C) Δ
Q≈−27.2
J/g.
The contribution of thermal and radiative treatments as well as the presence of some excipients, namely glycerol, carboxymethylcellulose (CMC), pectin, and agar, on the formation of protein−protein ...interactions as well as the formation and loss of protein−water interactions was investigated by means of differential scanning calorimetry in an isothermal mode. Protein−water interactions were assessed through measurement of the heat of the wetting parameter. Isothermal calorimetry measurements pointed out that γ-irradiation does not favor protein−water interactions, as reflected by its endothermic contribution (P ≤ 0.05) to the heat of wetting values. Although significant (P ≤ 0.05), the effect of the thermal treatment on endothermic responses using isothermal calorimetry was found to be somewhat lower. Among excipients added to biofilm formulations, glycerol generated the most important losses of protein−water interactions, as inferred by its significant (P ≤ 0.05) endothermic impact on the heat of wetting values. Keywords: Milk protein; isothermal calorimetry; heat of wetting; cross-links; glycerol; carboxymethylcellulose; polysaccharides; edible films
Calmodulin (CaM) is a ubiquitous calcium-binding protein responsible for the binding and activation of a vast number of enzymes and signaling pathways. It contains two lobes that bind two calcium ...ions each, separated by a flexible central linker. This structural flexibility allows CaM to bind and regulate a large number of diverse protein targets within the cell in response to Ca2+ gradients.
Voltage gated calcium channels (CaVs), as main sources of extracellular Ca2+, are crucial for a number of physiological processes, from muscle contraction to neurotransmission and endocrine function. These large transmembrane proteins open in response to membrane depolarization and allow gated entry of Ca2+ ions into the cytoplasm. Their regulation is currently the subject of intense investigation due to its pharmacological and scientific importance.
CaM has been previously shown to pre-associate and act as a potent inhibitor of one class of high-voltage activated (HVA) channels called L-type channels via its interaction with their C-terminal cytoplasmic region. This interaction is primarily mediated by a conserved CaM-binding motif called the ‘IQ’ motif (for conserved isoleucine and glutamine residues), although the exact molecular details of its involvement in inactivation are currently unclear. Elucidation of these details was the primary objective of this dissertation.
Recently, a novel sequence motif within this channel called ‘NSCaTE’ (N-terminal spatial calcium transforming element) has been described as an important contributor to calcium-dependent inactivation (CDI) of L-type channels. It was presumed to be unique to vertebrates, but we also show its conservation in a distantly related L-type channel homolog of Lymnaea stagnalis (pond snail). The interaction of CaM with a number of peptides representing the different regulatory motifs (IQ and NSCaTE) for both mammalian and snail isoforms was characterized in an attempt to better understand their role in CDI. Biophysical work with peptides as well as electrophysiology recordings with an N-terminal truncation mutant of Lymnaea CaV1 homolog were performed to expand our understanding of how the interplay between these channel elements might occur. In brief, the most striking feature of the interaction concerns the strong evidence for a CaM-mediated bridge between the N- and C-terminal elements of L-type channels.
Further investigation of the CaM interaction with both IQ and NSCaTE peptides using Ca2+-deficient CaM mutants reveals a preference of both peptides for the Ca2+-C-lobe of CaM, and a much higher affinity of CaM for the IQ peptide, suggesting that the N-lobe of CaM is the main interaction responsible for the physiological effects of NSCaTE. These results are consistent with our electrophysiology findings that reveal a distinct buffer-sensitive CDI in wild type LCaV1 that can be abolished by the N-terminal truncation spanning the NSCaTE region.
In addition to L-type channels, CaM has also been shown to have an indirect role in the regulation of low-voltage activated (LVA) or T-type channels (CaV3.x), via their phosphorylation by CaM-dependent protein kinase II (CaMKII). Using a primary sequence scanning algorithm, a CaM-binding site was predicted in a cytoplasmic region of these channels that was also previously shown to be important in channel gating. Biophysical experiments with synthetic peptides spanning this gating brake region from the three human and the single Lymnaea isoform strongly suggest that there is a novel, bona fide CaM interaction in this channel region, and also hint that this interaction may be a Ca2+-dependent switch of some sort. The results confirm a possible new role for CaM in the direct regulation of these channels, although the exact mechanism remains to be elucidated.
Isothermal calorimetry has been used to examine the effect of thermodynamic non-ideality on the kinetics of catalysis by rabbit muscle pyruvate kinase as the result of molecular crowding by inert ...cosolutes. The investigation, designed to detect substrate-mediated isomerization of pyruvate kinase, has revealed a 15% enhancement of maximal velocity by supplementation of reaction mixtures with 0.1 M proline, glycine or sorbitol. This effect of thermodynamic non-ideality implicates the existence of a substrate-induced conformational change that is governed by a minor volume decrease and a very small isomerization constant; and hence, substantiates earlier inferences that the rate-determining step in pyruvate kinase kinetics is isomerization of the ternary enzyme product complex rather than the release of products.
