Partial Heat Capacities Research Articles

Thermodynamics of barnase unfolding.

The thermodynamics of barnase denaturation has been studied calorimetrically over a broad range of temperature and pH. It is shown that in acidic solutions the heat denaturation of barnase is well approximated by a 2-state transition. The heat denaturation of barnase proceeds with a significant increase of heat capacity, which determines the temperature dependencies of the enthalpy and entropy of its denaturation. The partial specific heat capacity of denatured barnase is very close to that expected for the completely unfolded protein. The specific denaturation enthalpy value extrapolated to 130 degrees C is also close to the value expected for the full unfolding. Therefore, the calorimetrically determined thermodynamic characteristics of barnase denaturation can be considered as characteristics of its complete unfolding and can be correlated with structural features--the number of hydrogen bonds, extent of van der Waals contacts, and the surface areas of polar and nonpolar groups. Using this information and thermodynamic information on transfer of protein groups into water, the contribution of various factors to the stabilization of the native structure of barnase has been estimated. The main contributors to the stabilization of the native state of barnase appear to be intramolecular hydrogen bonds. The contributions of van der Waals interactions between nonpolar groups and those of hydration effects of these groups are not as large if considered separately, but the combination of these 2 factors, known as hydrophobic interactions, is of the same order of magnitude as the contribution of hydrogen bonding.

Thermodynamic Studies of the Collagen-like Region of Human Subcomponent C1q: A Water-containing Structural Model

Thermal transitions of C1q were investigated by methods of differential scanning calorimetry, circular dichroism and fluorescence. The melting curves of C1q display two pronounced heat absorption peaks which enables determination of the thermodynamic parameters characterizing each transition. The low temperature peak was assigned to melting of the C1q collagenous part. Analysis of the data has revealed unusual, as compared with the monomeric collagen molecules, thermodynamic features of the C1q collagenous part: (1) higher thermal stability strongly dependent on pH; (2) less linear co-operative regions; and (3) a noticeable change in the partial specific heat capacity (ΔCp) in contrast to both the monomeric collagen and the collagen fibrils. This unusually large ΔCp value suggested a conclusion that the fibril-like endpiece of C1q may have a cavity filled with ice-like ordered water molecules.

Thermodynamic study of poly(N-vinyl caprolactam) hydration at temperatures close to lower critical solution temperature

The lower critical solution temperature of aqueous solutions of poly(N-vinyl caprolactam) falls in the 305–307 K range and depends on the molecular weight of the polymer. The thermodynamic functions of mixing at 298 K have been calculated from measurements of vapor pressures and heats of dissolution and dilution. Partial Gibbs energy, partial enthalpy, and partial entropy of mixing were negative over the entire range of composition. Increasing temperature resulted in a decrease in the exothermal character of mixing. Excessive heat capacity values, calculated from the dependencies of enthalpy of mixing on temperature, were positive over the entire composition range. Heat capacity of dilute solutions was measured at 298 K and partial heat capacity of poly(N-vinyl caprolactam) at infinite dilution was shown to be positive. The data obtained point out the hydrophilic and hydrophobic hydration of poly(N-vinyl caprolactam) in aqueous solutions. Hydrophobic hydration dominates at temperatures close to binodal curve. As a result, the mutual mixing of the polymer with water is decreased and phase separation takes place.

Scanning microcalorimetry and circular dichroism study of melting of the natural polypeptides in the left-handed helical conformation.

It has been shown that in aqueous solution histone H1 and H5 C-terminal fragments and peptide hormones beta-endorphin and ACTH adopt preferably the left-handed helical conformation of the poly-L-proline II type. Scanning microcalorimetry and circular dichroism have been used to show that the linear temperature dependence of CD maximum amplitude and partial heat capacity value are broken in the temperature interval between 50 and 60 degrees C, after which [C]p reaches the constant level. It was proposed to be due to noncooperative disordering of the conformation caused by the destruction of the polypeptide hydration shell.

Calorimetric study of the heat and cold denaturation of beta-lactoglobulin.

Temperature-induced changes of the states of beta-lactoglobulin have been studied calorimetrically. In the presence of a high concentration of urea this protein shows not only heat but also cold denaturation. Its heat denaturation is approximated very closely by a two-state transition, while the cold denaturation deviates considerably from the two-state transition and this deviation increases as the temperature decreases. The heat effect of cold denaturation is opposite in sign to that of heat denaturation and is noticeably larger in magnitude. This difference in magnitude is caused by the temperature-dependent negative heat effect of additional binding of urea to the polypeptide chain of the protein upon its unfolding, which decreases the positive enthalpy of heat denaturation and increases the negative enthalpy of cold denaturation. The binding of urea considerably increases the partial heat capacity of the protein, especially in the denatured state. However, when corrected for the heat capacity effect of urea binding, the partial heat capacity of the denatured protein is close in magnitude to that expected for the unfolded polypeptide chain in aqueous solution without urea but only for temperatures below 10 degrees C. At higher temperatures, the heat capacity of the denatured protein is lower than that expected for the unfolded polypeptide chain. It appears that at temperatures above 10 degrees C not all the surface of the beta-lactoglobulin polypeptide chain is exposed to the solvent, even in the presence of 6 M urea; i.e., the denatured protein is not completely unfolded and unfolds only at temperatures lower than 10 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

