Two-state irreversible thermal denaturation of anionic peanut (Arachis hypogaea L.) peroxidase

Detailed differential scanning calorimetry (DSC), steady-state tryptophan fluorescence and far-UV circular dichroism (CD) studies, together with enzymatic assays, were carried out to monitor the thermal stability of anionic peanut peroxidase (aPrx) at pH 3.0. The spectral parameters were seen to be good complements to the highly sensitive but integral method of DSC. Thus, changes in far-UV CD corresponded to changes in the overall secondary structure of the enzyme, while changes in intrinsic tryptophan fluorescence emission corresponded to changes in the tertiary structure of the enzyme. The results, supported with data concerning changes in enzymatic activity with temperature, show that thermally induced transitions for aPrx are irreversible and strongly dependent upon the scan rate, suggesting that denaturation is under kinetic control. It is shown that the process of aPrx denaturation can be interpreted with sufficient accuracy in terms of the simple kinetic scheme, , where k is a first-order kinetic constant that changes with temperature, as given by the Arrhenius equation; N is the native state, and D is the denatured state. On the basis of this model, the parameters of the Arrhenius equation were calculated.     

Thermoanalytical study of linkage isomerism in coordination compounds: Part I. Reinvestigation of thermodynamic and thermokinetic of solid state interconversion of nitrito (ONO) and nitro (NO2) isomers of pentaaminecobalt(III) chloride by means of DSC

Solid state thermal isomerization of [Co(NH₃)₅(ONO)]Cl₂ (nitrito isomer) to [Co(NH₃)₅(NO₂)]Cl₂ (nitro isomer) and reverse reaction were investigated by non-isothermal differential scanning calorimetry (DSC) and found to be essentially an equilibrium process. The interconversions are accelerated at above 65 °C and reach to equilibrium state at about 155 °C. After establishment of the equilibrium the relative amounts of two isomers at any temperature are governed by Gibbs free energy relationship. The experimental enthalpy changes of isomerization of pure nitrito and nitro solid samples to the equilibrium state are −4.67 (±0.19) and 0.99 (±0.05) kJ mol⁻¹, respectively. From these values, total enthalpy change was calculated as: ΔH°=−5.66(±0.20) kJ mol⁻¹. Using Gibbs free energy relationship, equilibrium constant, total free energy and entropy changes were estimated at 60 °C as: K=7.72(±0.8),  kJ mol⁻¹ and  J K⁻¹ mol⁻¹.An initial rate method has been developed to determine the kinetic parameters of these reactions from non-isothermal DSC data. Both nitro to nitrito and reverse reactions obey first order kinetic law in solid state. Estimated activation parameters of forward and reverse paths at 60 °C are , , and , respectively. The negative activation entropy of both directions support the intramolecular mechanism of isomerization, including formation of a seven coordinate transition state, which formerly suggested based on spectral and X-ray methods.     

Temperature-induced denaturation of Aes acetyl-esterase from Escherichiacoli

The thermal stability of the Aes acetyl esterase from Escherichia coli has been investigated by means of differential scanning calorimetry and circular dichroism measurements. The calorimetric curves show a denaturation temperature of 68 °C for Aes and 61 °C for the single point mutant V20D-Aes. The same values are obtained from CD denaturation curves of the two proteins recorded in both the far-UV and near-UV regions. Even if the denaturation process is irreversible and characterized by a single calorimetric peak and a single inflection point in both far- and near-UV CD curves, the overall data indicate that the process is more complex than a two-state transition. This is in line with the presence of two structural domains in the 3D model of Aes, according to homology modelling. A comparison of the thermal stability of Aes with those of the homologous thermophilic EST2 and hyperthermophilic AFEST suggests that the optimization of charge-charge interactions should not be so effective in the case of the mesophilic enzyme.     

Phase polymorphism of [Zn(DMSO)6](ClO4)2 studied by differential scanning calorimetry

Four solid phases of [Zn(DMSO)₆](ClO₄)₂ have been detected by differential scanning calorimetry (DSC). Specifically, the phase transitions were detected between: metastable phase KII ↔ supercooled phase K0 at , stable phase KIb ↔ stable phase KIa at , stable phase KIa ↔ stable phase K0 at . At T_m2 = 389 K crystals partially and at T_m1 = 465 K completely melts. From the entropy change values it was concluded that the phases: K0 and K0′ are the orientationally dynamically disordered phases, so called ODDIC crystals, and phases KIa, KIb and metastable KII are dynamically ordered but with some degree of positional disorder.     

