TSC study of the dielectric relaxations of human‐bone collagen

Differential scanning calorimetry (DSC) and thermally stimulated current (TSC) were used to characterize human‐bone collagen. DSC glass‐transition and denaturation temperatures of the collagen in a dehydrated state were 90 and 215 °C, respectively. By TSC, the main relaxation mode, labeled α and located around 90 °C, could be attributed to the dielectric manifestation of the glass transition. The corresponding molecular movements are cooperative with a compensation temperature close to the denaturation temperature. At low temperatures and in a hydrated state, a second mode labeled β₂ was observed at −110 °C. Dehydration shifted this mode to higher temperatures, revealing a weak mode labeled γ at −150 °C. This γ mode was attributed to motions of aliphatic side chains. An analysis of low‐temperature elementary spectra allowed us to assign the β₂ mode to structural water movements and revealed an additional compensation phenomenon in the temperature range (−80 to −50 °C). Because the compensation temperature of this mode was close to the collagen glass‐transition temperature, the corresponding mode β₁ was attributed to polar side‐chain motions, precursors of a collagen glass transition. Finally, around ambient temperature, three sharp peaks were attributed to hydrogen bonds breaking. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 987–992, 2000     

荧光光谱法研究牛血清蛋白在盐酸胍体系中的变性

应用荧光光谱技术,对盐酸胍与牛血清蛋白在30℃水溶液中的结合作用及造成牛血清蛋白变性的过程进行了研究,考察了盐酸胍诱导牛血清蛋白变性时荧光强度和峰位的变化规律,并计算出伸展分数fu,变性平衡常数Ku,伸展吉布斯自由能△Gu,衡量蛋白质对变性剂稳定性的参量△GH2o,衡量蛋白质变性协同性的参量m和变性中点C1/2.研究结...     

On the Use of the Hypothesis of Statistically Homogeneous Suspensions in the Calculation of Their Conductivity

Electrostatic effects on protein adsorption were investigated using differential scanning calorimetry (DSC) and adsorption isotherms. The thermal denaturation of lysozyme, ribonuclease A (RNase), and alpha-lactalbumin in solution and adsorbed onto silica nanoparticles was examined at three concentrations of cations: 10 and 100 mM of sodium and 100 mM of sodium to which 10 mM of calcium was added. The parameters investigated were the denaturation enthalpy (DeltaH), the temperature at which the denaturation transition was half-completed (T(m)), and the temperature range of the denaturation transition. For lysozyme and RNase, adsorption isotherms depend strongly on the ionic strength. At low ionic strength both proteins have a high affinity for the silica particles and adsorption is accompanied by a 15-25% reduction in DeltaH and a 3-6 degrees C decrease in T(m), indicating that the adsorbed state of the proteins is destabilized. Also, an increase in the width of the denaturation transition is observed, signifying a larger conformational heterogeneity of the surface bound proteins. At higher ionic strengths, both with and without the addition of calcium, no significant adsorption-induced alteration in DeltaH was observed for all three proteins. The addition of calcium, however, decreases the width of the denaturation transition for lysozyme and RNase in the adsorbed state. Copyright 2001 Academic Press.     

Ab initio characterization of ring-opening H-transfer in ionized cyclopentanone: Similarity to ion-neutral complex-mediated alkane eliminations

Ab initio procedures were used to characterize the transition states for -cleavage and subsequent H-transfer starting with ionized cyclopentanone (1). The objective was to determine whether the parts of the ion disconnected by the bond cleavage remain in close association through the H-transfer. The transition state for C-C bond cleavage is close in energy to the resulting distonic ion, which is a stable species, and far in geometry from any transition state involving H-transfer. We find no evidence for any concertedness or “quasiconcertedness” in this reaction. Ring-opening H-transfer in 1 and ion-neutral complex-mediated alkane eliminations are compared.     

Low-temperature and high-pressure induced swelling of a hydrophobic polymer-chain in aqueous solution

We report molecular dynamics simulations of a hydrophobic polymer-chain in aqueous solution between 260 K and 420 K at pressures of 1 bar, 3000 bar, and 4500 bar. The simulations reveal a hydrophobically collapsed structure at low pressures and high temperatures. At 3000 bar and about 260 K and at 4500 bar and about 260 K, however, an abrupt transition to a swelled state is observed. The transition is driven by a smaller volume and a remarkably strong lower enthalpy of the swelled state, indicating a steep positive slope of the corresponding transition line. The swelling is strongly stabilized by the energetically favorable state of water in the polymer's hydrophobic first hydration shell at low temperatures. This finding is consistent with the observation of a positive heat capacity of hydrophobic solvation. Moreover, the slope and location of the estimated swelling transition line for the collapsed hydrophobic chain coincides remarkably well with the cold denaturation transition of proteins.     

