Enthalpy Release Research Articles

Direct evidence of the enthalpy release being accompanied by first-order phase transitions of monolayers at the air/water interface by compression

Insoluble monolayers at the air/water interface change their phases with isothermal compression. Previously, however, it has been difficult to detect the temperature change of the monolayers by the enthalpy release in their first-order phase transition regions because the amount of film materials on the water surface was much too small. We constructed a new instrument, which controlled the temperatures of the water surface and the air above the water within 0.02°C, to detect this temperature change. Direct evidence is given for the enthalpy release which is accompanied by the first-order phase transitions caused by the compression of monolayers of arachidic acid and stearic acid at the air/water interface, and the possibility is discussed that the so-called “solid films” of these monolayers do not correspond to the two-dimensional solids but to the first-order phase transition regions from two-dimensional liquids to solids.

Energetic and kinetic evaluations conducted in Cu-3.34 at % Sn through differential scanning calorimetry

Microcalorimetric enthalpy measurements associated with the different peaks appearing during linear heating of Cu-3.34 at % Sn were made in a 50% cold-rolled alloy. Unlike the situation in the deformed material, quenched and furnace-cooled alloys do not exhibit thermal events at any of the heating rates employed in the temperature range scanned. In the cold-rolled condition, from the energetic and kinetic analysis of the first exothermic peak, designated Stage 1, and of the endothermic peak, designated Stage 3, it was consistently inferred that they correspond, respectively, to the growth on dislocations of a metastable phase e′ and to its subsequent dissolution prior to recrystallization. Such inference was also supported by Vickers microhardness and yield-stress determinations. The calculated volume fraction for e′ after Stage 2 goes to completion, is about 0.02. A suitable expression previously developed for enthalpy release due to the pinning of solute atoms to partial dislocations was applied to compute dislocation density from the exothermic peak (Stage 2). The calculated value is in excellent agreement with those obtained from the analysis of the recrystallization trace (Stage 4) and from tensile tests, thus confirming that the second DSC trace actually corresponds to the solute segregation process. It is also suggeseted that additional dislocation-induced formation of e′ might take place as a consequence of the enhanced solute concentration around partial dislocations. The non-isothermal dissolution kinetics of e′ was adequately described by an integrated kinetic model function essentially appropriate for application in one-dimensional diffusion situations.

Calorimetric investigation of structural relaxation in supercooledNi75Al22Zr2B amorphous alloy

A differential scanning calorimetric (DSC) study is performed on a supercooled ${\mathrm{Ni}}_{75}$${\mathrm{Al}}_{22}$${\mathrm{Zr}}_{2}$B alloy to present an example for identification between amorphous and microcrystalline structures of a structurally disordered solid. The supercooled ${\mathrm{Ni}}_{75}$${\mathrm{Al}}_{22}$${\mathrm{Zr}}_{2}$B alloy exhibits a weak and broad exo- therm (which is followed by the pronounced crystallization exotherm) due to enthalpy or structural relaxation, which masks the glass-liquid transition (${\mathit{T}}_{\mathit{g}}$) characteristic of an amorphous solid. Thermal treatment of the alloy (performed at T=${\mathit{T}}_{\mathit{g}}$) removes the exothermic step, and the DSC output recorded after the thermal treatment showed a characteristic ${\mathit{T}}_{\mathit{g}}$ at around 228 \ifmmode^\circ\else\textdegree\fi{}C. The signal presents a monotonically decreasing enthalpy release (\ensuremath{\partial}H/\ensuremath{\partial}t${)}_{\mathit{T}}$ in an isothermal calorimetric measurement, which is modeled successfully by using the existing mechanism of structural relaxation. The microstructure of the specimens studied before and after the thermal treatments confirms the origin of the DSC exotherm and isotherm signal to be structural relaxation.

Second‐order kinetics of enthalpy recovery in polystyrene

AbstractSecond‐order kinetics were found applicable to the isothermal enthalpy release after quenching from a high temperature. The kinetic behavior was unaffected by different amounts of initial excess enthalpy. The activation enthalpy obtained from the temperature dependence of the second order rate constant was 200 kcal/mol or 837 kJ/mol which could be compared with 736 kJ/mol for volume recovery in a similar temperature range relative to Tg.

Calorimetric studies of flavin binding protein: Flavin analog binding

The effect of flavin structure variation upon the overall binding reaction of flavin to hen egg white riboflavin binding protein (WRBP) was correlated to thermodynamic parameters obtained via titration calorimetry. This effect was measured by determining a reference binding enthalpy (Δ H ref) for 3-carboxymethylriboflavin and subsequently comparing (Δ H ref) to the binding enthalpies determined for all other flavin analogs in this study. The reference enthalpy change was measured in detail under a variety of temperatures (14, 25, and 38 °C), pH's (5.5, 6.5, 7.4, 8.5), and buffer types. The reference enthalpy for the binding of N(3)-carboxymethylriboflavin to WRBP was measured as (Δ H) = −20.9 kcal/mol at 25.0 °C and pH 7.4. Evaluation of the binding thermodynamic values for the reference flavin indicates that there is no H + flux concomitant with binding. A small negative change in heat capacity ((Δ H Cp ) occurs during the binding process suggesting a contraction of the protein. Finally, the reaction appears to be independent of pH and buffer type indicating little, if any, involvement of charged residues in the binding of flavin to apoWRBP. Thermodynamic values for the binding of an additional eight flavin analogs were then measured at pH 7.4 and 25.0 °C. The thermodynamic binding parameters for these analogs were evaluated by comparison to those determined for the reference flavin. The results indicate that the ribityl C2′ position is a major influence in the interaction between the ribityl side chain and the protein; the C9 position of the isoalloxazine ring is sterically restricted in a manner similar to positions C7 and C8; and the positions C7 and C8 appear to be unequal with regard to enthalpy release, suggesting that the C8 position is the most restricted region in the aromatic ring. These various findings indicate the unique ability of titration calorimetry to evaluate structural variation of ligands to their corresponding binding site interactions.

