Showing papers on "Enthalpy published in 1971"

Thermodynamic analysis of thermal transitions in globular proteins. I. Calorimetric study of ribotrypsinogen, ribonuclease, and myoglobin

Abstract: Thermal confonnalional transformations of globular proteins, chymotrypsinogen, ribo-nudcase and myoglobin in solutions at different pH values were studied microcalorimetrically. It was shown that the heat effects observed in the process of heating have a complicated form and their explanation necessitates an assumption on the existence of two stages of the process separated by temperature: (I) a pre-denaturational stage where the protein partial heat capacity changes are probably connected with a labilization of the globule structure and (II) a denaturational stage representing a single-step transition of the protein into a state with a higher enthalpy. Transitions without enthalpy change, the existence of which is shown for the first time in globular proteins, necessitates a more cautions approach to the thennudynamical analysis of different physical parameters on the basis of the van't Hoff equation.

Investigation of the 2050 Å system of the nitric oxide dimer

Abstract: The far ultra-violet continuum of nitric oxide was assigned to the dimer by obtaining the following experimental evidence : (a) the intensity of the spectrum is proportional to [NO]2; (b) the possibility that the carrier may be some other known oxide of nitrogen was eliminated; (c) from the temperature dependence the enthalpy change accompanying dissociation was 2.24 (±0.1 ) kcal/mol (100–140 K), in good agreement with 2.4 (±0.2) kcal/mol deduced from the dimer infra-red intensities4; (d) approximate upper and lower limits to the dissociation constant, and the temperature dependence, were consistent with Guggenheim's5 method of analyzing the second virial coefficients by means of the principle of corresponding states. An absolute scale of dissociation constants was fixed from the excess of the reduced second virial coefficient at its normal boiling point, and the temperature dependence found spectroscopically. The dissociation energy of the dimer is 1.6 (±0.1) kcal/mol. Evidence for substantial population of excited states of the dimer at 300 K was discussed. The dimer continuum exhibits a maximum at 2050 A, and the oscillator strength of the system was recorded as 0.26.

Measurement of the relative enthalpy of pure a-Al2O3 (NBS heat capacity and enthalpy Standard Reference Material No. 720) from 273 to 1173 K

Abstract: The relative enthalpy of NBS Standard Reference Material No. 720 (99.98 percent pure, single-crystal α-Al2O3, a calorimetrie heat-capacity standard) was measured over the range 273 to 1173 K by the drop method using a highly precise Bunsen ice calorimeter. Enthalpy data over the same temperature interval were obtained also on the Calorimetry Conference Sample of this substance. These results are believed to be more accurate than similar NBS results on the latter sample published in 1956, and show no significant discontinuity with the NBS data on the same substance that covered the ranges 13 to 380 K and 1173 to 2257 K. The average deviation from the mean for all enthalpy measurements on the SKM 720 sample was 0.017 percent, and the smooth enthalpy values derived from the data were estimated to be accurate to 0.1 percent. The precautions observed in order to minimize measuring errors are described in detail. The data are compared with many sets of the most reliable published data available and new recommended values for the thermodynamic functions of α-Al2O3 are presented for the interval 0 to 1200 K.

Experimental thermodynamics of the helix–random coil transition. IV. Influence of the base composition of DNA on the transition enthalpy

Abstract: The helix–coil transition of DNA of various base compositions has been followed by measuring the heat capacity and the ultraviolet absorption as a function of temperature through the transition region. The results of these measurements indicate that the transition enthalphy ΔH for a given pH value and salt concentration depends considerably on the base composition of the various DNA samples. The values of the transition enthalpies vary from ΔH = 7.73 kcal/mole base pair (MBP) for 31% GC base pairs to ΔH = 8.52 kcal/MBP for 72% GC base pairs. The results of the heat capacity measurements are discussed in relationship to the theoretical models suggested by various authors. Some peculiarities of the measured heat capacity versus temperature curves are reported. The correlation between these peculiarities and the results of the transcription experiments of DNA is discussed.