The nature and function of the catalytic centres of the DMSO reductases

Density functional theory calculations have been performed to probe aspects of the function of the reaction centres of the DMSO reductase enzymes, in respect of catalysis of oxygen atom transfer (OAT). The first comparison between Mo and W at the active site of these enzymes has been accomplished by a consideration of the reaction profile for OAT from DMSO to [MoIV(OMe)(S2C2H2)2]1- versus that for the corresponding reaction with [WIV(OMe)(S2C2H2)2]1-. Both reaction profiles involve two transition states separated by a well-defined intermediate; however, whilst the second transition state (TS2) is clearly rate-limiting for the Mo system, the two transition states have a similar energy for the W system. The activation energy for OAT from DMSO to [WIV(OMe)(S2C2H2)2]1- is ca. 23 kJ mol-1 lower for the corresponding reaction with Mo, consistent with the significantly faster rate of reduction of DMSO by Rhodobacter capsulatus W-DMSO reductase than by its Mo counterpart. Consistent with the principle of the entatic state, the geometrical constraints imposed by the protein on the metal centre of the Mo- and W-DMSO reductases facilitate OAT by favouring a trigonal prismatic geometry for the transition state TS2 that is close to that observed for the metal in the oxidised form of each of these enzymes. The effects of different tautomers of a simplified form of the pyran ring-opened, dihydropterin state of the molybdopterin cofactor on the reaction profile for OAT have been considered. The major effect, a significant lowering of the activation barrier associated with TS2, is observed for a protonated form of a tautomer that involves conjugation between the pyrazine and metallodithiolene rings.     

A differential microcalorimetric study of whey proteins and their behaviour in oil-in-water emulsions

Oil-in-water emulsions (20% soya oil, 1% protein) have been prepared containing lysozyme or isolates of -lactalbumin and β-lactoglobulin from whey protein. The structural characteristics of these proteins adsorbed at an oil/water interface were determined by following their thermal transitions using differential scanning microcalorimetry. Thermograms of the proteins in the adsorbed state differed markedly from the corresponding transitions seen for the proteins in solution. This suggests that the proteins underwent substantial changes in secondary and tertiary structure upon adsorption. In general, for all the proteins studied, a net decrease in the total energy absorbed during denaturation was found when the proteins were in an adsorbed state. Both lysozyme and -lactalbumin were in part “surface denatured”, and they showed a certain degree of reversibility between solution and the adsorbed state. β-Lactoglobulin showed the highest degree of denaturation upon adsorption and the conformational change was irreversible.     

一种确定反应中间态几何特征和能量的综合性方法

通过综合使用传统的过渡态优化算法、数学统计工具以及人工神经网络算法(ANN)找到一种不依赖于反应物起始构象而得到化学反应中过渡态结构和能量的方法. 在两个反应物互相接近的过程中, 每一步的几何构象都对应着一个系统能量值. 本研究的目的是尽可能地收集处在反应能量面上的这种能量点值. 通过采用几何参数作为自变量对势能面进行模拟研究, 得到了势能面上对应过渡态结构的一阶鞍点. 采用乙醛负离子和甲醛作为反应物, 对经典的醛醇缩合反应中的亲核进攻步骤进行了研究. 对内禀反应坐标(IRC)路径的计算是从反应物的三组不同起始构象出发, 最终获得了反应势能面上的96个点. 本研究中的势能面采用人工神经网络算法进行模拟研究, 并利用交叉验证方法评估得到的结果, 避免了采用人工神经网络算法时过度拟合情况的发生.     

