Local spin configurations of Fe atoms in the Rh1−x Fex (x=0.1, 0.2, and 0.3) system with competing exchange interactions

The local spin configurations of Fe atoms in the magnetically ordered alloys Rh_1?x Fe_x (x=0.1, 0.2, and 0.3) have been investigated by Mössbauer spectroscopy. The Mössbauer absorption spectra are measured in the range from 5 K to temperatures of the transition to the paramagnetic state. The measurements in magnetic fields with a strength up to 5 T are carried out at a temperature of 4.2 K. Analysis of the magnetic-hyperfinefield distribution functions demonstrates that Fe atoms form discrete sets of collinear spin configurations corresponding to different net moments of the nearest coordination sphere. The spin structure of the alloys is governed by a random distribution of Fe atoms over the lattice sites and the competition between the Fe-Rh ferromagnetic exchange interaction and the antiferromagnetic interaction of the neighboring Fe atoms. No spin frustration and spin “melting” effects characteristic of spin glasses are revealed in the Rh-Fe alloys.     

Influence of screw extrusion on the atomic order in constructional steel

The atomic order in low-carbon steel strained by screw extrusion making the microstructure finer and changing the phase ratio in the material is studied by X-ray diffraction analysis. The steel is found to consist of multiscale structural fractions: a finely crystalline (300–600 Å) α-Fe matrix, nanodimensional (180–250 Å) Fe₃C and Fe₃Si or Fe₅Si₃ phases, and a fraction with randomly arranged atoms. A second-order order-disorder phase transition is revealed with order prevailing in the longitudinal (‖) section of the samples and disorder prevailing in their cross (⊥) section. Sets of “extended” and “compressed” planes are found to coexist in the structure for the first time, with extended planes prevailing in the (‖) section and compressed planes prevailing in the (⊥) one.     

pH对中国人乳β-酪蛋白二级结构的圆二色和拉曼特征影响

人乳中β-酪蛋白为婴儿的快速生长提供了最适宜的氨基酸、钙磷等。以往的研究均是对牛乳的β-酪蛋白二级结构进行分析,而牛乳β-酪蛋白对婴儿配方乳的指导意义不如人乳β-酪蛋白,且人乳β-酪蛋白难以获得合适的蛋白晶体,其三级结构不清楚。为了获得中国人乳β-酪蛋白二级结构的相关基础信息,采用圆二色光谱(CD)法和拉曼光谱(Raman)法分析在不同pH条件下其二级结构的变化,结果表明：pH值的改变能够诱导中国人乳β-酪蛋白二级结构的改变,CD分析表明β-酪蛋白各部分的二级结构分别是：0.5%~2% α-螺旋,16%~18% β-折叠,30%~34% β-转角,49%~51% 无规则卷曲;随着pH的增加,部分结构发生了变化,α-螺旋在pH 8处含量增加,而在pH 10处含量减小。拉曼光谱分析表明：β-酪蛋白在酰胺I的特征峰在1 662 cm-1处,分析出β-酪蛋白的无规则卷曲含量较高。同时,根据I₈₅₀/I₈₃₀的比值计算出β-酪蛋白的酪氨酸残基趋向于“暴露式”。圆二色光谱和拉曼光谱都证明了人乳β-酪蛋白二级结构中无规则卷曲的含量最高,而β-折叠和β-转角含量保持相对的稳定。并且,在pH 8条件下,α-螺旋含量高于在其他pH条件下的含量。     

