Partial Specific Heat Capacity Research Articles

Studies of relationship between polymer structure and hydration environment in amphiphilic polytartaramides

ABSTRACTThe relationships between the chemical structures and hydration environment of the polymers can provide significant insight into the water‐amphiphilic polymer interactions. Here, the hydrophobicity of amphiphilic block copolymers poly(ethylene tartaramide‐b‐alkyl isocyanate) is gradually tuned by using of a series of pendant alkyl (isopropyl, n‐butyl, cyclopentyl, and cyclohexyl) groups. Dynamics of hydration probed by low‐field NMR relaxometry exhibits a heterogeneous environment of water molecules, corresponding to tightly bound water with slow re‐orientational mobility and loosely bound water with fast re‐orientational mobility. Progressively larger amounts of bound water are present in the copolymers, ongoing from pendant isopropyl, n‐butyl, cyclopentyl, and finally to cyclohexyl group. Water in the copolymer bearing the cyclohexyl group has a significantly high partial specific heat capacity. Therefore, hydrophobic interaction between the polymer and water is enhanced when the hydrophobicity of the polymer is increased, resulting in considerable hydrophobic hydration with decreased mobility of the bound water. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 138–145

Denaturation of type I collagen fibrils is an endothermic process accompanied by a noticeable change in the partial heat capacity.

Thermal transitions of type I collagen fibrils were investigated by differential scanning calorimetry and spectrophotometry of turbidity within a wide range of external conditions. The advanced microcalorimeter allowed us to carry out the measurements at low concentrations of collagen (0.15-0.3 mg/mL). At these concentrations of collagen and under fibril-forming conditions, the melting curves display two pronounced heat adsorption peaks (at 40 and 55 degreesC). The low-temperature peak was assigned to the melting of monomeric collagen, while the high-temperature peak was assigned to the denaturation of collagen fibrils. It was shown that the denaturation of fibrils, in contrast to the monomeric collagen, is accompanied by a noticeable change in the partial specific heat capacity. Surprisingly, comparison of the collagen calorimetric curves in the fibril-forming and nonforming conditions revealed that DeltaCp of fibril denaturation is caused by a decrease in the Cp of collagen at premelting temperatures. This suggests the existence of an intermediate structural state of collagen in a transparent solution preceding fibril formation. Our study also shows that collagen fibrils formed prior to heating have thermodynamic parameters different from those of fibrils formed and denatured during heating in the calorimeter. Analysis of the data allowed us to determine the denaturation enthalpy of the mature fibrils and to conclude that the enthalpy plays a more important role in fibril stabilization than was previously assumed. The observed large DeltaCp value of fibril denaturation as well as the difference between thermodynamic parameters of the mature and newly formed fibrils is readily explained by the presence of water molecules in the fibril structure.

Structural energetics of barstar studied by differential scanning microcalorimetry.

The energetics of barstar denaturation have been studied by CD and scanning microcalorimetry in an extended range of pH and salt concentration. It was shown that, upon increasing temperature, barstar undergoes a transition to the denatured state that is well approximated by a two-state transition in solutions of high ionic strength. This transition is accompanied by significant heat absorption and an increase in heat capacity. The denaturational heat capacity increment at approximately 75 degrees C was found to be 5.6 +/- 0.3 kJ K-1 mol-1. In all cases, the value of the measured enthalpy of denaturation was notably lower than those observed for other small globular proteins. In order to explain this observation, the relative contributions of hydration and the disruption of internal interactions to the total enthalpy and entropy of unfolding were calculated. The enthalpy and entropy of hydration were found to be in good agreement with those calculated for other proteins, but the enthalpy and entropy of breaking internal interactions were found to be among the lowest for all globular proteins that have been studied. Additionally, the partial specific heat capacity of barstar in the native state was found to be 0.37 +/- 0.03 cal K-1 g-1, which is higher than what is observed for most globular proteins and suggests significant flexibility in the native state. It is known from structural data that barstar undergoes a conformational change upon binding to its natural substrate barnase.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

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.

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)

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.

Calorimetric study of tryptophan synthase alpha-subunit and two mutant proteins.

Heat-denaturation of tryptophan synthase alpha-subunit from E. coli and two mutant proteins (Glu 49 leads to Gln or Ser; called Gln 49 or Ser 49, respectively) has been studied by the scanning microcalorimetric method at various pH, in an attempt to elucidate the role of individual amino acid residues in the conformational stability of a protein. The partial specific heat capacity in the native state at 20 degrees, Cp20, has been found to be (0.43 +/- 0.02) cal . k-1 . g-1, the unfolding heat capacity change, delta dCp, (0.10 +/- 0.01) cal . K-1 . g-1, and the unfolding enthalpy value extrapolated to 110 degrees, delta dh110, (9.3 +/- 0.5) cal . g-1 for the three proteins. The value of Cp20 was larger than those found for "fully compact protein" and that of delta dh110 was smaller. Unfolding Gibbs energy, delta dG at 25 degrees for Wild-type, Gln 49, and Ser 49 were 5.8, 8.4, and 7.1 kcal . mol-1 at pH 9.3, respectively. Unfolding enthalpy, delta dH, of the three proteins seemed to be the same and equal to (23.2 +/- 1.2) kcal . mol-1 at 25 degrees. As a consequence of the same value of delta dH and the different value in delta dG, substantial differences in unfolding entropy, delta dS, were found for the three proteins. The values of delta dG for the three proteins at 25 degrees coincided with those from equilibrium methods of denaturation by guanidine hydrochloride.