A study of the heat capacity of starch/water mixtures

The heat capacity of starch/water mixtures was investigated as a function of temperature and composition by differential scanning calorimetry. For amorphous amylopectin/water mixtures, the polysaccharide made the major contribution to the observed heat capacity increment at the glass-transition temperature of the mixture. In the glass, the apparent partial specific heat capacity of the water was comparable to that of liquid water. The heat capacity increment observed on gelatinisation of maize and potato starches was compared to that observed on melting and dissolution of highly crystalline A and B polymorphs of short-chain amylose. The role of melting and glass transitions in starch gelatinisation is discussed.

Conformational properties of streptokinase—differential scanning calorimetric investigations

The two-domain structure of streptokinase (Sk) was demonstrated by scanning calorimetric investigations at neutral pH and low ionic strength. The melting pattern of the protein is composed of two two-state transitions at TtrS1 = 45.9 +/- 0.4 degrees C with delta H1 = 431 +/- 18 kJ/mol, and TtrS2 = 60.1 +/- 1.3 degrees C with delta H2 = 306 +/- 16 kJ/mol. The partial specific heat capacity of native Sk was determined to be Cp = 1.42 +/- 0.17 J/K/g and the denaturational heat capacity change associated with the two transitions, delta Cp1 = 0.21 J/K/g and delta Cp2 = 0.38 J/K/g, respectively. The overall melting pattern of Sk remains almost unchanged at a variety of tested solvent compositions, except at pH 4 (and below) and in the presence of denaturants. The two domains show different susceptibility to urea. It is proposed that the less thermostable domain is located within the N-terminal part (residues 1-230), and the more thermostable one, within the C-terminal region.

Apparent molar heat capacities and volumes of some alkylated derivatives of uracil and adenine in aqueous solution at 25�C

Densities and apparent molar heat capacities of some alkylated derivatives of uracil and adenine: 1-methyluracil, 1,3-dimethyluracil, 1,3-diethylthymine, 5,6-trimethylene-1,3-dimethyluracil, 5,6-tetramethylene-1,3-dimethyluracil, 5,6-pentamethylene-1,3-dimethyluracil, 2,9-dimethyladenine, 2-ethyl-9-methyladenine, 2-propyl-9-methyladenine, 8-ethyl-9-methyladenine, 6,8,9-trimethyladenine and 8-ethyl-6,9-dimethyladenine were determined using flow calorimetry and flow densimetry at 25°C. It was found that the partial molar volumes and heat capacities correlate linearly with the number of substituted methylene groups-CH 2 -as well as to the number of hydrogen atoms, n H , belonging to the skeleton of the molecule. In the case of alkylated uracils a difference was observed in the values at infinite dilution V 2 o and C p2 o , depending on the substitution of alkyl and cyclooligomethylene groups.

Cold destabilisation of enzymes.

The thermal stability profile of chymotrypsinogen has been investigated in the temperature range 230-340 K, with special emphasis on the phenomenon of cold instability. Differential scanning calorimetry was used to study the heat capacities of the native and denatured protein in undercooled solution and the results were combined with those obtained by spectrophotometry at ordinary temperatures. The partial heat capacities of both forms decrease with decreasing temperature, assuming negative values. In the experimentally accessible temperature range (above the spontaneous nucleation temperature of ice) the heat capacity difference delta C is found to be positive with a non-linear temperature dependence. delta C is predicted to change sign at some low temperature which cannot, however, be reached by experiment for chymotrypsinogen. In contrast to earlier studies, covering a much more limited temperature range and having to employ an additional destabilisation by means of pH and/or chaotropes, the present findings permit the construction of a more reliable thermodynamic stability profile and related properties. These differ in important details from those reported for other proteins, but based on measurements only in the neighbourhood of the heat-denaturation temperature. The thermodynamic characteristics are, however, in good agreement with earlier predictions and with recent low-temperature measurements on the tetrameric enzyme lactate dehydrogenase.