There and back again: Two views on the protein folding puzzle

The ability of protein chains to spontaneously form their spatial structures is a long-standing puzzle in molecular biology. Experimentally measured folding times of single-domain globular proteins range from microseconds to hours: the difference (10–11 orders of magnitude) is the same as that between the life span of a mosquito and the age of the universe. This review describes physical theories of rates of overcoming the free-energy barrier separating the natively folded (N) and unfolded (U) states of protein chains in both directions: “U-to-N” and “N-to-U”. In the theory of protein folding rates a special role is played by the point of thermodynamic (and kinetic) equilibrium between the native and unfolded state of the chain; here, the theory obtains the simplest form. Paradoxically, a theoretical estimate of the folding time is easier to get from consideration of protein unfolding (the “N-to-U” transition) rather than folding, because it is easier to outline a good unfolding pathway of any structure than a good folding pathway that leads to the stable fold, which is yet unknown to the folding protein chain. And since the rates of direct and reverse reactions are equal at the equilibrium point (as follows from the physical “detailed balance” principle), the estimated folding time can be derived from the estimated unfolding time. Theoretical analysis of the “N-to-U” transition outlines the range of protein folding rates in a good agreement with experiment. Theoretical analysis of folding (the “U-to-N” transition), performed at the level of formation and assembly of protein secondary structures, outlines the upper limit of protein folding times (i.e., of the time of search for the most stable fold). Both theories come to essentially the same results; this is not a surprise, because they describe overcoming one and the same free-energy barrier, although the way to the top of this barrier from the side of the unfolded state is very different from the way from the side of the native state; and both theories agree with experiment. In addition, they predict the maximal size of protein domains that fold under solely thermodynamic (rather than kinetic) control and explain the observed maximal size of the “foldable” protein domains.     

On hydrodynamic interpretation of folding of an α-helical protein

Using the method of molecular dynamics, the simulation of folding of an α-helical protein from the unfolded to compact and functional (native) state is performed. The protein folding is interpreted as a stationary motion of a compressible “folding fluid”. It is shown that the densities of folding fluxes obey the same similarity relations as the velocities of an incompressible fluid in the Kolmogorov’s turbulence theory, except that instead of the rate of change of kinetic energy per mass unit, the rate of change of flux variance per volume unit plays the role of the key parameter.     

Dynamics of polyglutamic acids in α-helical and coil states. Comparison with dynamics of some globular proteins. Rayleigh scattering of Mössbauer radiation (RSMR) data

Summary The classical model system poly-L-glutamic acid (poly-Glu), was investigated in a disordered coil state (atpH=7.0) and in helix state (atpH=2.0) by the RSMR technique. By considering that the coil state of poly-Glu models unfolded (random coil) state and α-helix state models the fluctuating secondary structure (during consequent folding of protein), a comparative analysis of the dynamical properties of poly-Glu in different states with the dynamical properties of different proteins in the native state (α-helical myoglobin and HSA, partially β-sheet lysozyme) and in intermediate (molten globule) state (α-lactalbumin) was performed. This comparison brings some unpredicted results: native α-helical proteins behave close to random coil, native partially β-sheet proteins behave close to fluctuating secondary structure (α-helix) and the dynamic behaviour of molten-globule state (partially β-sheet α-lactalbumin) is not different from the behaviour of lysozyme and much more rigid than that of native α-helical proteins. Paper presented at ICAME-95, Rimini, 10–16 September 1995.     

Soluble renormalization groups and scaling fields for low-dimensional Ising systems

A variety of one-dimensional Ising spin systems, including staggered and parallel magnetic fields, alternating and second neighbor interactions, four-spin coupling, etc., are discussed in terms of renormalization group theory. A continuous range of distinct renormalization groups is constructed in exact closed form, analyzed in detail, and compared with exactly calculated thermodynamic properties. Fixed point linearization yields relevant, irrelevant, and marginal operators. All groups yield identical “critical” behavior (at T = 0) with η = 1, δ = ∞, γ = ν = 2 ? α, and with identical linear scaling fields. A generalization of Wegner's analysis to discrete groups yields explicit power series for the nonlinear scaling fields; these are seen to depend on the particular renormalization group and, hence, are physically nonunique. A planar, multiconnected “truncated tetrahedron” model of effective dimensionality log₂ 3 is analyzed via a dedecoration and star-triangle group revealing highly singular behavior as T → T_c = 0.     

