Heat Capacity Increment Research Articles

Thermodynamics of the Double Helix‐Coil Equilibrium in Tetramethylammonium Gellan: High‐Sensitivity Differential Scanning Calorimetry Data

AbstractThe order–disorder conformational transition in the tetramethylammonium salt of gellan has been studied by high‐sensitivity differential scanning calorimetry as a function of concentration of the background electrolyte, Me4NCl (0.0025–0.5 M). The transition temperature and enthalpy increase with increasing salt concentration following a logarithmic relation. At the same time, there is a linear correlation between the transition enthalpy and temperature with a positive slope of 0.18 J g−1 K−1. According to Kirchhoff's law, the slope can be considered as a heat capacity increment of the transition. This value is much larger than that expected to arise from electrostatic effects. Thus, it has been suggested that the transition is accompanied by an increase in water accessibility to some less hydrophilic fragments of the primary structure of gellan buried in its ordered conformation. The transition has an λ‐like heat capacity profile and can be therefore defined as a second‐order phase transition. This assignment is in agreement with predictions of a matching model of the double helix‐coil transition. The model fits well to the integral transition curve at 0.5 M Me4NCl, assuming a cooperative length of gellan to be about eight repeating units. This value is close to the size of chain corresponding to the persistent length of gellan in the coil state. The cooperativity parameter in the matching model (σ = 0.62 ± 0.01) indicates a small contribution of the stacking effect into the cooperativity of the transition, while a larger contribution apparently originates from the loop factor. The free energy of transition at a reference temperature (273 K) is a logarithmic function of the concentration of the salt, ΔtG(Cs), as predicted by the counter condensation theory and the Poisson–Boltzmann (PB) model. The latter is preferable because of the low linear charge density parameter (ξ < 1) for the double helix of gellan. Analysis of the ΔtG(Cs) function in terms of the PB model showed that the linear charge density of gellan in the coil conformation needs to be higher than that estimated for the fully stretched chain.Correlation between the transition enthalpy and the transition temperature for the conformational order–disorder transition in the TMA gellan.magnified imageCorrelation between the transition enthalpy and the transition temperature for the conformational order–disorder transition in the TMA gellan.

Study of the Dual Amorphous Phases in Semicrystalline Poly(ethylene terephthalate) Using the Heat Capacity Increment at the Glass Transition

Study of the Dual Amorphous Phases in Semicrystalline Poly(ethylene terephthalate) Using the Heat Capacity Increment at the Glass Transition

Physical Aging in Side-Chain Liquid Crystal Polymers: A DSC Investigation of the Enthalpy Relaxation

The enthalpy relaxation of the glasses of two side-chain liquid crystal polymethacrylates was studied with physical aging experiments detected with differential scanning calorimetry. The experimental traces were fairly well reproduced in the framework of a recent configurational entropic model. The temperature dependence of the equilibrium structural relaxation times was obtained with a simultaneous fit of several different thermograms and compared with the behavior found for the poly(methyl methacrylate) (PMMA). The comparison evidenced the stronger character of the two liquid crystal polymers, in the frame of the strong/fragile classification of glass-forming systems. At variance with the prediction of the energy landscape model, the heat capacity increment at the glass transition was greater than in PMMA, confirming some recent results on polymeric glass formers.

Relationships between the temperature dependence of solvent denaturation and the denaturant dependence of protein stability curves