Effect of Lewis acid catalysts on Diels-Alder and hetero-Diels-Alder cycloadditions sharing a common transition state

The thermal and Lewis acid catalyzed cycloadditions of beta,gamma-unsaturated alpha-ketophosphonates and nitroalkenes with cyclopentadiene have been explored by using density functional theory (DFT) methods. In both cases, only a single highly asynchronous bis-pericyclic transition state yielding both Diels-Alder and hetero-Diels-Alder cycloadducts could be located. Stepwise pathways were found to be higher in energy. On the potential energy surface, the bis-pericyclic cycloaddition transition state is followed by the Claisen rearrangement transition state. No intermediates were located between these transition states. Claisen rearrangement transition states are also highly asynchronous, but bond lengths are skewed in the opposite direction compared to the bis-pericyclic transition states. The relative positions of the bis-pericyclic and Claisen rearrangement transition states may control periselectivity due to the shape of the potential energy surface and corresponding dynamical influences. Inspection of the thermal potential energy surface (PES) indicates that a majority of downhill paths after the bis-pericyclic transition state lead to the Diels-Alder cycloadducts, whereas a smaller number of downhill paths reach the hetero-Diels-Alder products with no intervening energy barrier. Lewis acid catalysts alter the shape of the surface by shifting the cycloaddition and the Claisen rearrangement transition states in opposite directions. This topographical change qualitatively affects the branching ratio after the bis-pericyclic transition state and ultimately reverses the periselectivity of the cycloaddition giving a preference for hetero-Diels-Alder cycloadducts.     

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 T _m) 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. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Kinetic Isotope Effect as a Probe of Spin Crossover in the CH Activation of Methane by the FeO+ Cation

Two‐state reactivity (TSR) is often used to explain the reaction of transition‐metal–oxo reagents in the bare form or in the complex form. The evidence of the TSR model typically comes from quantum‐mechanical calculations for energy profiles with a spin crossover in the rate‐limiting step. To prove the TSR concept, kinetic profiles for C? H activation by the FeO⁺ cation were explored. A direct dynamics approach was used to generate potential energy surfaces of the sextet and quartet H‐transfers and rate constants and kinetic isotope effects (KIEs) were calculated using variational transition‐state theory including multidimensional tunneling. The minimum energy crossing point with very large spin–orbit coupling matrix element was very close to the intrinsic reaction paths of both sextet and quartet H‐transfers. Excellent agreement with experiments were obtained when the sextet reactant and quartet transition state were used with a spin crossover, which strongly support the TSR model.     

Nonperfect synchronization of reaction center rehybridization in the transition state of the hydride transfer catalyzed by dihydrofolate reductase

It has been suggested that the magnitudes of secondary kinetic isotope effects (2 degrees KIEs) of enzyme-catalyzed reactions are an indicator of the extent of reaction-center rehybridization at the transition state. A 2 degrees KIE value close to the corresponding secondary equilibrium isotope effects (2 degrees EIE) is conventionally interpreted as indicating a late transition state that resembles the final product. The reliability of using this criterion to infer the structure of the transition state is examined by carrying out a theoretical investigation of the hybridization states of the hydride donor and acceptor in the Escherichia coli dihydrofolate reductase (ecDHFR)-catalyzed reaction for which a 2 degrees KIE close to the 2 degrees EIE was reported. Our results show that the donor carbon at the hydride transfer transition state resembles the reactant state more than the product state, whereas the acceptor carbon is more productlike, which is a symptom of transition state imbalance. The conclusion that the isotopically substituted carbon is reactant-like disagrees with the conclusion that would have been derived from the criterion of 2 degrees KIEs and 2 degrees EIEs, but the breakdown of the correlation with the equilibrium isotope effect can be explained by considering the effect of tunneling.     

牛血清IgG热化学变性和热变性的研究

使用差示扫描量热仪(DSC)和荧光光谱法研究了在pH 7.4时牛血清IgG (bIgG)热变性, 热化学变性和等温化学变性过程(变性剂为尿素和盐酸胍), 首次报道了bIgG在热化学变性和等温化学变性过程中的相关热力学参数. DSC和荧光光谱实验结果表明, bIgG的热变性和热化学变性过程都是较复杂的不可逆过程, 这个过程可被看作一个三态变构过程. DSC实验表明在热化学变性过程中bIgG的变性温度和焓变值会随着环境中的变性剂浓度的升高而降低. 使用荧光光谱法对bIgG在尿素或盐酸胍存在下的等温化学变性过程进行了研究, 结果显示bIgG的化学变性过程也是一个较复杂的非二态过程. 实验数据分析表明, 变性剂尿素和盐酸胍与bIgG之间主要是依靠氢键相互作用的, 而热变性过程中bIgG的凝集是由于bIgG热变性时结构改变后暴露出的疏水结构互相作用造成的. 实验结果还表明单纯的热变性只能导致bIgG的不完全变性, 而即使是在高浓度变性剂存在时的bIgG热化学变性, 尿素和盐酸胍分别导致的bIgG热化学变性的去折叠态也是不同的.     