Methanol: Heat Capacity, Enthalpies of Transition and Melting, and Thermodynamic Properties from 5–300°K

Abstract: Thermal properties of methanol were studied by adiabatic calorimetry. The first‐order nature of the phase transition at 157.4°K with an entropy increment of 0.97 cal mole−1·°K−1 was confirmed. The heat capacity of the crystalline phase stable just below the triple point was defined and shown to be extremely sensitive to impurity. No evidence for a second previously‐reported phase transition could be detected. The standard entropy (S°) and Gibbs energy function (− [G° − H°0] / T) for the liquid at 298.15°K are 30.40 and 15.18 cal mole−1·°K−1, respectively. The proposed classification of methanol as a plastic crystal on the basis of its small entropy of melting (4.38 cal mole−1·°K−1) is considered with respect to hydrogen bonding in the liquid phase.

The kinetics of reduction of hematite by carbon

Abstract: Reduction kinetics of mixtures of hematite and carbon powders were investigated in the temperature range of 850° to 1087°C. Experiments were carried out under argon atmosphere and the isothermal weight loss of the samples was determined as a function of time. The effects of carbon particle size, hematite/carbon ratio of the mixture, and addition of promotive or inhibitive reagents were also investigated. The results were summarized in the form of fractional reaction vs time plots. A kinetic model developed on the basis of carbon solution-loss reaction as rate-controlling represented the results fairly well. An enthalpy of activation of 72 kcal/per mole was calculated, within the range of 957° to 1087°C. The observed effects of Li2O and FeS on the reduction kinetics are consistent with the influence these reagents are known to exercise on the solution-loss reaction.

Thermodynamics of polystyrene solutions. Part 3.—Polystyrene and cyclohexane

Abstract: The excess volumes VE of mixtures of polystyrene with cyclohexane at 25°C are negative; at a segment fraction of ϕ2= 0.5, VE amounts to ca. 0.14 % of the total volume. This result in conjunction with chemical potentials and excess enthalpies determined by Palmen, Schmoll and Jenckel, by Krigbaum and Geymer, and by Scholte are interrelated here by the statistical thermodynamic theory employed in the preceding papers. On assignment of X12= 42 J cm–3 and Q12= 0.023 J cm–3 deg–1 the excess volume VE, the reduced residual chemical potential χ, and the reduced partial molar excess enthalpy χH are well represented at all concentrations. The theory also predicts a lower critical solution temperature in agreement with experiments of Allen et al. and of Tager et al. indicating an LCST in the range 180°C to 210°C for this system.

Thermodynamic parameters of helix-coil transition in polypeptide chains. II. Poly-L-lysine.

Abstract: The helix–coil transitions for poly-L-lysine (PL) were investigated by the methods of spectropolarimetry, viscometry and potentiometric titration in 02M NaCl at different temperatures as well as in 02MNaBr, 1MKCl, and in mixtures of 02MNaCl or NaBr with methanol at room temperature The enthalpy and entropy differences between the helical and coillike states of uncharged PL molecules in 02M NaCl were determined from the potentiometric titration curves The cooperativity parameters σ for PL in different solvents were determined by two methods (from the sharpness of the transition and from the dependence of the intrinsic viscosity on the helical content in the transition region) In 02MNaCl σ has a value of (23 ± 05) × 10−4 and does not depend on temperature, ie, the cooperativity of the helix-coil transition, as for PGA, is mainly of an entropy origin (the initiating of the helical region is accompanied by the entropy decrease ΔSi = −12 eu/mole of helical regions) A comparison of the obtained results for PGA and PL with the molecular theories of the helix-coil transitions shows that the role of dipole-dipole interactions of nonneighboring peptide groups is greatly overestimated in these theories, leading to a considerable enthalpy contribution to the free energy of initiating helical regions which is not observed in the experiment

Thermodynamic study on aqueous dilute solutions of organic compounds. Part 2.—Cyclic ethers