Heat capacities of water swollen hydrophilic polymers above and below 0°C

AbstractThe heat capacities of water swollen poly[2‐(2‐hydroxyethoxy)ethyl methacrylate] were determined in a DSC‐2 calorimeter within the temperature range 220–350K for concentrations from 0 to 1,2g of water per 1g of the polymer. At temperatures above 0°C the partial specific heat capacity of water in gel is concentration independent and equal to the specific heat capacity of pure liquid water. It seems, therefore, that water does not form stable icelike structures near polymer chains. To analyze phase transformations of water in gel below 0°C, general thermodynamic equations were derived and used as a basis for suggesting criteria which allow to decide whether in a given experiment the phase transformation proceeded in an equilibrium way. In measurements below the melting point of ice the conditions for an equilibrium process consist in the preceding heating of the frozen sample to a temperature close to the melting point, followed by cooling to 220K. The assessed composition dependence of the melting point depression is consistent with the dependence of activity on concentration obtained from measurements of water vapour sorption at 35°C. Analysis of data on the heat capacity below 0°C led to the conclusion that at a water content of 0,4g/g and lower, or at temperatures below 250K the crystallization of water from gel was inhibited by kinetic factors originating probably in the reduced diffusivity of water in gel, due to the reduced mobility of polymer chains. Hence, non‐freezing water need not be identical with “strongly bound” water; in the study of water structure in polymers based on heat capacities, preference should be given to data obtained at usual and elevated temperatures.

The heat capacity of water swollen poly(2-hydroxyethyl acrylate) above and below 0 °C

The heat capacities of water swollen poly(2-hydroxyethyl acrylate) were determined with a DSC-2 calorimeter in the range 220-350 K for concentrations between 0 and 1.95 g water per 1 g polymer. The results confirm conclusions obtained in a preceding paper for the system poly[2-(2-hydroxyethoxy)ethyl methacrylate]-water. At temperatures above 0°C the partial specific heat capacity of water in the polymer is independent of composition and equal to the specific heat capacity of pure liquid water. This indicates that water sorbed in the polymer is not strongly bound. Analysis of data on the heat capacity below 0°C has led to a conclusion that at a water content below 0.2 g/g or at temperatures 230 K and lower, the crystallization of water from the gel is inhibited by factors which make impossible the attainment of thermodynamic equilibrium. Non-freezing water should not be identified with strongly bound water.

Partial specific volume, expansibility, compressibility, and heat capacity of aqueous lysozyme solutions.

Density measurements have been made on aqueous lysozyme solutions at 20, 25, and 30 degrees. The apparent specific volumes, phi v, and expansibilities, phi e, have been determined from the density measurements and fitted to a function of concentration (weight per cent). Sound velocities and heat capacities have also been measured for various concentrations of lysozyme-water solutions at 25 degrees. From the density, expansibility, heat capacity, and sound velocity data at 25 degrees, the isothermal compressibility, phi k, for the lysozyme solutions have been calculated over a range of concentrations. All the physicochemical properties measured were found to be a linear function of the weight per cent of lysozyme. The number of water molecules hydrated to 1 mol of lysozyme was estimated from the volume and compressibility and found to be 162 at 25 degrees.

Calorimetric determination of enthalpies of solution of slightly soluble liquids I. Application to benzene in water

The enthalpies of solution of benzene in water at 288.15, 293.15, 298.15, and 303.15 K have been determined by a novel flow-microcalorimetric technique. The apparatus is so designed that precisely measured aliquots (1 to 5 mm 3) of the slightly soluble solute are injected into the solvent flow in the calorimeter. From the results ΔC p and the partial molar specific heat capacity, C p, 2 o(198.15 K), for benzene have been calculated.

Design and testing of an improved precise drop calorimeter for the measurement of the heat capacity of small samples

An improved version of an earlier drop heat-capacity calorimeter has been designed and tested. The calorimeter is intended for precise work with small solid or liquid samples (⩽ 1 g) and is designed primarily for the temperature range 273 to 343 K. Precision with 1 g samples is 0.01 per cent, whereas the accuracy is judged to be better than 0.1 per cent at 298 K. As part of the testing the partial molar heat capacity for diethoxyethane in 0.05 mol dm −3 aqueous solution was determined to be (571 ± 7) J K −1 mol −1, at 298.15 K. The partial specific heat capacity for lysozyme in 1.1 to 9.5 mass per cent solutions and at pH = 4.6 and 298.15 K was found to be (1.494 ± 0.007) J K −1 g −1.