Water behavior in the neighborhood of hydrophilic and hydrophobic membranes: Lessons from molecular dynamics simulations

The study of properties of water in the vicinity of surfaces poses a fascinating challenge. In this article we studied the behavior of water molecules in the neighborhood of membranes. We addressed the question of how these water molecules are influenced by the membranes’ hydrophilicity. Three systems were studied through molecular dynamics simulations: water in the presence of a hydrophilic membrane (PL), water in the presence of a hydrophobic (PB) one and water in the absence of membranes (BULK). Additionally, in order to study the dependence of the effect of the membrane on the behavior of neighboring water molecules with temperature, each system was simulated at three different temperatures (K): 250, 300 and 350. For each condition, kinetic and structural features were studied. The first feature involved the calculation of diffusion coefficients and activation energy. The second feature was evaluated through the study of water density and hydrogen bond distribution. From the present study we concluded that: (1) density studies underestimate the influence of both hydrophilic and hydrophobic membranes on the neighboring water molecules; (2) the hydrophilic and hydrophobic membranes disturb the hydrogen bond network within distances ranging from 1 to 8 nm, depending on the nature of the membrane and the temperature conditions; (3) the presence of a hydrophobic surface results in an enhancement of the natural hydrogen bond network present in liquid water, to a greater extent than what even an ordered Ih ice structure is able to achieve (i.e. PL membrane); (4) the structural enhancement due to the presence of a hydrophobic surface involves roughly 18 to 24 water hydration layers, for ambient and above temperature conditions.     

The role of Gauss curvature in a membrane phase separation problem

Consider a two-phase lipid vesicle. Below the transition temperature, the phases separate into non-connecting domains that coarsen into larger areas. The free energy of phase properties determines the length of the boundaries separating the regions. The two phases correspond to different lipid compositions, and in cells, this fluctuation in composition is a dynamic process vital to its function. We prove that a small patch of the minority lipids forms at a point of the membrane where the Gauss curvature attains a maximum. This patch has a round shape approximately and its boundary has a constant geodesic curvature. The proof consists of three steps. The construction of a family of good approximate solutions, an improvement of the approximate solutions so that their geodesic curvature is a constant modulo translation, and the identification of an exact solution from the family of the improved approximate solutions. Our theoretical results are supported by vesicle experiments.     

The effect of screening on entropy production in pattern formation

Information theory is used to study the effects of screening on the rate of entropy production during pattern formation. Screening is an effect where the outermost parts of a growing fractal pattern influence the growth probability at interior sites. The results demonstrate that a state of maximum entropy production does exist for dynamical systems which generate patterns based on simple screening rules alone. This state corresponds to a critical point where the pattern exhibits self-similarity and fractal properties typical of random aggregates. Scaling occurs because the screening transmits information from the smallest to the largest scales of the system.     

含有Dzyaloshinskii-Moriya相互作用的自旋1键交替海森伯模型的量子相变和拓扑序标度

利用张量网络表示的无限矩阵乘积态算法研究了含有Dzyaloshinskii-Moriya (DM)相互作用的键交替海森伯模型的量子相变和临界标度行为.基于矩阵乘积态的基态波函数计算了系统的量子纠缠熵及非局域拓扑序.数据表明,随着键交替强度变化,系统从拓扑有序的Haldane相转变为局域有序的二聚化相.同时DM相互作用抑制了系统的二聚化,并最终打破系统的完全二聚化.另外,通过对相变点附近二聚化序的一阶导数和长程弦序的数值拟合,分别得到了此模型相变的特征临界指数a和b的值.结果表明,随着DM相互作用强度的增强, a逐渐减小,同时b逐渐增大. DM相互作用强度影响着此模型的临界行为.针对此模型的临界性质的研究,揭示了量子自旋相互作用的彼此竞争机制,对今后研究含有DM相互作用的自旋多体系统中拓扑量子相变临界行为提供一定的借鉴与参考.     

Investigating lipid interactions and the process of raft formation in cellular membranes using ToF-SIMS

There is an increased interest in how lipids interact with each other, especially in the lateral separation of lipids into coexisting liquid phases as this is believed to be an attribute of raft formation in cell membranes. ToF-SIMS has shown itself to be an excellent tool for investigating cellular and model membrane systems and will be perhaps the most powerful one for investigating raft formation. Results from our laboratory show the capability of ToF-SIMS at identifying unequivocally the content of coexisting liquid lipid phases. Using supported lipid monolayers we find that the inclusion of dipalmitoylphosphatidylethanolamine (DPPE) to a homogeneous dipalmitoyl-phosphatidylcholine (DPPC)/cholesterol phase results in the formation of cholesterol-rich domains [A.G. Sostarecz, C.M. McQuaw, A.G. Ewing, N. Winograd, J. Am. Chem. Soc. 126 (2004) 13882]. Also, for DPPE/cholesterol systems a single homogeneous DPPE/cholesterol phase is formed at ∼50 mol% cholesterol, whereas DPPC/cholesterol systems form a single phase at 30 mol% cholesterol [C.M. McQuaw, A. Sostarecz, L. Zheng, A.G. Ewing, N. Winograd, Langmuir 21 (2005) 807]. Currently we are exploring the incorporation of sphingomyelin into phospholipid-cholesterol mixtures in an effort to gain a better understanding of its role in raft formation.     