Abstract: Free energy, enthalpy and entropy variations associated with solution and hydration processes have been determined for the following compounds: tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, tetrahydropyran, 1,3-dioxolan and 1,4-dioxan. Negative ΔSh and ΔHh values result from the transfer of the above-mentioned ethers from the gaseous state to dilute aqueous solutions. Their absolute values, however, are markedly smaller for the diethers than for monoethers, this being particularly so for ΔSh values. Comparisons are made between the thermodynamic behaviour in aqueous solutions of cyclic ethers and amines and the analogue, open-chain derivatives. On the basis of views currently held on water structure, hypotheses are suggested to explain effects associated with (i) changing from open to cyclic systems, (ii) lengthening of the chain and methyl substitution, and (iii) the presence, in cyclic systems, of several hydrophilic centres.

Thermodynamic parameters of helix-coil transition in polypeptide chains. I. Poly-(L-glutamic acid).

Abstract: The helix-coil transitions for poly(L-glutamic acid) (PGA) in 0.2M NaCl and in its mixture with dioxane were studied by the methods of spectropolarimetry, viscometry, and potentiometric titration at different temperatures from 8 to 50°C. The enthalpy and entropy differences between the helical and coillike states of uncharged PGA molecules were determined from the curves of potentiometric titration. The temperature dependence of the cooperativity parameter σ was determined by two methods: from the sharpness of transition and from the dependence of the intrinsic viscosity on the helical content in the transition region. In 0.2MNaCl, σ= (2.5 ± 0.5) × 10−3 and practically does not depend on temperature, i.e., the cooperativity of the helix-coil transition is connected mainly with the entropy decrease in initiating helical regions (ΔSi ≈ −12 is mole of helical regions). On the contrary, initiation of a helical region in the water-organic solvent mixture is accompanied by a considerable enthalpy increase.

Potentiometric titration of poly‐L‐lysine: the coil‐to‐β transition

Abstract: We have performed potentiometric titrations of poly-L-lysine. From these data we have calculated the free energy and enthalpy changes for the folding of the random coil to the α-helix in 10% ethanol (−120 and −120 cal/mole) and from the random coil to the β-structure in water (−140 and 870 cal/mole) and in 10% ethanol (−180 and 980 cal mole). Comparison of these values with each other and with values for the coil α- helix transition in water (−78 and −880 cal/mole) led to the following conclusions. The stabilization by ethanol of ethanol of the α-helix with respect to the coil is that predicted from the known free energy of transfer of the peptide group from water to 10% ethanol. Similar data to explain the enthalpy difference are not available. The thermodynamic functions for the transition from α-helix to β-structure, obtained by subtracting those for the coil α-helix and coil β-structure transitions, are explained from a consideration of the structural differences: non bonded interactions of the polypeptide backbone are less favorable in the β-structure than in the α-helix, causing an increase in the energy, while hydrophobic contacts between side chains raise the entropy of the β-structure as compared with the α-helix, so that the free energy difference between the two structures is small, but enthalpy and entropy differences are large. The observation of only small differences in the free energy and enthalpy changes for the transition from coil β-structure upon going from water to 10% ethanol is expected by considering both the free energy of transfer of the peptide group (as for the α-helix) and the free energy and enthalpy of transfer of the apolar part of the side chain involved in hydrophobic bond formation.

Calorimetric Study of the Glassy State. VI. Phase Changes in Crystalline and Glassy-Crystalline 2,3-Dimethylbutane

Abstract: 2,3-Dimethylbutane has two crystalline phases, high temperature form (crystal-I) and low temperature form (crystal-II). On account of the supercooling effect of crystal-I, its glassy crystalline state (non-equilibrium frozen-in state) is established below Tg (76 K) by rapid cooling. The heat capacities for various phases were determined from 13 to 300 K from principal interest in the thermodynamic properties of the glassy crystal. The transition temperature from crystal-II to crystal-I and melting point were determined to be 136.02 K and 145.05 K, respectively. The heats of transition and of fusion were determined to be 6427 J mol−1 and 788 J mol−1, respectively. From these data, the residual entropy and T2 temperature were determined to be 7.4 JK−1 mol−1 and 65 K, respectively. We also calculated the ratio of Tg to T2 for several glassy crystals known hitherto, which amounts to 1.15–1.20. The stabilization effect was studied by the measurement of the change of the configurational enthalpy in the glassy c...

