电势法测定KI在CH3OH-H2O混合溶剂中的活度系数

用无液接电势电池测定了303.15 K时,KI在甲醇-水混合溶剂中的电动势,用扩展的Debye-Hückel方程及Pitzer方程关联不同溶剂组成下电动势的实验值,得到电池的标准电动势E°和KI在混合溶剂中的活度系数.结果表明,在一定的溶剂比例下,随KI浓度的增大,其平均活度系数先随它的浓度增加而下降,经过一个最低点后又上升;或者随浓度增加而单调递减.当KI浓度固定时,它的平均活度系数随溶剂中甲醇的含量增加而下降.表明KI在甲醇中有微弱的缔合.     

Dissolved organic carbon--contaminant interaction descriptors found by 3D force field calculations

Enthalpies of transfer at 300 K of various partitioning processes were calculated in order to study the suitability of 3D force fields for the calculation of partitioning constants. A 3D fulvic acid (FA) model of dissolved organic carbon (DOC) was built in a MM+ force field using AMI atomic charges and geometrical optimization (GO). 3,5-Dichlorobiphenyl (PCB14), 4,4'-dichlorobiphenyl (PCB15), 1,1,1-trichloro-2,2-bis-(4-chlorophenyl)-ethane (PPDDT) and 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (Atrazine) were inserted into different sites and their interaction energies with FA were calculated. Energies of hydration were calculated and subtracted from FA-contaminant interactions of selected sites. The resulting values for the enthalpies of transfer from water to DOC were 2.8, -1.4, -6.4 and 0.0 kcal/mol for PCB 14, PCB15, PPDDT and Atrazine, respectively. The value of PPDDT compared favorably with the experimental value of -5.0 kcal/mol. Prior to this, the method was studied by the calculation of the enthalpies of vaporization and aqueous solution using various force fields. In the MM + force field GO predicted enthalpies of vaporization deviated by +0.7 (PCB14), +3.6 (PCB15) and -0.7 (PPDDT)kcal/mol from experimental data, whereas enthalpies of aqueous solution deviated by -3.6 (PCB14), +5.8 (PCB15) and +3.7 (PPDDT) kcal/mol. Only for PCB14 the wrong sign of this enthalpy value was predicted. Potential advantages and limitations of the approach were discussed.     

A model system to study the insertion of cholesterol into a phospholipid monolayer

Colloidal probe atomic force microscopy (AFM) was used to study the interaction between a surface bearing tethered cholesterol groups and an egg phosphatidylcholine (egg-PC) monolayer. The cholesterol bearing surface was comprised of a mixed self-assembled monolayer comprised of O-cholesteryl N-(8'-mecapto-3',6'-dioxaoctyl)carbamate (CPEO3) molecules and beta-mercaptoethanol formed on a 20 mum diameter gold-coated silica particle. The egg-PC monolayer was adsorbed onto an octadecylthiol monolayer formed on template-stripped gold. The force between the surfaces, as a function of separation, was measured for surface concentrations of CPEO3 from 0 to 100 mol %. At all concentrations there was a long-range repulsive double-layer force due to weak surface charges. At surface concentrations of CPEO3 from 1 to 29 mol % the interaction on the approach of the surfaces showed a maximum in the repulsive force, followed by a small (2-5 nm) jump into a force minimum corresponding to adhesion of the surfaces. On separation, a normalized pull-off force of 1.0-1.6 mN m(-1) was measured. Over the same concentration range, the calculated interaction energy per CPEO3 molecule decreased from 1.1 +/- 0.2 kT to 0.04 kT. At surface concentrations of 35 mol % and above there was no reproducible adhesion between the cholesterol-bearing surface and the phospholipid monolayer. We attribute the occurrence of short-range attraction and adhesion in the 1-29 mol % regime to the insertion of (some) cholesterol groups into the phospholipid monolayer. At higher surface concentrations the efficiency of insertion is reduced due to steric effects. We discuss the experimental results in the light of the energetics of the insertion of a cholesterol molecule into a lipid bilayer.     