We have used a simple binding model to consider how the thermodynamics of denaturant–protein interactions might influence the shape of protein stability curves (free energy change as a function of temperature), and how these effects translate into a temperature dependence of the apparent m-value (sensitivity of unfolding free energy to denaturant). We find that for an exothermic binding reaction with no binding heat capacity increment, increasing denaturant concentrations produces an apparent increase in curvature in the protein stability curve, giving rise to an increase in the heat capacity increment of unfolding. Similar increases are seen if the binding heat capacity increment is taken as positive. However, for a negative binding heat capacity increment, increasing denaturant concentrations decreases the curvature of the stability curve, giving rise to a decrease in the heat capacity of unfolding. At very high denaturant concentrations (above which the heat capacity of denaturation becomes negative) the stability curve becomes dimpled, showing two separate maxima rather than one. These three models result in very different temperature dependencies of apparent m-values. For urea-induced unfolding of the ankyrin-domain of the Drosophila Notch protein, we find a dependence of experimental m-values on temperature that is similar to that produced by a negative binding heat capacity increment. This temperature dependence is consistent with the observed decrease in heat capacity of unfolding as denaturant is added.

Effects of some metal ions on the denaturational heat capacity increments in dilute solutions of ds-DNA

The native DNA duplex may be viewed as a cooperatively-ordered H bonded structure, including well localized Watson–Crick base pairs and H-bounded networks of hydration parts of the DNA–solvent interface in the grooves of the helix. In this paper, we present the effect of different metal ions (Li +, Mg 2+ and Cu 2+) on the denaturational heat capacity increment (Δ C p ) for calf thymus DNA. Since the contribution from the ordered hydration water fraction disruption energy to the total enthalpy and heat capacity increment values of double helix melting is significant, it must be possible to detect the effect of metal ions on the structural ordering/disordering of the H-bounded network in the hydration shell of the DNA duplex. Experimental results suggest that Li +, which is preferentially adsorbed in the minor groove of B-DNA and should contribute significantly to the stabilization of B-form, has a pronounced influence on the value of Δ C p . For the system Mg 2+–DNA, the values of Δ C p are also significant, which can be explained by the formation of inner hydration sphere complexes and immobilization of structural water by the grooves of the duplex, stabilizing the helix. The effect of Cu 2+ ions is much more pronounced. The satellite peaks of calf thymus DNA become lower as the concentration of Cu 2+ increases and for concentrations of Cu 2+ higher than 0.010 M, they disappear and Δ C p =0. This is indicative of the known preference of Cu 2+ for purins and GC rich sites of DNA, binding to the N7 of guanine. As a result, disruption of the base stacking and hydrogen bounded water networks in the grooves of the double helix takes place.

Structural Basis for Difference in Heat Capacity Increments for Ca 2+ Binding to Two α-Lactalbumins

Thermodynamic parameters for the unfolding of as well as for the binding of Ca 2+ to goat α-lactalbumin (GLA) and bovine α-lactalbumin (BLA) are deduced from isothermal titration calorimetry in a buffer containing 10 mM Tris-HCl, pH 7.5 near 25°C. Among the different parameters available, the heat capacity increments (Δ C p) offer the most direct information for the associated conformational changes of the protein variants. The Δ C p values for the transition from the native to the molten globule state are rather similar for both proteins, indicating that the extent of the corresponding conformational change is nearly identical. However, the respective Δ C p values for the binding of Ca 2+ are clearly different. The data suggest that a distinct protein region is more sensitive to a Ca 2+-dependent conformational change in BLA than is the case in GLA. By analysis of the tertiary structure we observed an extensive accumulation of negatively charged amino acids near the Ca 2+-binding site of BLA. In GLA, the cluster of negative charges is reduced by the substitution of Glu-11 by Lys. The observed difference in Δ C p values for the binding of Ca 2+ is presumably in part related to this difference in charge distribution.

A study on the thermodynamic properties of polyimide BTDA-ODA by adiabatic calorimetry and thermal analysis

Polyimide BTDA-ODA sample was prepared by polycondensation or step-growth polymerization method. Its low temperature heat capacities were measured by an adiabatic calorimeter in the temperature range between 80 and 400 K. No thermal anomaly was found in this temperature range. A DSC experiment was conducted in the temperature region from 373 to 673 K. There was not phase change or decomposition phenomena in this temperature range. However two glass transitions were found at 420.16 and 564.38 K. Corresponding heat capacity increments were 0.068 and 0.824 J g−1 K−1, respectively. To study the decomposition characteristics of BTDA-ODA, a TG experiment was carried out and it was found that this polyimide started to decompose at ca 673 K.

