重离子碰撞中熔合位垒的研究

熔合位垒的研究对熔合反应以及超重核合成有重要的意义。在改进的同位旋相关的量子分子动力学(ImIQMD)模型框架下,提取了熔合反应体系40Ca+40Ca,48Ca+208Pb,48Ca+204Pb和16O+154Sm的熔合位垒。研究了壳修正能对熔合位垒的影响、动力学位垒的能量依赖性、同位旋效应以及形变核的方向效应。计算发现壳修正能降低了熔合反应的位垒。在研究动力学位垒的能量依赖性时,发现位垒高度和位垒半径表现出相反的能量依赖行为。在动力学反应中,当两个核距离接近时,缺中子体系的库仑势同样表现出一定的能量依赖性。对于丰中子体系,由于中子屏蔽作用,库仑势基本没有能量依赖性。     

Effects of charge fluctuation on two-membrane instability and fusion

Membrane fusion is fundamental to diverse biological processes ranging from intercellular and intracellular transport to egg fertilization. We study the effects of coupling between membrane undulation and charge fluctuation on its fusion. We find that, at concentrations of millimolar range, multivalent cations such as calcium in solution induce a strong correlated-charge fluctuation on each membrane, leading to inversion and overcondensation of surface charges. When the charge fluctuation is cooperatively coupled to undulation, two apposing membranes undergo a dynamic instability to spontaneous growth of in-phase undulation with submicron wavelengths, thereby greatly reducing fusion barrier.     

Barrier fluctuations and stochastic resonance in membrane transport

The role of barrier fluctuations in membrane enzymatic processes, in particular in the active transport of ions through cell membranes, is examined. For enzymes embedded in the cell membrane the role of the barrier height (activation energy) is played by the membrane electric potential. This barrier height can be modulated either by internal fluctuations or by external electrical fields, either random or periodic. Existing experimental data on active transport of Na(+) and Rb(+) in human erythrocytes (catalyzed by Na(+)-K(+)-ATPase) can be interpreted as evidence of stochastic resonance between the external ac field and the fluctuations of the membrane potential. The obtained results suggest that the significant part of these fluctuations is supplied by the stimulated action of neighbor voltage-gated ionic channels. This supports the idea that intrinsic noise plays a constructive role in one of most important and most frequent biophysical processs, viz. ion transmission through cell membranes. Means of further experimental verification of this conjecture are proposed. (c) 1998 American Institute of Physics.     

Minimum energy path to membrane pore formation and rupture

We combine dynamic self-consistent field theory with the string method to calculate the minimum energy path to membrane pore formation and rupture. In the regime where nucleation can occur on experimentally relevant time scales, the structure of the critical nucleus is between a solvophilic stalk and a locally thinned membrane. Classical nucleation theory fails to capture these molecular details and significantly overestimates the free energy barrier. Our results suggest that thermally nucleated rupture may be an important factor for the low rupture strains observed in lipid membranes.     

Topological relaxation of a shear-induced lamellar phase to sponge equilibrium and the energetics of membrane fusion

We report time-resolved small angle neutron scattering (t-SANS) measurements of the topological relaxation of Couette shear-induced stacked L(alpha) lamellar states to their multiconnected isotropic L3 sponge equilibrium phases in a surfactant bilayer membrane system. Comparison of this structural relaxation time to the interval between diffusive membrane contacts, as determined from dynamic light scattering or estimated from the shear rates required for L(alpha) saturation, allows us to determine the activation energy barrier to the membrane fusion process reestablishing the solution channel handles that characterize the sponge phase.     

Transmembrane transport of multicomponent liposome-nanoparticles into giant vesicles

With the emergence and rapid development of nanotechnology, the nanoparticles hybridized with multicomponent lipids are more and more used in gene delivery. These vectors interact with the cell membrane before entering into the cell. Therefore, the nature of this interaction is important in investigating multicomponent liposome-nanoparticle (MLP) transport across the cell membrane. In this paper the transport of MLPs across the membranes of giant vesicles (GVs) in solvents is studied by using the self-consistent field theory (SCFT). Based on the analysis of the MLP permeating the GV membranes, a simple transport model is proposed. The effects of the difference in membrane morphology and the size of the nanoparticle on the endocytosis are discussed systematically. The role of energy barriers in quasi-equilibrium is also examined. The results indicate that the interaction between MLP and GV is a spontaneous process and the energy barrier needs overcoming to form metastable intermediates. The results provide theoretical reference for better understanding the transmembrane transport process of nanoparticles, and guidance for relevant experimental studies as well.     