相结构形成因子统计值对钢中化合物形成的影响

本文根据宏观物理量是微观原子状态统计值的思想,将原子组态最可几值改用统计值代替,利用固体与分子经验电子理论(EET)计算钢中FeS、MnS、TiC(N)、NbC(N)、VC(N)、AlN的相结构形成因子的统计值S′,探讨了各种化合物的S′对钢中组成原子自发凝聚成各种化合物能力的影响.研究发现S′McNS′McC,可以从电子层面合理解释钢在平衡状态下由高温向低温转变时,FeS先于MnS析出,合金碳化物先于合金氮化物析出,在非平衡状态下,析出顺序相反的事实.并成功解释了铁碳相图共晶转变和共析转变各个相的形成顺序.从电子层次得出,在平衡条件下,S′较大的相先形成,在非平衡条件下,S′较小的相先形成.在某一适当冷却速度范围内,S′相差较小的相可以共生,这一冷却速度区间将是S′差值的函数.     

相结构形成因子统计值对钢中化合物形成的影响

本文根据宏观物理量是微观原子状态统计值的思想,将原子组态最可几值改用统计值代替,利用固体与分子经验电子理论(EET)计算钢中FeS、MnS、TiC(N)、NbC(N)、VC(N)、AlN的相结构形成因子的统计值S^',探讨了各种化合物的S^'对钢中组成原子自发凝聚成各种化合物能力的影响。研究发现S_MnS^'相似文献   

The role of 18-methyleicosanoic acid in the structure and formation of mammalian hair fibres

Although branched chain fatty acids perform many functions in biological systems, the importance of the anteiso 18 methyleicosanoic acid (MEA) has only recently been recognized. In this first review on MEA its role and distribtuion is explored MEA has been found in minor amounts in the fatty acid components of a wide range of biological materials, but the current interest results from it being the major covalently bound fatty acid in mammalian hair fibres, a finding which is unusual because protein-bound fatty acids are typically straight-chain, even-numbered acids (C14–C18). MEA is released by surface restricted reagents indicating that it is located exclusively in or on the surface of the cuticle cells, a conclusion that has been verified by analysis of isolated cuticle cells. X-ray photoelectron spectroscopy (XPS) and secondary-ion mass spectroscopy (SIMS) studies support these results in that they show the surface of the cuticle to be predominantly hydrocarbon. When either neutral hydroxylamine or acidic chlorine solutions are applied to hair and wool fibres fatty acids are liberated, indicating the presence of thioester bonds. Calculations, based on fatty acid and amino acid analysis, indicate that approximately one residue in 10 of the cuticular membrane protein is a fatty acid thioester of cysteine. Removal of this covalently linked fatty acid renders the fibre hydrophilic, thus offering a chemical explanation for many technological and cosmetic treatments of mammalian fibres. Examination of the fibre surface and that of isolated cuticle cells by transmission electron microscopy (TEM) confirms the presence of a thin non-staining continuous layer surrounding the cuticle cells. Alkaline treatments which remove the bound fatty acids were found to disrupt this layer. TEM examination of developing hair fibres has indicated that the fatty acid layer on the upper surface and scale edges of the cuticle cell differs from that of the underside of the cell. Similar structural studies of hair from patients with maple syrup urine disease (MSUD) support the findings that thioester-bound MEA is limited to the upper surface of fibre cuticle cells. The current model proposed for the boundary layer consists of crosslinked protein with surface thioester-linked fatty acids. forming a continuous hydrophobic layer on the upper surface and scale edges of the cells.     

The dynamic evolutions of the generalized Hobbit model and structure formation

In this paper a more general phenomenological model is proposed on the basis of the Hobbit model in Cardone et al. (Phys Rev D 69:083517, 2004). The main purpose of constructing our model is to make this model can not only mimic the ΛCDM model, but also describe a four-phase smooth transition of the universe, namely the inflationary phase, a radiation-dominated phase, a matter-dominated phase and a de Sitter-like phase. In order to check whether this model is a viable one, the evolutions of the universe are respectively discussed in the two cases, and the possible physical interpretations for this model are also respectively shown by using the scalar fields. Finally, the structure formations in our model are simply discussed for the both cases, and the results given by us reconfirm that our model can be regarded as a fit to the ΛCDM model, if we choose the proper conditions.     