Heat capacity of solid zinc from 298.15 to 692.68 K and of liquid zinc from 692.68 to 940 K: thermodynamic function values

The heat capacity of zinc has been determined by adiabatic calorimetry from 298.15 to 940 K. For crystalline zinc C p ,m increases from 25.47 J K −1 mol −1 at the former temperature to an evenly extrapolated value of 30.96 J K −1 mol −1 at the triple-point temperature 692.681 K (ITS-90). The estimated heat capacity increment of fusion is 2.46 J K −1 mol −1. For liquid zinc the heat capacity decreases from 33.42 J K −1 mol −1 at T fus to 32.31 J K −1 mol −1 at 940 K. Thermodynamic function values have been derived and are tabulated for selected temperatures after revaluation of earlier low temperature results.

Temperature-induced denaturation and renaturation of triosephosphate isomerase from Saccharomyces cerevisiae: evidence of dimerization coupled to refolding of the thermally unfolded protein.

The thermal denaturation of the dimeric enzyme triosephosphate isomerase (TIM) from Saccharomyces cerevisiae was studied by spectroscopic and calorimetric methods. At low protein concentration the structural transition proved to be reversible in thermal scannings conducted at a rate greater than 1.0 degrees C min(-1). Under these conditions, however, the denaturation-renaturation cycle exhibited marked hysteresis. The use of lower scanning rates lead to pronounced irreversibility. Kinetic studies indicated that denaturation of the enzyme likely consists of an initial first-order reaction that forms thermally unfolded (U) TIM, followed by irreversibility-inducing reactions which are probably linked to aggregation of the unfolded protein. As judged from CD measurements, U possesses residual secondary structure but lacks most of the tertiary interactions present in native TIM. Furthermore, the large increment in heat capacity upon denaturation suggests that extensive exposure of surface area occurs when U is formed. Above 63 degrees C, reactions leading to irreversibility were much slower than the unfolding process; as a result, U was sufficiently long-lived as to allow an investigation of its refolding kinetics. We found that U transforms into nativelike TIM through a second-order reaction in which association is coupled to the regain of secondary structure. The rate constants for unfolding and refolding of TIM displayed temperature dependences resembling those reported for monomeric proteins but with considerably larger activation enthalpies. Such large temperature dependences seem to be determinant for the occurrence of kinetically controlled transitions and thus constitute a simple explanation for the hysteresis observed in thermal scannings.

Rigid amorphous phase and low temperature melting endotherm of poly(ethylene terephthalate) studied by modulated differential scanning calorimetry

AbstractThe rigid amorphous phase, the low temperature melting endotherm, and their development with thermal treatment in poly(ethylene terephthalate) (PET) were investigated by means of modulated differential scanning calorimetry. The differential of the reversing heat capacity and nonreversing heat flow signals were used to analyze the behavior of the glass transition and the low temperature melting endotherm. With increasing annealing time, the increment of the heat capacity at the glass‐transition temperature decreased and the increment of heat capacity at the annealing temperature increased. It was suggested that the origin of the low temperature melting endotherm mainly resulted from the transition of the rigid amorphous fraction for the PET used. The glasslike transition of the rigid amorphous fraction occurred between the glass transition and melting. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2779–2785, 2001

PVT Properties of Dimethyl(isopropenylethynyl)carbinol at Temperatures from 293 to 363 K and Pressures from 0.1 to 350 MPa, Studied Using the Micro-PVT System