Heat capacity of proteins: I. Partial molar heat capacity of individual amino acid residues in aqueous solution: Hydration effect

The partial molar heat capacities of various peptides and various organic compounds that model the amino acid side-chains or their parts in aqueous solution have been determined by precise scanning microcalorimetry in the temperature range from 5 to 125 degrees C. This provides an estimate of the partial molar heat capacity of the peptide--CHCONH--group and the side-chains of all amino acid residues. The values obtained are compared with the values found for these substances in the gaseous phase, in order to define the hydration effect. It has been shown that the partial heat capacity of the non-polar groups is positive at low temperature (5 degrees C) and decreases with increasing temperature, while for the polar and charged groups it is negative at low temperature, becomes zero at room temperature and increases further with increasing temperature. This leads to a hydrophobicity scale of the amino acid side-chains based upon the temperature dependences of their heat capacities. Due to the observed specificity in the temperature dependence, at room temperature, the heat capacities of amino acid side-chains correlate well with the non-polar surface areas.

Heat capacity of proteins: II. Partial molar heat capacity of the unfolded polypeptide chain of proteins: Protein unfolding effects

Using the heat capacity values for amino acid side-chains and the peptide unit determined in the accompanying paper, we calculated the partial heat capacities of the unfolded state for four proteins (apomyoglobin, apocytochrome c, ribonuclease A, lysozyme) in aqueous solution in the temperature range from 5 to 125 degrees C, with an assumption that the constituent amino acid residues contribute additively to the integral heat capacity of a polypeptide chain. These ideal heat capacity functions of the extended polypeptide chains were compared with the calorimetrically determined heat capacity functions of the heat and acid-denatured proteins. The average deviation of the experimental functions from the calculated ideal ones in the whole studied temperature range does not exceed the experimental error (5%). Therefore, the heat-denatured state of a protein, in solutions with acidic pH preventing aggregation, approximates well the completely unfolded state of this macromolecule. The heat capacity change caused by hydration of amino acid residues upon protein unfolding was also determined and it was shown that this is the major contributor to the observed heat capacity effect of unfolding. Its value is different for different proteins and correlates well with the surface area of non-polar groups exposed upon unfolding. The heat capacity effect due to the configurational freedom gain by the polypeptide chain was found to contribute only a small part of the overall heat capacity change on unfolding.

Thermodynamics of aqueous solutions of polyvinylcaprolactam

The thermodynamic properties of aqueous solutions of polyvinylcalprolactam have been explored. The diagram of state for this system representing a binodal with a lower critical dissolution temperature lying in the interval 305–307 K for samples with different MM has been obtained. At 298 K the authors measured the Gibbs energy and the mixing enthalpy and entropy of polyvinylcaprolactan with water. All the thermodynamic mixing functions are negative over the whole region of compositions. With rise in temperature the exothermicity of mixing falls. From the temperature dependence of the enthalpy the values of the mean excess heat capacity were calculated in the interval 293–303 K over the whole region of compositions. At 298 K the heat capacity of dilute aqueous polyvinylcaprolactam solutions was measured and the limiting partial heat capacity of this polymer in an infinitely dilute solution calculated. The results show that in aqueous polyvinylcaprolactam solutions both hydrophobic and hydrophilic hydration is observed. In the temperature and composition regions close to the binodal hydrophobic hydration predominates so that the mutual miscibility of the polymer with water decreases and the solution undergoes phase separation.

Heat capacity and conformation of proteins in the denatured state

Heat capacity, intrinsic viscosity and ellipticity of a number of globular proteins (pancreatic ribonuclease A, staphylococcal nuclease, hen egg-white lysozyme, myoglobin and cytochrome c) and a fibrillar protein (collagen) in various states (native, denatured, with and without disulfide crosslinks or a heme) have been studied experimentally over a broad range of temperatures. It is shown that the partial heat capacity of denatured protein significantly exceeds the heat capacity of native protein, especially in the case of globular proteins, and is close to the value calculated for an extended polypeptide chain from the known heat capacities of individual amino acid residues. The significant residual structure that appears at room temperature in the denatured states of some globular proteins (e.g. myoglobin and lysozyme) at neutral pH results in a slight decrease of the heat capacity, probably due to partial screening of the protein non-polar groups from water. The heat capacity of the unfolded state increases asymptotically, approaching a constant value at about 100 °C. The temperature dependence of the heat capacity of the native state, which can be determined over a much shorter range of temperature than that of the denatured state and, correspondingly, is less certain, appears to be linear up to 80 °C. Therefore, the denaturational heat capacity increment seems to be temperature-dependent and is likely to decrease to zero at about 140 °C.

The character of hydration in aqueous solutions of poly(1-vinylazoles)

The heat capacity of aqueous dilute solutions of poly(1-vinylimidazole) and poly(1-vinyl-1,2,4-triazole) and the diffusion coefficient of water in films made from these polymers were measured at 298 K as a function of concentration. The specific heat capacity of dilute aqueous solutions decreases with increasing polymer concentration. The partial heat capacities of the two polymers in the limit of infinite dilution were calculated. The diffusion coefficient increases with increasing concentration of water in the film. The results are discussed from the point of view of hydrophobic and hydrophilic interactions. The latter predominates in the poly(1-vinylazole)-water systems investigated.