Ligand-linked phase equilibria of sickle cell hemoglobin

This paper extends the concepts of linkage and control, previously studied in single phase allosteric and polysteric systems, to multiple phase (polyphasic) systems. In particular, a study has been made of the dependence of the solubility of sickle cell hemoglobin on oxygen partial pressure. Phase diagrams are obtained from observations of birefringence changes of hemoglobin solutions in a thin film optical cell. The effects of temperature and pH are found to be correlated largely with oxygen binding curves for non-gelling solutions. This suggests only small enthalpy and proton release changes for the gelation process. Variable time delays for the onset of birefringence were observed for partial deoxygenation of a fully oxygenated sample. The reciprocal of the time delay depends on a high power of the supersaturation ratio. The nucleation kinetics are, thereby, similar to those found in fully deoxygenated solutions in temperature-jump studies. Oxygen binding curves for non-gelling solutions of sickle cell hemoglobin were used in conjunction with the phase diagram results to evaluate oxygen binding curves for the polymer gel. Account was taken of the water content of the gel and of the large non-ideality of the solution. Analysis of the phase diagram data based on polyphasic linkage relationships suggests that some reversible oxygen-binding by the gel is present. The difference in oxygen binding between solution and gel obtained in this way is similar to that found by Hofrichter (1979) for carbon monoxide.

The sliding filament model of muscle contraction: II. The energetic and dynamical predictions of a quantum mechanical transducer model

A theoretical model of a molecular energy transducing unit designed for the production of mechanical work is constructed and its consequences examined and compared with the experimentally determined myothermal and dynamic properties of vertebrate striated muscle. The model rests on a number of independent assumptions which include: the almost instantaneous generation of mechanical force by the occurrence of a radiationless transition between vibronic states of the transducer (crossbridge) at a point of potential energy surface crossing; transmission of this force to the load via the active sites on the thin filament by means of non-bonding repulsive forces, no energy being required for detachment; “detachment” consists of a second radiationless transition at a lower energy point than the first force generating transition, the energy difference appearing largely as work. The method of force generation completely avoids problems such as the “force-rate dilemma” which occur repeatedly in any discussion where state populations are near-Boltzmann and also leads without further arbitrary assumptions to such concepts as “attached but non force-producing states” and strongly position dependent “attachment” and “detachment” rate constants since these can only be appreciable near potential energy surface crossings. The kinetics and energetics of a transducer of this type operating cyclically and converting ATP → ADP + P i are considered and shown to lead to length-tension and energetic behaviour very similar to that exhibited by vertebrate striated muscle, both for contraction and stretching. The existence of a limiting tension for stretching is predicted by the model as is the decrease of the rate of enthalpy release rate below the isometric value. At the limiting tension the rate of enthalpy release by the transducers is virtually zero, as observed. However, the stretching only inhibits the ATP hydrolysis, the cyclic synthesis from ADP and work being impossible with this model. The response to rapid length step changes automatically contains the asymmetry observed experimentally (with respect to lengthening and shortening) and arbitrary assumptions over and above those giving adequate explanation of the steady-state properties are not required. The asymmetry arises mainly as a consequence of the non-bonded pushing action of the crossbridges. This same assumption predicts the occurrence of an asymmetric thermoelastic ratio for active muscle with respect to stretching and contraction. The quantitative aspects of the model are satisfactory as it simultaneously reconciles the numerical magnitudes of macroscopic quantities such as isometric tension, maximum contraction velocity, limiting tension sustainable on stretching, isometric heat rate and resting heat rate with molecular parameters such as the filament and crossbridge periodicities, molecular vibrational relaxation rates, recurrence times for the radiationless transitions occurring, etc. This is achieved without any parameter optimization and only a very much smaller number of unknown parameters than the number of observed results accounted for. Many of the entities occurring in the model cycle (vibronic states of crossbridges, ATP, etc.) appear to be in one-to-one correspondence with many of the kinetic entities postulated to account for the biochemical kinetic results obtained for the actomyosin ATPase system in vitro. Finally, the rigor state has to be viewed in a different way from the conventional one; on the basis that the present model states which are part of the contraction cycle but sparsely populated during the latter (and hence are of chemical kinetic but not dynamical importance) are heavily populated during the rigor state. The mechanical properties of the rigor state would then be determined by these molecular states which would be very short-lived during the contraction cycle. If this is correct the rigor state could yield much more information about inaccessible parts of the contraction cycle than is presently supposed. The model leads one to expect a rather different response to quick length step changes in the rigor state from that of the active state, in contrast to current interpretations in terms of a large number of attached crossbridges, unable to detach due to the absence of ATP.