The effect of pressure on the volume of dimethyl(isopropenylethynyl)carbinol, (CH3)2C(OH)C⋮CC(CH3)CH2, has been measured over the pressure range (0.1 to 350) MPa and the temperature range (293.15 to 363.15) K using the Micro-PVT system, a new universal instrument for measuring the thermodynamic characteristics of materials. Experimental data on the variation of the volume ratio k = Vp/V0 (Vp = sample volume at given pressure P; V0 = sample volume at 0.1 MPa) with pressure have been approximated by two equations allowing the interpolation and extrapolation of experimental data with an acceptable uncertainty. The temperature dependence of the coefficients of the Tait equation has been studied, enabling the calculation of the volume ratio with an acceptable uncertainty for a wider temperature and pressure range. The isothermal compressibility, the isobaric thermal expansivity, and the increment of isobaric molar heat capacity from its value at 0.1 MPa have been calculated using the experimental data obtained.

Thermodynamics of trimer-of-hairpins formation by the SIV gp41 envelope protein

The gp41 envelope protein mediates the entry of primate immunodeficiency viruses into target cells by promoting the fusion of viral and cellular membranes. The structure of the gp41 ectodomain core represents a trimer of identical helical hairpins in which a central trimeric coiled-coil made up of three amino-terminal helices is wrapped in an outer layer of three antiparallel carboxyl-terminal helices. Triggering formation of this fusion-active gp41 conformation appears to cause close membrane apposition and thus overcome the activation energy barrier for lipid bilayer fusion. We present a detailed description of the folding thermodynamics of the simian immunodeficiency virus (SIV) gp41 core by using a recombinant trimeric N34(L6)C28 model. Differential scanning calorimetry and spectroscopic experiments on denaturant-induced and thermal unfolding indicate that the free energy of association of three 68 residue N34(L6)C28 peptides to a trimer-of-hairpins is 76 kJ mol−1 at pH 7.0 and 25 °C in physiological buffer. The associated enthalpy change, ΔHunf, is 177 kJ mol−1, while the entropy of unfolding, ΔSunf, is 0.32 kJ K−1 mol−1. The temperature of maximal stability is close to 20 °C. The unfolding heat capacity increment is ∼9 kJ K−1 mol−1 (∼45 J K−1 mol residue−1), which is lower than expected for unfolding of the trimer to an extended and fully hydrated polypeptide chain. Replacement by isoleucine of the polar residues Thr582 or Thr586 buried in the N-terminal trimeric coiled-coil interface leads to very strong stabilization of the trimer-of-hairpins, 30–35 kJ mol−1. Single-point mutations in the central coiled-coil thus strongly stabilize the gp41 core structure. These thermodynamic characteristics may be important for the refolding of the gp41 envelope protein into its fusion-active conformation during membrane fusion.

Studies of the Thermal Volume Transition of Poly(N-isopropylacrylamide) Hydrogels by High-Sensitivity Differential Scanning Microcalorimetry. 2. Thermodynamic Functions

We report the first accurate measurements of the partial heat capacity of poly(N-isopropylacrylamide) hydrogels with varying cross-link density. When the cross-link density is increased, the transition broadens and the transition temperature decreases, while the enthalpy, entropy, and heat capacity increment of the transition do not practically change. The transition heat capacity increment is negative, Δtcp = −0.63 ± 0.04 J/g/K. This indicates the formation of a hydrophobic core of the gel upon the transition. The partial heat capacity of polymer network in the gel approaches the partial heat capacity of the unfolded linear poly(N-isopropylacrylamide) at low temperatures, indicating a complete disordering of the gel under these conditions. On the basis of the calorimetric data, thermodynamic functions of the transition were calculated from 0 to 150 °C. They allow one to compare enthalpic and entropic contributions to the stabilization of the collapsed gel. This state is found to be most stable at about 100 °C, and a reswelling transition could be expected only above 150 °C. Contributions of the dehydration of apolar and polar groups as well as residual factors to the transition enthalpy, entropy, and free energy were calculated. The role of apolar dehydration, i.e., of the hydrophobic effect, was not found to be predominant. Apparently, interactions of residues (van der Waals interactions and/or hydrogen bonding) contribute mainly to the stabilization of the collapsed state.