A calorimetric approach is described for analysing the kinetics of periodate oxidation on a series of monosaccharidic substrates. Rate constants at several temperatures were calculated from the ...calorimetric decay curves that are proportional to the rate of conversion. Arrhenius plots provided the activation parameters for the various carbohydrates and a linear correlation was found between the values of enthalpy and entropy of activation. The dependence of the values of kinetic rates on stereochemistry is interpreted in terms of conformational probability of the reactive state. The suitability of the calorimetric method to track the kinetic process of slow reactions is emphasised, in particular its ability to monitor, directly and continuously, the course of the reaction.
The terbenzimidazoles are a class of synthetic ligands that poison the human topoisomerase I (TOP1) enzyme and promote cancer cell death. It has been proposed that drugs of this class act as TOP1 ...poisons by binding to the minor groove of the DNA substrate of TOP1 and altering its structure in a manner that results in enzyme-mediated DNA cleavage. To test this hypothesis, we characterize and compare the binding properties of a 5-phenylterbenzimidazole derivative (5PTB) to the d(GA4T4C)2and d(GT4A4C)2duplexes. The d(GA4T4C)2duplex contains an uninterrupted 8-bp A· T domain, which, on the basis of x-ray crystallographic data, should induce a highly hydrated ``A-tract'' conformation. This duplex also exhibits anomalously slow migration in a polyacrylamide gel, a feature characteristic of a noncanonical global conformational state frequently described as ``bent.'' By contrast, the d(GT4A4C)2duplex contains two 4-bp A· T tracts separated by a TpA dinucleotide step, which should induce a less hydrated ``B-like'' conformation. This duplex also migrates normally in a polyacrylamide gel, a feature further characteristic of a global, canonical B-form duplex. Our data reveal that, at 20 degrees C, 5PTB exhibits an ≈ 2.3 kcal/mol greater affinity for the d(GA4T4C)2duplex than for the d(GT4A4C)2duplex. Significantly, we find this sequence/conformational binding specificity of 5PTB to be entropic in origin, an observation consistent with a greater degree of drug binding-induced dehydration of the more solvated d(GA4T4C)2duplex. By contrast with the differential duplex affinity exhibited by 5PTB, netropsin and 4′,6-diamidino-2-phenylindole (DAPI), two AT-specific minor groove binding ligands that are inactive as human TOP1 poisons, bind to both duplexes with similar affinities. The electrophoretic behaviors of the ligand-free and ligand-bound duplexes are consistent with 5PTB-induced bending and/or unwinding of both duplexes, which, for the d(GA4T4C)2duplex, is synergistic with the endogenous sequence-directed electrophoretic properties of the ligand-free duplex state. By contrast, the binding to either duplex of netropsin or DAPI induces little or no change in the electrophoretic mobilities of the duplexes. Our results demonstrate that the TOP1 poison 5PTB binds differentially to and alters the structures of the two duplexes, in contrast to netropsin and DAPI, which bind with similar affinities to the two duplexes and do not significantly alter their structures. These results are consistent with a mechanism for TOP1 poisoning in which drugs such as 5PTB differentially target conformationally distinct DNA sites and induce structural changes that promote enzyme-mediated DNA cleavage.
Using isothermal calorimetry (ITC), we have found one case where a well-characterized allosteric activator showed no sign of allostery in its ΔG° of binding to successive sites on multiple subunits ...and another case where successive binding showed no ΔG° binding allostery but did show large entropy-compensated flip-flopping enthalpy changes. This behavior, which we have termed "isoergonic cooperativity" and others have referred to as "silent coupling" is quite simply explained by basic linkage theory when reactions are considered beyond the ΔG° level. Thus, direct calorimetric determination of all thermodynamic parameters including ΔH°, ΔS°, ΔG°, ΔC (p)°, and d(ΔC (p)°/dt) provides a more informative depiction of a ligand binding event and its consequences than does the mere measurement of ΔG° alone. We further discuss the benefits and limitations of methods that have previously been used to study silent coupling. In particular, ITC is free of the numerous pitfalls inherent in the application of van't Hoff and Årrhenius plots to allosteric phenomena. Aside from having a 30-fold advantage in precision, ITC is capable of measuring changes in enthalpy directly at five more levels of mathematical differentiation than are available to van't Hoff type approaches.
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