Elastic energy of tilt and bending of fluid membranes

Tilt of hydrocarbon chains of lipid molecules with respect to membrane plane is commonly considered to characterize the internal structure of a membrane in the crystalline state. However, membranes in the liquid state can also exhibit tilt resulting from packing constraints imposed on the lipid molecules in diverse biologically relevant structures such as intermediates of membrane fusion, pores in lipid bilayers and others. We analyze the energetics of tilt in liquid membranes and its coupling with membrane bending. We consider three contributions to the elastic energy: constant tilt, variation of tilt along the membrane surface and membrane bending. The major assumption of the model is that the core of a liquid membrane has the common properties of an elastic continuum. We show that the variation of tilt and membrane bending are additive and that their energy contributions are determined by the same elastic coefficient: the Helfrich bending modulus, the modulus of Gaussian curvature and the spontaneous curvature known from previous studies of pure bending. The energy of a combined deformation of bending and varying tilt is determined by an effective tensor accounting for the two factors. In contrast, the deformation of constant tilt does not couple with bending and its contribution to the elastic energy is determined by an independent elastic constant. While accurate determination of this constant requires additional measurements, we estimate its value using a simplified approach. We discuss the relationships between the obtained elastic Hamiltonian of a membrane and the previous models of membrane elasticity. Received 10 February 2000 and Received in final form 19 June 2000     

熔合反应中动力学势垒的研究

用改进的量子分子动力学模型研究了与入射能量相关的重离子熔合势垒. 随着 入射能的降低可以观察到动力学势垒的最低值, 这个最低动力学势垒与绝热势垒 非常接近；动力学势垒随着入射能的增加而升高, 最终接近于静态势垒(非绝热势垒). 基于动力学势垒的研究, 对于重离子熔合反应的额外推动(extra-push)给出了微观理解,对势垒贯穿给出了一种新的解释. 为进一步理解动力学势垒, 还研究了颈部的形成和体系的动力学形变, 分析了动力学势垒降低的原因.     

Adhesion of membranes with competing specific and generic interactions

Biomimetic membranes in contact with a planar substrate or a second membrane are studied theoretically. The membranes contain specific adhesion molecules (stickers) which are attracted by the second surface. In the absence of stickers, the trans-interaction between the membrane and the second surface is assumed to be repulsive at short separations. It is shown that the interplay of specific attractive and generic repulsive interactions can lead to the formation of a potential barrier. This barrier induces a line tension between bound and unbound membrane segments which results in lateral phase separation during adhesion. The mechanism for adhesion-induced phase separation is rather general, as is demonstrated by considering two distinct cases involving: i) stickers with a linear attractive potential, and ii) stickers with a short-ranged square-well potential. In both cases, membrane fluctuations reduce the potential barrier and, therefore, decrease the tendency of phase separation. Received 24 January 2002 and Received in final form 24 April 2002     

Fluctuation spectrum of fluid membranes coupled to an elastic meshwork: jump of the effective surface tension at the mesh size

We identify a class of composite membranes: fluid bilayers coupled to an elastic meshwork that are such that the meshwork's energy is a function F(el)[A(xi)] not of the real microscopic membrane area A, but of a smoothed membrane's area A(xi), which corresponds to the area of the membrane coarse grained at the mesh size xi. We show that the meshwork modifies the membrane tension sigma both below and above the scale xi, inducing a steep crossover of amplitude deltasigma=dF(el)/dA(xi). The predictions of our model account for the fluctuation spectrum of red blood cell membranes coupled to their cytoskeleton. Our results indicate that the cytoskeleton might be under extensional stress, which would provide a means to regulate available membrane areas. We also predict an observable tension jump for membranes decorated with polymer "brushes."     

Distribution of fusion barriers

Heavy ion fusion cross sections and compound nucleus average spin values obtained from distribution of fusion barriers are discussed. Various shapes of distribution functions are studied using a truncated Gaussian distribution function (TGD). It is shown that fusion cross section and average spin values are less sensitive to different parametrization of TGD function, whereas the second derivative of the product of energy and fusion cross sections (w.r.t. energy), obtained from the corresponding TGD functions are significantly different depending on the shape of the barrier distribution function. It is also shown byχ ² analysis of fusion cross section data that some systems favour a narrow Gaussian distribution function whereas others, for which the vibrational and rotational collective states are less important, favour a flat barrier distribution. A physical interpretation of the dynamical process that gives rise to different barrier distribution is given in the framework of microscopic coupled channel calculations.     

Enhancement of macroscopic quantum tunneling by Landau-Zener transitions

Motivated by recent realizations of qubits with a readout by macroscopic quantum tunneling in a Josephson junction, we study the problem of barrier penetration in the presence of coupling to a spin-1 / 2 system. It is shown that, when the diabatic potentials for fixed spin intersect in the barrier region, Landau-Zener transitions lead to an enhancement of the tunneling rate. The effect of these spin flips in imaginary time is in qualitative agreement with experimental observations.     

重核融合反应动力学位垒的微观研究

在简要评述重核融合过程中几种主要理论模型的基础上,提出了微观输运动力学模型,即改进的量子分子动力学模型.在这个模型的框架内,我们研究了重核融合位垒的动力学行为.我们发现,随入射能量的减少,可以得到最低的动力学位垒,它趋近于绝热静态位垒.而随入射能量的增加,动力学位垒增加,最后趋近于非绝热静态位垒,这给出了位垒分布的两个边缘.在微观输运动力学模型基础上,我们还研究了在融合路径上,动力学位垒与融合体系微观构型的关系.考虑到融合过程不同时刻的单粒子位势与双中心壳模型位势的相似性,我们可以很好的研究融合过程中,在构型空间里单粒子态及相关量的时间演化行为.     