Thermochemistry of the specific binding of C12 surfactants to bovine serum albumin

The specific binding to bovine serum albumin (BSA) of anionic and non-ionic surfactants with C12 acyl chains has been studied by high sensitivity isothermal titration calorimetry. This method proved particularly effective in resolving the binding of anionic surfactants into separate classes of sites with different affinity. For sodium dodecylsulfate (SDS) the measured binding curves could be rationalized as association to two classes (high affinity/low affinity) of sites comprising, respectively, three and six similar (i.e. thermodynamically equivalent), independent sites. Changes in the thermodynamic functions enthalpy, standard free energy, standard entropy and heat capacity could be discerned for each class of binding site, as well as for micelle formation. These data suggest that binding to low affinity sites (in analogy with micelle formation) exhibits energetic parameters; in particular, a large negative change in heat capacity, which is characteristic of hydrophobic interactions. The thermodynamics of high affinity binding, on the other hand, is indicative of other dominant forces; most likely electrostatic interactions. Other anionic ligands investigated (laurate and dodecyl benzylsulfonate) showed a behavior similar to SDS, the most significant difference being the high affinity binding of the alkylbenzyl sulfonate. For this ligand, the thermodynamic data is indicative of a more loosely associated complex than for SDS and laurate. BSA was found to bind one or two of the non-ionic surfactants (NIS) hepta- or penta(ethylene glycol) monododecyl ether (C12EO7 and C12EO5) with binding constants about three orders of magnitude lower than for SDS. Hence, the free energy of the surfactant in the weakly bound BSA-NIS complex is only slightly favored over the micellar state. The binding process is characterized by very large exothermic enthalpy changes (larger than for the charged surfactants) and a large, positive increment in heat capacity. These observations cannot be reconciled with a molecular picture based on simple hydrophobic condensation onto non-polar patches on the protein surface.

Calorimetric evidence for a native-like conformation of hen egg-white lysozyme dissolved in glycerol

Hen egg-white lysozyme, lyophilized from aqueous solutions of different pH (from pH 2.5 to 10.0) and then dissolved in water and in anhydrous glycerol, has been studied by high-sensitivity differential scanning microcalorimetry over the temperature range from 10 to 150°C. All lysozyme samples exhibit a cooperative conformational transition in both solvents occurring between 10 and 100°C. The transition temperatures in glycerol are similar to those in water at the corresponding pHs. The transition enthalpies in glycerol are substantially lower than in water but follow similar pH dependences. The transition heat capacity increment in glycerol does not depend on the pH and is 1.25±0.31 kJ mol −1 K −1, which is less than one fifth of that in water (6.72±0.23 kJ mol −1 K −1). The thermal transition in glycerol is reversible and equilibrium, as demonstrated for the pH 8.0 sample, and follows the classical two-state mechanism. In contrast to lysozyme in water, the protein dissolved in glycerol undergoes an additional, irreversible cooperative transition with a marginal endothermic heat effect at temperatures of 120–130°C. The transition temperature of this second transition increases with the heating rate which is characteristic of kinetically controlled processes. Thermodynamic analysis of the calorimetric data reveals that the stability of the folded conformation of lysozyme in glycerol is similar to that in water at 20–80°C but exceeds it at lower and higher temperatures. It is hypothesized that the thermal unfolding in glycerol follows the scheme: N⇔ho-MG⇒U, where N is a native-like conformation, ho-MG is a highly ordered molten globule state, and U is the unfolded state of the protein.