Magnetic and Thermal Properties of Crystals Including Isolated Clusters. I. Heat Capacity and Infrared Spectrum of [Cr3O(CH3COO)6(H2O)3]Cl·6H2O Crystal between 1.5 and 280 K

Abstract: The heat capacity was measured between 1.5 and 280 K. At liquid helium temperatures a heat capacity anomaly was observed and a new phase transition was found at 211.4 K with a secondary C p maximum at 215.5 K. The entropy and the enthalpy changes for the overall transition are Δ S t =13.778 J K -1 mol -1 and Δ H t =3322 J mol -1 , respectively. The infrared spectra were measured at 120,225 and 295 K. Many new absorption peaks appeared below the transition point. A mechanism for the phase transition is suggested in terms of ordering processes of crystalline water. A new model is proposed for explaining the heat capacity anomaly at liquid helium temperature: The cluster ions with equilateral triangle symmetry at room temperature are slightly distorted through the phase transition processes resulting two kinds of cluster ions forming isosceles triangles with different exchange parameters. The closest agreement with the experimental data was obtained by assuming J 0 =30 k and J 1 =4.5 k for half moles of the ...

Thermodynamic properties of the molten thallium + tellurium system

Abstract: The thermodynamic properties of molten Tl + Te alloys have been determined by the e.m.f. method. The observed integral entropy of mixing becomes practically zero and enthalpy shows a minimum at the stoichiometric composition Tl2Te. The excess stability also exhibits a strong tendency to compound formation at this composition. It is concluded from these observations, together with those of the electrical properties,1–5 that the molten Tl + Te system can be treated as two pseudo-binary systems, Tl + Tl2Te and Tl2Te + Te, and that around the stoichiometric composition Tl2Te a high local ordering can be considered because of the ionic character of the chemical bonding.

Drop Calorimetric Measurements on some Chlorides, Sulfides, and Binary Melts

Abstract: Enthalpy content measurements on pure KCl, AgCl, CuCl, PbCl2, Ag2S, and PbS were made at temperatures up to 1000 °C using a sapphire calibrated drop calorimeter. Similar measurements were performed on AgCl–Ag2S and AgCl–KCl melts. These results determined values for the integral enthalpy of liquid–liquid mixing.

Thermodynamics of polystyrene solutions. Part 2.—Polystyrene and ethylbenzene

Abstract: The volume change on mixing polystyrene with ethylbenzene has been determined directly at 25°C. It is negative and amounts to ca. 0.30 % of the total volume at ϕ2= 0.5. Osmotic pressures have been measured at 10°, 35°, and 60°C in the concentration range ϕ2= 0.09 to 0.27. The temperature coefficient of the reduced residual chemical potential χ is close to zero; hence the heat of dilution is very small. The entropy of dilution is positive but small. The statistical thermodynamic theory employed in the preceding paper accounts for the sign and magnitude of the excess volume. The experimental partial molar entropies are much smaller than the combinatory contribution alone would allow. This finding and the small negative enthalpy of dilution indicated by recent work of Palmen are shown to arise mainly from equation-of-state terms of the newer theory. The parameter X12 representing the exchange enthalpy is positive and small, as should be expected for this system. The observed increase in χ with concentration is well reproduced by the theory. Thermal expansivities and thermal pressure coefficients of ethylbenzene have been determined at atmospheric pressure from 10° to 90°C.