Tetrahydrides of third-row transition elements: spin-orbit coupling effects on the stability of rhenium tetrahydride

The potential energy surfaces of low-lying states in rhenium tetrahydride (ReH(4)) were explored by using the multiconfiguration self-consistent field (MCSCF) method together with the SBKJC effective core potentials and the associated basis sets augmented by a set of f functions on rhenium atom and by a set of p functions on hydrogen atoms, followed by spin-orbit coupling (SOC) calculations to incorporate nonscalar relativistic effects. The most stable structure of ReH(4) was found to have a D(2d) symmetry and its ground state is (4)A(2). It is found that this is lower in energy than the dissociation limit, ReH(2)+H(2), after dynamic correlation effects are taken into account by using second-order multireference M?ller-Plesset perturbation (MRMP2) calculations. This reasonably agrees with previous results reported by Andrews et al. [J. Phys. Chem. 107, 4081 (2003)]. The present investigation further revealed that the dissociation reaction of ReH(4) cannot occur without electronic transition from the lowest quartet state to the lowest sextet state. This spin-forbidden transition can easily occur because of large SOC effects among low-lying states in such heavy metal-containing compounds. The minimum-energy crossing (MEX) point between the lowest quartet and sextet states is proved to be energetically and geometrically close to the transition state for the dissociation reaction on the potential energy surface of the lowest spin-mixed state. The MEX point (C(2) symmetry) was estimated to be 9184?cm(-1) (26.3 kcal/mol) higher than the (4)A(2) state in D(2d) symmetry at the MRMP2 level of theory. After inclusion of SOC effects, an energy maximum on the lowest spin-mixed state appears near the MEX point and is recognized as the transition state for the dissociation reaction to ReH(2)+H(2). The energy barrier for the dissociation, evaluated to be MEX in the adiabatic picture, was calculated to be 5643?cm(-1) (16.1 kcal/mol) on the lowest spin-mixed state when SOC effects were estimated at the MCSCF level of theory.     

First-principle studies of intermolecular and intramolecular catalysis of protonated cocaine

We have performed a series of first-principles electronic structure calculations to examine the reaction pathways and the corresponding free energy barriers for the ester hydrolysis of protonated cocaine in its chair and boat conformations. The calculated free energy barriers for the benzoyl ester hydrolysis of protonated chair cocaine are close to the corresponding barriers calculated for the benzoyl ester hydrolysis of neutral cocaine. However, the free energy barrier calculated for the methyl ester hydrolysis of protonated cocaine in its chair conformation is significantly lower than for the methyl ester hydrolysis of neutral cocaine and for the dominant pathway of the benzoyl ester hydrolysis of protonated cocaine. The significant decrease of the free energy barrier, approximately 4 kcal/mol, is attributed to the intramolecular acid catalysis of the methyl ester hydrolysis of protonated cocaine, because the transition state structure is stabilized by the strong hydrogen bond between the carbonyl oxygen of the methyl ester moiety and the protonated tropane N. The relative magnitudes of the free energy barriers calculated for different pathways of the ester hydrolysis of protonated chair cocaine are consistent with the experimental kinetic data for cocaine hydrolysis under physiologic conditions. Similar intramolecular acid catalysis also occurs for the benzoyl ester hydrolysis of (protonated) boat cocaine in the physiologic condition, although the contribution of the intramolecular hydrogen bonding to transition state stabilization is negligible. Nonetheless, the predictability of the intramolecular hydrogen bonding could be useful in generating antibody-based catalysts that recruit cocaine to the boat conformation and an analog that elicited antibodies to approximate the protonated tropane N and the benzoyl O more closely than the natural boat conformer might increase the contribution from hydrogen bonding. Such a stable analog of the transition state for intramolecular catalysis of cocaine benzoyl-ester hydrolysis was synthesized and used to successfully elicit a number of anticocaine catalytic antibodies.     