Cooperative rearranging region size determination by temperature modulated DSC in semi-crystalline poly(l-lactide acid)

Thermal analyzes have been performed on poly(l-lactide acid) (PLLA) annealed at 353 K for different durations in order to obtain a crystallinity degree varying between 0% and 42%. From temperature modulated differential scanning calorimetry investigations, we have calculated the average volume of a cooperative rearranging region (CRR) at the glass transition temperature , according to the method developed by Donth. The results show that the presence of crystalline phase in PLLA amorphous matrix implies modifications on structural relaxation phenomena, in particular on the average number of monomer units relaxing in the same time at the glass transition temperature.     

Heat capacity and standard molar enthalpy of formation of crystalline 2,6-dicarboxypyridine (C7H5NO4)

Low-temperature heat capacity C_p,m of 2,6-dicarboxypyridine (C₇H₅NO₄; CAS 499-83-2) was precisely measured in the temperature range from (80 to 378) K with a high precision automated adiabatic calorimeter. No phase transition or thermal anomaly was observed in this range. The thermodynamic functions [H_T − H_298.15] and [S_T − S_298.15] were calculated in the range from (80 to 378) K. The standard molar enthalpy of combustion and the standard molar enthalpy of formation of the compound have been determined, and , by means of a precision oxygen-bomb combustion calorimeter at T = 298.15 K. The thermodynamic properties of the compound were further investigated through differential scanning calorimeter (DSC) and the thermogravimetric (TG) analysis.     

Effect of cation site-disorder on the structure and magneto-transport properties of Ln5/8M3/8MnO3 manganites

Five members of Ln_5/8M_3/8MnO₃ series with A-cation size variance (σ²) ranging between 3×10⁻⁴ and 71×10⁻⁴ Å², and the same A-cation size , have been synthesized by the ceramic method. The five manganites are single phase and they crystallize in the Pnma perovskite superstructure. The five compositions display ferromagnetic-paramagnetic transitions at temperatures ranging between 130 and 270 K, for the highest and lowest variance sample, respectively. The samples with smaller variances show sharp magnetization transitions and the samples with the larger variances display broad transitions. These transitions have also been studied by differential scanning calorimetry, DSC, and some enthalpy changes are reported. The resistivity study indicates that all samples display the expected metal-to-insulator transitions at temperatures ranging between 140 and 270 K. The samples have been analysed at room temperature by ultra-high-resolution synchrotron powder diffraction and the structural and microstructural features are reported. Furthermore, Nd_5/8Sr_0.255Ca_0.12MnO₃ () and Sm_0.225Nd_0.4Sr_0.308Ca_0.067MnO₃ () samples have also been studied by synchrotron powder diffraction at 140 K, below the transition temperatures. Both samples are found to be single phase above and below the transition by ultra-high-resolution synchrotron powder diffraction. The microstructure of the samples has been investigated through Williamson-Hall plots. Sample broadenings are markedly anisotropic and strongly dominated by microstrains with average values of the Δd/d term close to 14×10⁻⁴. A direct correlation is found between the microstrain values and the widths of the magnetization transitions.     

Structural and conductivity study of a new protonic conductor Cs0.86(NH4)1.14SO4·Te(OH)6

The crystal structure of Cs_0.86(NH₄)_1.14SO₄·Te(OH)₆ is determined by X-ray diffraction analysis. The space group is P2₁/c with , , , β=106.65(3)° and Z=4 at 293 K. The structure is refined to R=2.9%. The distribution of atoms can be described as isolated TeO₆ octahedra and SO₄ tetrahedra. The Cs⁺ and NH₄⁺ cations, occupying the same positions, are located between these polyhedra. The main feature of this structure is the coexistence of two types of anions in the same crystal related by network hydrogen bonds.The mixed solid solution cesium ammonium sulphate tellurate exhibits two phase transitions at 470 and 500 K. These transitions, detected by differential scanning calorimetric, are analyzed by dielectric measurements using the impedance and modulus spectroscopy techniques.     