Correlations in quantum dots

The lowest excitations of a repulsively interacting few particle system are investigated within correlated “pocket state” basis functions. For long range interaction and non-isotropic confining potentials the method becomes exact, in the limit of large mean inter-particle distancesr _s. The multiplet structure of the many-electron energy levels is explained and the ratios δ between the lowest excitation energies, which are related to the electron spin, are determined quantitatively using group theoretical means. The δ are independent of the detailed form of the inter-particle repulsion and of sufficiently larger _s. The obtained δ-values are confirmed by available numerical data. The method is applied to 1D and 2D quantum dots.     

Stratification of Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> Single Crystals and Sintering of the Obtained Micro- and Nanoscale Plates

The use of surface active liquids facilitates intense stratification of mechanically strained Bi_0.5Sb_1.5Te₃ crystallites. A Bi_0.5Sb_1.5Te₃ heat element with specified thickness and structure is formed by layer-by-layer deposition of “thermoelectric ink” on its free surface. A heat treatment of the formed thermoelectric element in argon at a temperature of 800 K makes it possible to minimize radically the resistance of the grain boundaries introduced into its bulk.     

异质多晶SiGe栅应变Si NMOSFET物理模型研究

结合了“栅极工程”和“应变工程”二者的优点, 异质多晶SiGe栅应变Si MOSFET, 通过沿沟道方向使用不同功函数的多晶SiGe材料, 在应变的基础上进一步提高了MOSFET的性能. 本文结合其结构模型, 以应变Si NMOSFET为例, 建立了强反型时的准二维表面势模型, 并进一步获得了其阈值电压模型以及沟道电流的物理模型. 应用MATLAB对该器件模型进行了分析, 讨论了异质多晶SiGe栅功函数及栅长度、衬底SiGe中Ge组分等参数对器件阈值电压、沟道电流的影响, 获得了最优化的异质栅结构. 模型所得结果与仿真结果及相关文献给出的结论一致, 证明了该模型的正确性. 该研究为异质多晶SiGe栅应变Si MOSFET的设计制造提供了有价值的参考. 关键词： 异质多晶SiGe栅 应变Si NMOSFET 表面势 沟道电流     

圆二色谱表征芥蓝抗坏血酸过氧化物酶变性过程中的结构变化

圆二色谱(CD)是一种特殊的吸收光谱,蛋白质的二级结构如α-螺旋、β-折叠、β-转角在远紫外区(190～250 nm)都具有特征性的CD图谱。为了了解芥蓝抗坏血酸过氧化物酶(BaAPX)在变性过程中活性变化和结构变化的关系,动态监测了不同时间、温度及浓度条件下BaAPX的比酶活及远紫外CD图谱,并利用CD分析软件Dichroweb计算了BaAPX中四种二级结构的百分含量。结果表明：BaAPX属于全α型蛋白,α-螺旋的百分含量与比酶活之间存在比较明显的正相关关系。在低浓度低温的变性过程中,BaAPX发生了初始态(N态)→α-螺旋最少态(R态)一步结构变化;在高浓度低温的变性过程中,BaAPX发生了N态→平衡态(T态)一步结构变化;在热处理后低温复性的变性过程中,BaAPX发生了N态→R态→T态两步结构变化。     

Spin forbidden transitions in the Ka = 0 subband of NO2 in the λ = 592.5 nm region

In a high resolution laser excitation spectrum of NO₂, lines were recorded which do not follow the selection rule ΔN = ΔJ = ΔF of “spin allowed” transitions. Line positions and intensities of these “spin forbidden” lines were investigated for all rotational lines up to N″ = 12 of the K_a = 0 subband around λ = 592.5 nm. While the observed line intensities of “spin allowed” transitions can be well described by the J-coupling scheme, neither the J- nor the G-coupling scheme sufficiently describes the “spin forbidden” transitions. The observations can be fitted satisfactorily by perturbation theory, in which the Fermi interaction in ²A₁ is treated as the perturber. This looks similar to a superposition of J and G scheme in the ²A₁ ground state.     