Energetic basis of structural stability in the molten globule state: α-lactalbumin

The denatured states of α-lactalbumin, which have features of a molten globule state, have been studied to elucidate the energetics of the molten globule state and its contribution to the stability of the native conformation. Analysis of calorimetric and CD data shows that the heat capacity increment of α-lactalbumin denaturation highly correlates with the degree of disorder of the residual structure of the state. As a result, the denaturational transition of α-lactalbumin from the native to a highly ordered compact denatured state, and from the native to the disordered unfolded state are described by different thermodynamic functions. The enthalpy and entropy of the denaturation of α-lactalbumin to compact denatured state are always greater than the enthalpy and entropy of its unfolding. This difference represents the unfolding of the molten globule state. Calorimetric measurements of the heat effect associated with the unfolding of the molten globule state reveal that it is negative in sign over the temperature range of molten globule stability. This observation demonstrates the energetic specificity of the molten globule state, which, in contrast to a protein with unique tertiary structure, is stabilized by the dominance of negative entropy and enthalpy of hydration over the positive conformational entropy and enthalpy of internal interactions. It is concluded that at physiological temperatures the entropy of dehydration is the dominant factor providing stability for the compact intermediate state on the folding pathway, while for the stability of the native state, the conformational enthalpy is the dominant factor.

A calorimetric study of the influence of calcium on the stability of bovine α-lactalbumin

Bovine α-lactalbumin has been studied by differential scanning calorimetry with various concentrations of calcium to elucidate the effect of this ligand on its thermal properties. In the presence of excess calcium, α-lactalbumin unfolds upon heating with a single heat-absorption peak and a significant increase of heat capacity. Analysis of the observed heat effect shows that this temperature-induced process closely approximates a two-state transition. The transition temperature increases in proportion with the logarithm of the calcium concentration, which results in an increase in the transition enthalpy as expected from the observed heat capacity increment of denaturation. As the total concentration of free calcium in solution is decreased below that of the proteins, there are two temperature-induced heat absorption peaks whose relative area depends on the calcium concentration, such that further decrease of calcium concentration results in a increase of the low-temperature peak and a decrease of the high-temperature one. The high-temperature peak occurs at the same temperature as the unfolding of the holo-protein, while the low-temperature peak is within the temperature range associated with the unfolding of the apo-protein. Statistical thermodynamic modeling of this process shows that the bimodal character of the thermal denaturation of bovine α-lactalbumin at non-saturated calcium concentrations is due to a high affinity of Ca 2+ for α-lactalbumin and a low rate of calcium exchange between the holo- and apo-forms of this protein. Using calorimetric data, the calcium-binding constant for α-lactalbumin has been determined to be 2.9×10 8 M −1.

TRANSITION STATE OF HEAT DENATURATION OF METHIONINE AMINOPEPTIDASE FROM A HYPERTHERMOPHILE

Heat denaturation of methionine aminopeptidase from a hyperthermophile Pyrococcus furiosus (PfMAP) was studied by differential scanning calorimetry at acid pH. Analysis of the calorimetric data has shown that denaturation of PfMAP is non-equilibrium at heating rates from 0.125 to 2 K min–1. This means that the protein structure at these conditions is metastable and its stability (the apparent temperature of denaturation Tm) is under kinetic control. It was shown that heat denaturation of this protein is a one-step kinetic process. The enthalpy of the process and its activation energy were measured as functions of temperature. The obtained data allowed us to estimate the heat capacity increment and the change in the number of bound protons during activation of the molecule. The data also suggest that the conformation of PfMAP at the transition state only slightly differs from its native conformation with respect to compactness, hydration extent and hydroxyl protonation.

A quantitative estimation of the extent of compatibilization in heterogeneous polymer blends using their heat capacity increment at the glass transition

A quantitative estimation of the extent of compatibilization in heterogeneous polymer blends using their heat capacity increment at the glass transition

A quantitative estimation of the extent of compatibilization in heterogeneous polymer blends using their heat capacity increment at the glass transition

A quantitative estimation of the extent of compatibilization in heterogeneous polymer blends using their heat capacity increment at the glass transition