Exciton eigenstates and biexciton interaction energies in a spherical core/shell semiconductor hetero-nano structure

The exciton eigenstates and biexciton interaction energies in a spherical core–shell hetero-nano structure in the type II and the quasi-type II carrier localization regimes have been analyzed. For the analysis, we have evaluated the electron–hole overlap integral, the binding energy of exciton ground state, and the interaction energy of bi-exciton ground state in the structure. In the evaluation, the first order perturbation approach has been employed, where the direct Coulomb interaction energy, the surface polarization energy and the dielectric solvation energy are included. Our results show that the exciton eigenenergies and exciton–exciton interaction energy strongly depend on the choice of materials on which both the dielectric constants and the electron and hole effective masses rely.     

Fe-Cr-Ni合金中点缺陷形成及相互作用的第一性原理研究

奥氏体不锈钢由于具有良好的力学性能和耐腐蚀性,被广泛应用于核电工业。奥氏体不锈钢受辐照会产生多种点缺陷,进而影响材料力学性能。为了研究304NG奥氏体不锈钢中点缺陷的产生机理,以面心立方Fe-Cr-Ni合金为模型,利用基于密度泛函理论的第一性原理方法研究了多种点缺陷结构的晶格常数,形成能,结合能,扩散迁移及相互作用,并计算了γ-Fe中相应的的性质作为对照.研究结果表明：Fe空位和Fe-Fe <100>哑铃是Fe-Cr-Ni合金中容易形成的点缺陷.基于对这两种点缺陷的迁移能垒分析,发现Fe-Fe <100>哑铃要比空位更易迁移.在Fe-Cr-Ni合金中相邻Fe空位存在吸引作用,会促进孔洞的形成;Fe空位-Fe-Fe <100>哑铃及Fe空位-Fe-Cr <100>哑铃均存在较弱的排斥作用,此外,在第一近邻位置和第二近邻位置的八面体间隙Fe原子都会和空位结合,使体系恢复为完整晶胞.     

182Os 核yrast带相继SU(3)–U(5)–SU(3)结构相变势能曲面的一种可能理解

基于联合实施微观相互作用玻色子模型的最大F旋方案 (sdIBM-F_max)与γ射线能量-自旋曲线 (γ-ray energy over spin curves, E-GOS)方案, 成功描述了¹⁸²Os核yrast带相继的SU(3)–U(5)–SU(3)结构相变, 由于缺少直观解释而显得抽象. 本文借助微观sdIBM-F_max的微观参数与Bohr哈密顿量的势能曲面方程之间存在的泛函关系, 几何地给出了对这种相继相变途径的另外一种可能理解; 并阐述了在完全变形核的高角动量态中, 由于量子效应在高激发态与低激发态之间生成高简并的临界区, 提供了γ振动能量会变得低于转动能量的一个可能途径, 从而实现了SU(3)–U(5)的相变. 关键词： yrast带结构演化 势能曲面 相变临界区 182Os核')" href="#">¹⁸²Os核     

The changes in electronic structure and optical properties of magnetic semiconductors at magnetic phase transitions

The influence of magnetic phase transitions on electronic structure and optical properties of magnetic semiconductors is discussed. Europium chalcogenides and chromium chalcogenide spinels are the main subjects of the investigation. It is shown, that many-body effects are responsible for the changes of optical properties and non-rigid band behavior of electronic structure. Magnetic phase transition leads to energy shift of wide bands and change in density of states of “magnetic” d(f)-electrons without any significant shift of their energies. The influence of fluctuations at T ～ T _c and antiferromagnetic semiconductors are also considered.     

Quantum phase transition in XY spin chain with three-site interaction studied in terms of Loschmidt echo and Berry phase

By means of the Loschmidt Echo (LE) and Berry Phase (BP) calculations, quantum phase transition (QPT) of an XY spin chain with three-site interaction (α) in a transverse magnetic field (λ) is studied. Both the LE and BP?s λ derivative present anomaly behaviors at the critical regions λ₁,λ₂ and λ₃. The model is in the Ferromagnetic phase as λ>λ₁=1+α and in the Spin Liquid I phase as −1+α<λ<1+α. λ₁ and λ₂ are independent on the anisotropy parameter γ. But, the anisotropy interaction can shift the critical line λ₃ between the Spin Liquid II phase and the Ferromagnetic phase. The present work suggests that QPT of the XY spin chain with three-site interaction can be characterized by exploring the dynamical behaviors of the LE and BP.     