The microwave spectra of ortho-fluorotoluene with the asymmetric internal rotors CH2D and CD2H

The rotational spectra of αd₁- and αd₂-ortho-fluorotoluene in the ground state of the methyl group torsion have been measured. The evaluation of the spectra has been based on the theory for the internal rotation of an asymmetric internal top formulated earlier by several authors. The barrier potential being threefold symmetric (V₃), each torsional level consists of three nondegenerate substates, designated as sy and ±asy. The sy-state is assigned to the conformation with the unique methyl hydrogen isotope within the molecular heavy-atom plane (sy-rotamer), while the ±asy-states belong to the respective out-of-plane conformation (asy-rotamer). In the torsional ground state the level spacing between the ±asy substates is very small and numerous accidental close degeneracies are present between the rotational level systems based on these torsional substates. The rotational levels involved are strongly perturbed by the coupling between molecular overall rotation and internal rotation. Large deviations from a rigid rotor spectrum and (+) ? (?) intersystem (“tunneling”) transitions are observed. The spectrum of the asy-rotamer can be well reproduced by a “two-dimensional” Hamiltonian containing 11 “rotational constants,” 9 of which are determined by a fit to the spectrum. Several are sufficiently barrier-dependent to derive V₃. We obtain (in cal/mole) 567 ± 48 for αd₁-ortho-fluorotoluene, 711 ± 40 for the αd₂-isotope. The deviations from 649 cal/mole for the normal isotope are appreciable, probably indicating shortcomings of the semirigid model. The sy-rotamer presents a rigid rotor spectrum.     

Mössbauer spectroscopy on the reaction center of Rhodopseudomonas viridis

Proteins called “reaction centers” (RC) can be isolated from many photosynthetic bacteria. They have one non-heme iron in a quinone acceptor region. The RC of Rhodopseudomonas viridis contains an additional tightly bound tetra-heme cytochrome c subunit. The electronic configuration of both cytochrome and the non-heme iron has been studied in the crystallized protein by Mössbauer spectroscopy at different redox potentials, pH-values, and with an addition of o-phenanthroline. At high potentials (E_h=+500mV) all heme irons are in the low spin Fe³⁺-state, and at low potential (E_h=?150mV) they are low spin Fe²⁺ with the same Mössbauer parameters for all hemes independent of pH. Redox titrations change the relative area of the reduced and oxidized states in agreement with other methods. The non-heme iron shows a high spin Fe²⁺ configuration independent of E_h and pH with parameters comparable to those of Rhodopseudomonas sphaeroides. Surprisingly, there is strong evidence for another non-heme iron species in part of the molecules with a Fe²⁺ low spin configuration. Incubation with o-phenanthroline decreases the relative Fe²⁺ hs-area and increases the contribution of Fe²⁺ ls-area. Above 210K the mean square displacement, 2>, of the RC-crystals increases more than linearly with temperature. This may be correlated with the increase of the electron transfer rate and indicates that intramolecular mobility influences the functional activity of a protein.     

Models of spin structures in Sr<Subscript>2</Subscript>RuO<Subscript>4</Subscript>

The mean-field method is used to calculate the bands, Fermi surfaces, and spin susceptibilities of a three-band model of the RuO₄ plane of Sr₂RuO₄ rutinate for states with different spin structures. In particular, the spiral state is studied with the “incommensurate” vector Q=2π(1/3, 1/3) corresponding to the nesting of bands with the population n=4. This state proves to be the lowest with respect to energy among other (paramagnetic, ferromagnetic, antiferromagnetic, and periodic) solutions. In the spiral state, in addition to the main α, β, and γ sheets of the Fermi surface, shadow Fermi boundaries along the Γ(0, 0)-M(π, 0) line (previously observed in the ARPES experiments) are revealed and explained. This may change the interpretation of the data on dispersionless peaks in photoemission, previously ascribed to surface states. The spin susceptibilities of the spiral state exhibit peaks in the dependence Im?(q, ω) at q=Q in accordance with the observed magnetic peak in neutron scattering. The hypothesis of the presence of spin structures with q=Q in the normal state of Sr₂RuO₄ and the methods of checking this hypothesis are discussed.     