Skyrme能量密度泛函在深度垒下熔合反应中的初步应用

基于Skyrme能量密度泛函并对动能密度按扩展的Thomas-Fermi半经典近似展开到四阶项,探索了深度垒下熔合反应的熔合截面急剧下降现象。通过分析熔合反应的入射道势和熔合截面来选择Skyrme参数,进而探索了深度垒下熔合反应与核物质状态方程之间的关系。     

Analytical calculation of fusion cross-sections

We analyse the fusion cross-sections, calculated by using two different analytical parameterisations and compare them with the experimental data. Both the parameterisations are based on ion-ion potentials calculated within the framework of Skyrme energy density formalism. In the first case, the ion-ion potential (including the spin-density term) was parameterised and then, by adding the Coulomb potential, one could compute the fusion barrier analytically. In the second case, the calculated fusion barrier heights and positions were parameterised directly. Both of these (previously) reported parameterisations are used here to calculate the fusion barriers and fusion excitation functions for more than 50 reactions belonging to the s-d and f-shell nuclei. A detailed comparison of these parametrisations with the experimental and several other theoretical results shows that both of these parameterisations are able to reproduce the experimental data equally well. As the (second) direct parameterisation depends only on the charges and masses of colliding nuclei, it is very useful for predicting/ understanding the fusion process in low energy heavy-ion reactions. Received: 24 February 1999 / Accepted: 16 March 2000     

杆状病毒HaF融合肽在中性pH溶液中的NMR结构研究

膜融合蛋白在介导膜融合过程中会发生构象的转变. 以同核2D NMR为手段，测定了pH 7.0条件下棉铃虫核多角体病毒(Helicoverpa armigerasingle- nucleocapsid Nucleopolyhedrovirus，HearNPV)的HaF融合肽在类膜环境中的3级结构. 通过与酸性条件下该融合肽的结构作比较，证实了该融合肽在从融合蛋白内部暴露出来到插入宿主细胞膜的过程中发生了构象的转变. 并且这个构象的转变是一个结构趋向稳定、两亲性趋向完整的变化过程. 这些结论对研究其他融合肽的插膜过程有普遍的意义，为探索膜融合机制提供了信息.     

Antiproton-catalyzed fusion

Because of the potential application to power production, it is important to investigate a wide range of possible means to achieve nuclear fusion, even those initially appearing infeasible. In antiproton-catalyzed fusion, the negative antiproton shields the repulsion between the positively charged nuclei of hydrogen isotopes, allowing a much higher level of penetration through the repulsive Coulomb barrier and greatly enhancing the fusion cross section. With their more compact wave function, the more massive antiprotons offer much more shielding than negative muons. If the antiproton could exist in the ground state with a nucleus for a sufficient time without annihilating, the fusion cross sections are so enhanced at low energies that at room temperature, values up to about 1000 barns (d + t) would be possible. Unfortunately, the cross section for antiproton annihilation with the incoming nucleus is even higher. A model giving an upper bound for the fusion to annihilation cross section ratio for all relevant energies indicates that each antiproton will catalyze no more than about one fusion. Since the energy to make one antiproton greatly exceeds the fusion energy released, this level of catalysis is far from adequate for power production.     

Accurate coarse-grained modeling of red blood cells

We develop a systematic coarse-graining procedure for modeling red blood cells (RBCs) using arguments based on mean-field theory. The three-dimensional RBC membrane model takes into account the bending energy, in-plane shear energy, and constraints of fixed surface area and fixed enclosed volume. The coarse-graining procedure is general, it can be used for arbitrary level of coarse-graining and does not employ any fitting parameters. The sensitivity of the coarse-grained model is investigated and its behavior is validated against available experimental data and in dissipative particle dynamics (DPD) simulations of RBCs in capillary and shear flows.     

Fusion near the Coulomb barrier for the synthesis of heavy and superheavy elements: A theoretical approach

The compound nucleus formation is considered as a two-step process of touching and subsequent tunneling of the projectile into the target. The deep minima in the potential energy curve are due to shell effects in the experimental binding energies and give possible target-projectile combinations for the synthesis of heavy and superheavy elements. The asymmetric channels thus obtained are in remarkable agreement with the known experimental channels. In our model, the colliding partners are first shown to be captured in the pocket behind the outer (touching) barrier and the composite system so formed finally tunnels through the inner (fusion) barrier to form the resulting compound nucleus. These calculations reveal the importance of the fusion barrier, which occur only for the asymmetric target-projectile combinations. The calculated fusion cross-sections show a reasonable comparison with the observed one-neutron evaporation residue cross-sections. An estimate of the excitation energy carried by the compound nucleus is also obtained from our model calculations.