The role of a split injection strategy in the mixture formation and combustion of diesel spray: A large-eddy simulation

The role of a split injection in the mixture formation and combustion characteristics of a diesel spray in an engine-like condition is investigated. We use large-eddy simulations with finite rate chemistry in order to identify the main controlling mechanism that can potentially improve the mixture quality and reduces the combustion emissions. It is shown that the primary effect of the split injection is the reduction of the mass of the fuel-rich region where soot precursors can form.Furthermore, we investigate the interaction between different injections and explain the effects of the first injection on the mixing and combustion of the second injection. Results show that the penetration of the second injection is faster than that of the first injection. More importantly, it is shown that the ignition delay time of the second injection is much shorter than that of the first injection. This is due to the residual effects of the ignition of the first injection which increases the local temperature and maintains a certain level of combustion some intermediates or radical which in turn boosts the ignition of the second injection.     

The role of excluded volume and electrostatics from coarse-grain modeling of the interaction of gemini surfactants with like-charged membranes

Cationic systems composed of lipids and/or surfactants are of paramount importance in a variety of applications. Within these, gemini have attracted particular attention, mainly due to their improved aggregation properties and to the possibility of tuning offered by the presence of a spacer. In this work, a Monte Carlo simulation study with a coarse-grained model was employed to assess the interaction of cationic gemini surfactants with a like-charged model membrane. Separating the contribution of the excluded volume and that of the electrostatic effects in the organization of gemini–lipid membranes was the first goal of this work and the role of these factors was assessed varying the concentration, the spacer length and the headgroup charge of gemini surfactants. The results provide a new insight on the organization of lipid headgroups in the vicinity of gemini surfactants. It was found that the surfactant–lipid interaction is strongly affected by the surfactant spacer length, being controlled by an overall balance between excluded volume and surfactant–lipid and surfactant–surfactant electrostatic effects. It is also seen that the out-of-plane motion of the spacer has a significant effect upon membrane organization and counterion condensation. Good agreement was found with results previously obtained from atomistic simulation.     

1H NMR study of molecular dynamics and phase transition in (NH4)2ZnBr4

The proton second moment (M ₂) and spin-lattice relaxation time (T ₁) have been measured in (NH₄)₂ZnBr₄ in the range 77–300 K. The room-temperature spectrum shows a structure which disappears around 243 K. The signal is strong and narrow even at 77 K. Proton T ₁ shows a maximum at 263 K, caused by spin rotation interaction and decreases with decreasing temperature till 235 K, where it shows a sudden increase. Below 235 K, again it decreases and shows a slope change around 216.5 K (reported T_c ). From 216.5 K, T ₁ decreases continuously without exhibiting any minimum down to 77 K. The narrow line at 77 K, and absence of a T ₁ minimum down to 77 K indicate the possibility of quantum mechanical tunnelling in this system. Motional parameters such as activation energy and pre-exponential factor have been evaluated for the reorientational motion of the NH⁺ ₄ ion.     

LiNbO3∶Fe3+晶体的光谱精细结构、零场分裂参量及Jahn-Teller效应

应用不可约张量方法和群的理论构造了三角对称晶场中3d⁵组态离子的252阶可完全对角化的微扰哈密顿矩阵，利用该矩阵计算了LiNbO₃∶Fe³⁺晶体的光谱精细结构、零场分裂、晶体结构、Jahn-Teller(J-T)效应，其理论计算值与实验值相符合，并研究了自旋四重态、自旋二重态分别对基态能级的影响，证明了自旋四重态对基态能级的贡献是主要的，自旋二重态对基态能级的贡献虽很小，但却是不可忽略的.在此基础上,进一步研究了自旋-轨道耦合作用、自旋-自旋耦合作用对LiNbO₃∶Fe³⁺晶体的光谱精细结构和零场分裂参量的影响，发现自旋-轨道耦合作用是最主要的，自旋-自旋耦合作用也是不可忽略的. 研究表明，该种物质的四重态光谱结构中含有J-T效应. 其产生原因是自旋-轨道耦合及三角畸变的共同作用的结果，两者缺一不可.     

The atomic and electronic structure of the TiO2- and BaO-terminated BaTiO3(0 0 1) surfaces in a paraelectric phase

Relaxation and rumpling of BaTiO₃(0 0 1) surface with two different terminations have been investigated from ab initio local density approximation calculations. Large displacements of ions deviated from their crystalline sites have been obtained. These kinds of displacements lead to the formation the surface rumpling, dipole moments and electric field in the near-surface region. Band structures, density of states, bond population and electronic density redistributions have been obtained. Considerable enhancement of Ti–O chemical bond covalency nearby the surface, especially for the TiO₂ termination surface, has been found.