The Kinetic Isotope Effect as a Probe of Spin Crossover in the C?H Activation of Methane by the FeO+ Cation

Two‐state reactivity (TSR) is often used to explain the reaction of transition‐metal–oxo reagents in the bare form or in the complex form. The evidence of the TSR model typically comes from quantum‐mechanical calculations for energy profiles with a spin crossover in the rate‐limiting step. To prove the TSR concept, kinetic profiles for C H activation by the FeO⁺ cation were explored. A direct dynamics approach was used to generate potential energy surfaces of the sextet and quartet H‐transfers and rate constants and kinetic isotope effects (KIEs) were calculated using variational transition‐state theory including multidimensional tunneling. The minimum energy crossing point with very large spin–orbit coupling matrix element was very close to the intrinsic reaction paths of both sextet and quartet H‐transfers. Excellent agreement with experiments were obtained when the sextet reactant and quartet transition state were used with a spin crossover, which strongly support the TSR model.     

Thermal inactivation of alkali phosphatases under various conditions

The thermal inactivation of alkali phosphatases from bacteria Escherichia coli (ECAP), bovine intestines (bovine IAP), and chicken intestines (chicken IAP) was studied in different buffer solutions and in the solid state. The conclusion was made that these enzymes had maximum stability in the solid state, and, in a carbonate buffer solution, their activity decreased most rapidly. It was found that the bacterial enzyme was more stable than animal phosphatases. It was noted that, for ECAP, four intermediate stages preceded the loss of enzyme activity, and, for bovine and chicken IAPs, three intermediate stages were observed. The activation energy of thermal inactivation of ECAP over the range 25–70°C was determined to be 80 kJ/mol; it corresponded to the dissociation of active dimers into inactive monomers. Higher activation energies (∼200 kJ/mol) observed at the initial stage of thermal inactivation of animal phosphatases resulted from the simultaneous loss of enzyme activity caused by dimer dissociation and denaturation. It was shown that the activation energy of denaturation of monomeric animal alkali phosphatases ranged from 330 to 380 kJ/mol depending on buffer media. It was concluded that the inactivation of solid samples of alkali phosphatases at 95°C was accompanied by an about twofold decrease in the content of β structures in protein molecules. Original Russian Text ? L.F. Atyaksheva, B.N. Tarasevich, E.S. Chukhrai, O.M. Poltorak, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 2, pp. 391–396.     

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.     

势能面移动对三（2-苯基吡啶）合铱磷光光谱影响的理论研究

采用密度泛函理论及赝势基组对三（2-苯基吡啶）合铱（Ir（ppy）3）的基态及三重态结构进行优化,并分析了这两个态各自的振动模式,在此基础上计算了电子从三重态跃迁回基态的势能面移动,得到了该过程的重整能．由于0→1跃迁对Ir（ppy）s的磷光光谱有重要影响,我们利用Frank-Condon因子与势能面移动的联系,在给定的半高宽下,计算并得到了Ir（ppy）3的磷光光谱,结果与实验吻合较好．     

Modeling of phase equilibrium and vapor adsorption on carbon black based on a combination of a lattice theory and equation of state

A thermodynamic approach is developed in this paper to describe the behavior of a subcritical fluid in the neighborhood of vapor-liquid interface and close to a graphite surface. The fluid is modeled as a system of parallel molecular layers. The Helmholtz free energy of the fluid is expressed as the sum of the intrinsic Helmholtz free energies of separate layers and the potential energy of their mutual interactions calculated by the 10-4 potential. This Helmholtz free energy is described by an equation of state (such as the Bender or Peng-Robinson equation), which allows us a convenient means to obtain the intrinsic Helmholtz free energy of each molecular layer as a function of its two-dimensional density. All molecular layers of the bulk fluid are in mechanical equilibrium corresponding to the minimum of the total potential energy. In the case of adsorption the external potential exerted by the graphite layers is added to the free energy. The state of the interface zone between the liquid and the vapor phases or the state of the adsorbed phase is determined by the minimum of the grand potential. In the case of phase equilibrium the approach leads to the distribution of density and pressure over the transition zone. The interrelation between the collision diameter and the potential well depth was determined by the surface tension. It was shown that the distance between neighboring molecular layers substantially changes in the vapor-liquid transition zone and in the adsorbed phase with loading. The approach is considered in this paper for the case of adsorption of argon and nitrogen on carbon black. In both cases an excellent agreement with the experimental data was achieved without additional assumptions and fitting parameters, except for the fluid-solid potential well depth. The approach has far-reaching consequences and can be readily extended to the model of adsorption in slit pores of carbonaceous materials and to the analysis of multicomponent adsorption systems.