The thermodynamic properties of carbon in body-centered cubic iron

Abstract: The temperature dependence of the equilibrium between m ethane-hydrogen gas mixtures of varying composition and carbon dissolved in bcc iron has been investigated. The data obtained have been analyzed to give values of the partial enthalpy and excess entropy of carbon in ferrite. The results have been discussed and compared to other investigations of the thermodynamic properties of the C-bcc-Fe system. From this investigation the partial excess entropy of the carbon is 6.56k, the partial enthalpy with respect to a solute atom at rest in a vacuum is −144.06 kcal per mole and the corresponding relative partial enthalpy is 23.34 kcal per mole at 1000°K.

Calorimetric measurement of enthalpy change in the isothermal helix–coil transition of poly(L‐ornithine) in aqueous solution

Abstract: The enthalpy change associated with the isothermal pH-induced uncharged coil-to-helix transition ΔHh° in poly(L-ornithine) in 0.1 N KCl has been determnined calorimetrically to be −1530 ± 210 and −1270 ± 530 cal/mol at 10° and 25°C, respectively. Titration data provided information about the state of charge of the polymer in the calorimetric experiments, and optical rotatory dispersion data about its conformation. In order to compute ΔHh°, the observed calorimetric heat was corrected for the heat of breaking the sample cell, the heat of dilution of HCl, the heat of neutralization of the OH− ion, and the heat of ionization of the δ-amino group in the random coil. The latter was obtained from similar calorimetric measurements on poly(D,L-ornithine). Since it was discovered that poly(L-ornithine) undergoes chain cleavage at high pH, the calorimetric measurements were carried out under conditions where no degradation occurred. From the thermally induced uncharged helix–coil transition curve for poly(L-ornithine) at pH 11.68 in 0.1 N KCl in the 0°–40°C region, the transition temperature Ttr and the quantity (∂θh/∂T)Ttr have been obtained. From these values, together with the measured values of ΔHh°, the changes in the standard free energy ΔGh° and entropy ΔGh°, associated with the uncharged coil-to-helix transition at 10°C have been calculated to be −33 cal/mol and −5.3 cal/mol deg, respectively. The value of the Zimm–Bragg helix–coil stability constant σ has been calculated to be 1.4 × 10−2 and the value of s calculated to be 1.06 at 10°C, and between 0.60 and 0.92 at 25°C.

Cation self-diffusion in single crystal MgO

Abstract: Measurements of the self-diffusion of Mg2+ in single crystal MgO of two different purities have been made over the temperature range 1100°C to 1750°C. Above approximately 1300°C the results show direct evidence of the operation of both intrinsic and extrinsic diffusion. Below this temperature precipitation of the non-active impurities present appears to take place. Using the earlier similar but apparently purely intrinisic measurements of Lindner and Parfitt (1957) it is possible to evaluate both the enthalpy of motion for the cation vacancy, H m, and the enthalpy of formation of the complete Schottky defect, H f. The results obtained are: H m = 1·7±0·1 ev; H f = 3·4±0·2 ev.

Molten Fatty Acid Salts as Model Ionic Liquids. I. Thermodynamic and Transport Parameters of Some Organic Sodium Salts

Abstract: Procedures are described for maintaining good chemical stability in molten alkali-metal carboxylates, up to about 350 degrees C. Valid physical measurements can be made and the fluids can be used up to about this temperature, above which spontaneous decomposition of the anions is difficult to repress. Molten sodium propionate has a useful liquid range of about 60 degrees C and sodium isobutyrate of about 90 degrees C. Sodium n-butyrate transforms into a 'liquid crystal' at about 250 degrees C and into the isotropic liquid at 324 degrees C. For sodium isovalerate corresponding transition points are respectively 188 and 280 degrees C. Thermodynamic measurements are reported of volume and enthalpy changes at transition and melting points. Transport parameters measured include the viscosity and the electrical conductivity. The viscosity of these ionic melts undergoes a steep decrease at the transition from mesomorphic to isotropic liquids. Jumps in ionic conductivity are found both at the melting and clearing points. Even for the isotropic liquids, the ratio of viscosity to electrical conductivity is exceptionally high, compared with the other ionic melts. Mechanisms of melting for these ionic solids are discussed.