Neutron diffraction study of phase transitions in perovskite-type strontium molybdate SrMoO3

The structure of a polycrystalline sample of SrMoO₃ has been investigated using powder neutron diffraction from 5 to 300 K, to reveal two structural phase transitions, the first from the cubic structure with a=3.97629(3) Å to a tetragonal structure in I4/mcm near 266 K and the second to an orthorhombic Imma phase below 125 K. The average Mo-O distance is essentially independent of temperature. The temperature dependence of the octahedral tilting appears typical of a tricritical phase transition.     

Iron(II) spin crossover complexes with branched long alkyl chain

The bzimpy iron(II) complexes, 1-3, containing branched long alkyl chains were synthesized and characterized in detail. The temperature-dependant magnetic susceptibility of 1 showed gradual spin crossover behavior from low spin to high spin state, while 2 retained only low spin state in the same condition. Interestingly, 3 displayed an abrupt spin transition in temperature range from T_1/2↑ = 236 K to T_1/2↓ = 230 K with the thermal hysteresis loop about 6 K. The differential scanning calorimetric analysis of 3 revealed two species of liquid crystal phase transitions at 236 K and 351 K, respectively.     

Ferromagnetic-phase transition in the spinel-type CuCr2Te4

Ferromagnetic-phase transition in spinel-type CuCr₂Te₄ has been clearly observed. CuCr₂Te₄ is a telluride-spinel with the lattice constant , which has been synthesized successfully. The heat capacity exhibits a sharp peak due to the ferromagnetic-phase transition with the Curie temperature . This value of T_C corresponds exactly to that of the negative peak of dM/dT in low field of 1.0 Oe. The magnetic susceptibility shows the Curie-Weiss behavior between 380 and 650 K with the effective magnetic moment /Cr-ion and the Weiss constant . The low temperature magnetization indicates the spin-wave excitations, where the existence of first term of Bloch T^3/2 law and the next T^5/2 term are verified experimentally. This spin-wave excitation is detected up to approximately 250 K which is a fairly high temperature.     

Space group and crystal structure of the Perovskite CaTiO3 from 296 to 1720 K

The structural phase transitions of the calcium titanate perovskite CaTiO₃ were investigated by the Rietveld analysis of high-temperature neutron and X-ray powder diffraction data in the temperature range of 296-1720 K. The present work demonstrates that the compound exhibits two reversible phase transitions of orthorhombic Pbnm-tetragonal I4/mcm and of I4/mcm-cubic at 1498±25 K and 1634±13 K, respectively. No evidence of Cmcm phase is observed between the Pbnm and I4/mcm structures. The lattice parameters discontinuously change at the Pbnm-I4/mcm transition point, while a continuous change is observed for the transformation. These results indicate that the Pbnm-I4/mcm transition is of first order and that the transformation is of either second or higher order.     

A study of the NMR relaxation mechanisms of ABCl3 (A=Rb, Cs, B=Cd, Mn) single crystals with the electric quadrupole and the electric magnetic types

The ⁸⁷Rb and ¹³³Cs spin-lattice relaxation rates of RbCdCl₃ and CsCdCl₃ single crystals grown using the slow evaporation method were measured over the temperature range 160-400 K. The changes in the ⁸⁷Rb spin-lattice relaxation rate near 340, 363, and 395 K correspond to phase transitions of the RbCdCl₃ crystal. The jump in T₁⁻¹ at 395 K is due to a shortening in the c-direction as a result of a phase transition from a cubic to a tetragonal structure. We suggest that the cubic Rb environment is favored above 395 K due to the fast motions and soft modes, which cause relaxation and average out the quadrupolar splittings. The temperature dependence of the relaxation rate below 340 K in RbCdCl₃ can be represented by and is thus in accordance with a Raman process. The ¹³³Cs nuclei in the CsCdCl₃ crystal produce only one resonance line, which indicates that the local structure around the Cs atoms is cubic. The temperature dependence of the relaxation rate of the Cs nuclei can also be described with the quadratic equation . In the case of the RbCdCl₃ and CsCdCl₃ crystals, which are of electric quadrupolar type, their relaxations proceed via Raman processes, whereas in RbMnCl₃ and CsMnCl₃ crystals, which are of magnetic relaxation type, the relaxations proceed via single phonon processes. Therefore, the relaxation mechanisms of these different types of ABCl₃ crystals (quadrupolar and magnetic) are completely different NMR behavior.     