Variational principle for regular and random Ising models on the cactus tree or on the usual lattice in the “cactus approximation”

The distribution functions and the free energy are expressed in terms of the effective fields for the regular and random Ising models of an arbitrary spin S on the generalized cactus tree. The same expressions apply to systems on the usual lattice in the “cactus approximation” in the cluster variation method. For an ensemble of random Ising models of an arbitrary spin S on the generalized cactus tree, the equation for the probability distribution function of the effective fields is set up and the averaged free energy is expressed in terms of the probability distribution. The same expressions apply to the system on the usual lattice in the “cactus approximation”. We discuss the quantities on the usual lattice when the system or the ensemble of random systems has the translational symmetry. Variational properties of the free energy for a system and of the averaged free energy for an ensemble of random systems are noted. The “cactus approximations” are applicable to the Heisenberg model as well as to the Ising model of an arbitrary spin, and to ensembles of random systems of these models.     

Evaluation of Properties and Structural Transitions of Poly-L-lysine: Effects of pH and Temperature

Abstract

The structural response to temperature and pH changes of poly-l-lysine (PLL) has been studied by a variety of experimental methods including turbidimetry, dynamic light scattering, and Zeta potential analysis. The experimental results and the molecular dynamics simulations showed that the PLL structural transitions were a result of a competition between electrostatic repulsion, which promotes an extended state, and the hydrophobic effect, which favors a compact state. In fact, as the pH was decreased, the PLL conformation changed from α-helix to the random coil and the hydrophilic volume increases resulted in a transition to spherical micelles which then swelled due to charge-charge repulsions. Following a rise in temperature and/or at high pH, PLL undergoes the α-helix-to-β-sheet transition and reacted more rapidly to form hydrophobic aggregates.     

Magneto-optical signature of broken mirror symmetry of two-dimensional conductors

An unusual aspect of macroscopic electrodynamics of two-dimensional mirror-odd conducting structures bound up with the band spin–orbit coupling H _so = α(p × c) · σ of current carriers (where c is one of two none-quivalent normals to a given structure) is pointed out. Namely, it is shown that due to the spin–orbit coupling the presence of the in-plane magnetic field H ₀ gives rise to a dependence of the reflection/transmission amplitudes on the structure orientation c, the wave vector of the incident radiation q, and H ₀ of the form q · (c × H ₀). This q- and H ₀-odd dependence can be the foundation of the optical way to determine the value of the spin–orbit coupling α.     

Superstructure-mechanical property relationships of polyglutamic acid esters

A study has been made of the relaxation phenomena in a series of poly-γ-n-alkyl-D-glutamates having linear side chains of different lengths in the temperature range from ?170 to 250°C. The solid-state films of poly-γ-n-alkyl-D-glutamate were cast from solutions in α-helix and random coil forming solvents. The technique of dynamic mechanical relaxation at various frequencies up to 110 Hz was used in the investigation. X-ray, IR, and density measurements were used as supplements to the mechanical measurements in order to determine the relationships between the mechanical properties and superstructures. Six relaxation regions are discernible in the temperature range investigated, and these are labeled α, β, γ, δ, ε, and ζ in order of decreasing temperature. The following mechanisms are assigned to these relaxations: (1) The α-relaxation arises from the thermal molecular motion of the α-helices in the crystalline region. (2) The β relaxation arises from micro-Brownian segmental motion of the disordered region or the distorted part of the a-helices, (3) The γ and δ relaxations are associated with motions of the entire side-chain groups in the crystalline and the disordered regions, respectively. (4) The ε and ζ relaxations involve the conformational rearrangements of n-alkyl groups including carboxyl group and n-alkyl groups alone, respectively.     

载波傅里叶分析法中干涉图的预处理技术研究

提出了一种基于旋滤波法的干涉条纹预处理方法,并将其应用到载波傅里叶分析法中。该方法能较好地处理各种噪声而不会使条纹变模糊。实验证明该方法能有效地去除相干噪声、随机噪声、椒盐噪声、由于光照不均匀引起的较大面积的亮斑或暗斑等噪声,能较好地提高载波傅里叶分析法的精度和重复性。