Partial specific heat capacity of benzene and of toluene in aqueous solution determined calorimetrically for a broad temperature range

Using a precise scanning-microcalorimetric technique, the heat-capacity difference between water and an aqueous solution of benzene or of toluene has been measured for a broad temperature range (from 278 to 413 K) under excess pressure, and the partial specific heat capacity of these solutes has been determined. The hydration increment of the heat capacity of benzene and of toluene is proportional to the respective surfaces and decreases asymptotically as the temperature increases, as predicted by Gill et al. (1)

Temperature-induced phase transitions in proteins and lipids. Volume and heat capacity effects.

The partial specific heat capacity and volume of globular proteins and dispersions of phosphatidylcholines in aqueous solutions have been determined over a broad temperature range using a precise scanning microcalorimeter and a vibrational densimeter. It is shown that the temperature-induced, gel-to-liquid crystalline phase transition in phosphatidylcholines proceeds without a noticeable change in heat capacity but with a significant increase in the specific volume, whereas heat denaturation in proteins takes place without a noticeable change in the volume but with a significant increase in heat capacity. This principal difference between temperature-induced conformational phase transitions in proteins and lipids demonstrates clearly that heat denaturation of proteins, in contrast to the gel-to-liquid crystalline phase transition in lipids, cannot be regarded as a process similar to melting. Consequently, the 'molten globule' does not appear to be a suitable model for a heat-denatured protein.

Correlation of the character of intramolecular melting with digestibility by pepsin in precipitating and non-precipitating pig anti-DNP antibodies

Precipitating and non-precipitating pig anti-2,4-dinitrophenyl group (Dnp) antibodies were investigated by differential adiabatic scanning microcalorimetry in the pH range 3.7–4.5. The partial heat capacity functions obtained revealed a notable difference between the two antibody types when the analysis was done at pH 4.5 or 4.0. The transition observed at temps around 50°C in a non-precipitating antibody was absent in a precipitating antibody. Under analogous conditions, pH 4.5, the precipitating antibody was fully resistant to pepsin, while the non-precipitating antibody yielded appropriate F(ab') 2 and pFc'fragments. At pH 3.7 no substantial difference in the partial heat capacity function could be observed between the two antibody types. Below pH 4.0 the precipitating antibody became susceptible to peptic cleavage and yielded fragments of the same general character as the non-precipitating antibody. This finding lends support to the view that the structural block in immunoglobulin G that melts first, i.e. at temps near 50°C, is the C H2 domain or a part of it.

Application of a solvent-coupled model to a calculation of partial molar isobaric heat capacities of neutral and ionic solutes in aqueous solution. Estimates of isobaric heat capacities of activation

Partial molar isobaric heat capacities and volumes of apolar and ionic solutes in water have been examined using the Lumry–Frank two-state model for water in conjunction with the Grunwald extrathermodynamic assumption describing the impact of a solvent equilibrium on the properties of a solute. The analysis has been extended to a consideration of isobaric heat capacities of activation. The predicted dependence of the latter parameter on temperature for a first-order unimolecular reaction resembles that calculated using the Albery–Robinson mechanism for solvolytic reactions.

Evaluation of the Thermodynamic Functions for Aqueous Sodium Chloride from Equilibrium and Calorimetric Measurements below 154 °C

A new weighted least-squares method is described which is generally applicable for the nonsubjective evaluation of the best set of thermodynamic functions from a given data set of equilibrium (ΔG) and calorimetric (ΔH, Cp) measurements. The method, applied to model a wide range of 2428 measurements for the water-sodium chloride system between −21 and 154 °C, accurately represents all measurements within experimental error. The resulting model is used to predict the thermodynamic functions and their standard errors for aqueous sodium chloride up to 110 °C. Tables are given for freezing point, solubility, boiling point, osmotic and activity coefficients, vapor pressure, apparent molal relative enthalpy, partial molal relative enthalpies, integral heat of solution, specific heat, apparent molal heat capacity, partial molal heat capacities, apparent molal relative heat capacity, partial molal relative heat capacities, standard thermodynamic functions, and their changes for dissolution.

Heat capacity of water absorbed in methylcellulose

AbstractThe heat capacity of methylcellulose is determined at various water contents over a wide range of temperatures, down to 125 K. From the data, the partial specific heat capacity of water has been obtained. The results show that water should be considered as a single, mobile phase. No evidence exists for bound water. Near 130 K water becomes immobilized in a glassy state.