Kinetic nature of the thermal destabilization of LHCII macroaggregates

The main light-harvesting chl a/b pigment-protein complex of photosystem II (LHCII) in isolated state forms macroaggregates with different ultrastructure and lipid content [I. Simidjiev, V. Barzda, L. Mustardy, G. Garab, Anal. Biochem. 250 (1997) 169-175]. The thermodynamic stability of highly delipidated tightly bound LHCII macroaggregates is studied by differential scanning calorimetry and fluorescence spectroscopy. The calorimetric profile of LHCII is asymmetric, the denaturation transition is taking place at around 72 degrees C. A shoulder, which overlaps with the main denaturation transition, appears around 58 degrees C. The denaturation temperature strongly depends on the scanning rate indicating the kinetic nature of the thermal destabilization of LHCII macroaggregates. The fluorescence data prove that the thermal denaturation of LHCII is an irreversible and kinetically controlled process.     

Effects of branching characteristics and copolymer composition distribution on non-isothermal crystallization kinetics of metallocene LLDPEs

The effects of branch content (BC) and copolymer composition distribution (CCD) on the non-isothermal crystallization kinetics of metallocene m-LLDPEs were studied using modified Avrami analysis, modulated differential scanning calorimetry (MDSC), and Crystaf. Several m-LLDPEs and an m-HDPE - all having comparable M_w and PDI - were experimented. In addition, a ZN-LLDPE was used for comparison purposes. The branch content, unlike the used cooling rates (2-6 °C/min), significantly affected the crystallization behavior. Crystallization peak temperature, , decreased linearly with increasing BC. All the m-LLDPEs showed primary and secondary crystallizations. The secondary crystallization showed to be more pronounced at high BC. The primary crystallization Avrami parameter n for m-HDPE ranged between 3.72 and 4.50, indicating spherulitic crystal growth whereas that for the m-LLDPEs, varied from 2.02 to 5.70. The ZN-LLDPE (having broader composition distribution) offered higher values of and than the m-LLDPEs with similar BC, M_w, and PDI. It, unlike the m-LLDPEs and m-HDPE, fairly agreed with the crystallization kinetic model proposed by Liu et al. The lamella thickness of the m-LLDPEs, L, calculated as per Gibbs-Thomson equation, showed to be in the range 2-16 nm, depending on BC and it decreased approximately following the relationship: L (nm) = 15.0 e^{(−0.0498BC)}.     

High temperature phase transition in SOFC anodes based on Sr2MgMoO6−δ

The double perovskite Sr₂MgMoO_6−δ has been recently reported as an efficient anode material for solid oxide fuel cells (SOFCs). In the present work, this material have been investigated by high temperature X-ray diffraction (XRD), differential scanning calorimetry (DSC) and impedance spectroscopy to further characterise its properties as SOFC anode. DSC and XRD measurements indicate that Sr₂MgMoO_6−δ exhibits a reversible phase transition around 275 °C from triclinic () with an octahedral tilting distortion to cubic () without octahedral distortion. This phase transition is continuous with increasing temperature without any sudden cell volume change during the phase transformation. The main effect of the phase transformation is observed in the electrical conductivity with a change in the activation energy at low temperature. La³⁺ and Fe-substituted Sr₂MgMoO_6−δ phases were also investigated, however these materials are unstable under oxidising conditions due to phase segregations above 600 °C.     

The AFeO2 (A=K, Rb and Cs) family: A comparative study of structures and structural phase transitions

Structures and phase transitions for the isostructural series of compounds KFeO₂, RbFeO₂ and CsFeO₂ have been systematically studied by synchrotron X-ray high resolution powder diffraction experiments and in case of CsFeO₂ also by single crystal diffractometry. At room temperature, all of the three compounds crystallize in the orthorhombic (Pbca) KGaO₂ type of structure consisting of a three dimensional network of corner-sharing [FeO_4/2]⁻ tetrahedra, which at elevated temperatures shows a reversible phase transformation to a cubic structure (space group ). For KFeO₂, RbFeO₂ and CsFeO₂ this phase transformation takes place at 1003 K, 737 K and 350 K respectively, as confirmed by differential scanning calorimetry and X-ray diffraction. Upon heating through the transitions the major structural changes are driven by the onset or enhancement of librational motion of the FeO₄ tetrahedra. Due to this phenomenon the Fe-O-Fe bonds appear to step-wise getting straight, seemingly approaching 180° within the time and space averaged structure.