Dissolved organic carbon--contaminant interaction descriptors found by 3D force field calculations

Enthalpies of transfer at 300 K of various partitioning processes were calculated in order to study the suitability of 3D force fields for the calculation of partitioning constants. A 3D fulvic acid (FA) model of dissolved organic carbon (DOC) was built in a MM+ force field using AM1 atomic charges and geometrical optimization (GO). 3,5-Dichlorobiphenyl (PCB14), 4,4'-dichlorobiphenyl (PCB15), 1,1,1-trichloro-2,2-bis-(4-chlorophenyl)-ethane (PPDDT) and 2-chloro-4-ethylamino-6-isopropylamino- s -triazine (Atrazine) were inserted into different sites and their interaction energies with FA were calculated. Energies of hydration were calculated and subtracted from FA-contaminant interactions of selected sites. The resulting values for the enthalpies of transfer from water to DOC were 2.8, m 1.4, m 6.4 and 0.0 kcal/mol for PCB14, PCB15, PPDDT and Atrazine, respectively. The value of PPDDT compared favorably with the experimental value of m 5.0 kcal/mol. Prior to this, the method was studied by the calculation of the enthalpies of vaporization and aqueous solution using various force fields. In the MM+ force field GO predicted enthalpies of vaporization deviated by +0.7 (PCB14), +3.6 (PCB15) and m 0.7 (PPDDT) kcal/mol from experimental data, whereas enthalpies of aqueous solution deviated by m 3.6 (PCB14), +5.8 (PCB15) and +3.7 (PPDDT) kcal/mol. Only for PCB14 the wrong sign of this enthalpy value was predicted. Potential advantages and limitations of the approach were discussed.     

Alchemical Grid Dock (AlGDock): Binding Free Energy Calculations between Flexible Ligands and Rigid Receptors

Alchemical Grid Dock (AlGDock) is open-source software designed to compute the binding potential of mean force—the binding free energy between a flexible ligand and a rigid receptor—for a small organic ligand and a biological macromolecule. Multiple BPMFs can be used to rigorously compute binding affinities between flexible partners. AlGDock uses replica exchange between thermodynamic states at different temperatures and receptor–ligand interaction strengths. Receptor–ligand interaction energies are represented by interpolating precomputed grids. Thermodynamic states are adaptively initialized and adjusted on-the-fly to maintain adequate replica exchange rates. In demonstrative calculations, when the bound ligand is treated as fully solvated, AlGDock estimates BPMFs with a precision within 4 kT in 65% and within 8 kT for 91% of systems. It correctly identifies the native binding pose in 83% of simulations. Performance is sometimes limited by subtle differences in the important configuration space of sampled and targeted thermodynamic states. © 2019 Wiley Periodicals, Inc.     

Temperature-dependent intermolecular force measurement of poly(N-isopropylacrylamide) grafted surface with protein

We have investigated the temperature dependence of the intermolecular force between poly(N-isopropylacrylamide) (PNiPAM) grafted surface and bovine serum albumin (BSA) in phosphate buffer (pH 7.4) using atomic force microscopy at the nanonewton scale. These observations show that the interaction force is nearly zero below the phase transition temperature of PNiPAM and that it increases steeply during the phase transition. Since the PNiPAM chains are grafted onto the aminosilane (gamma-aminopropyltriethoxysilane)-treated silicon wafer, we measured the force-distance curve of BSA-immobilized tips for the bare and the aminosilane-treated silicon wafer. These surfaces show no temperature dependence and their values are different from those of the PNiPAM-grafted surfaces at 30 degrees C. The results indicate that the measured adhesion force is between the PNiPAM-grafted surface and the BSA-immobilized tip. Our studies on the intermolecular force between other surfaces (CH(3)- and COOH-terminated self-assembled monolayers) and the BSA-immobilized tip indicate that the variation in the intermolecular force between the PNiPAM surface and BSA with temperature can be attributed to the changes in the properties of the PNiPAM chains. From consideration of the PNiPAM phase transition mechanism, it is speculated that the intermolecular force between the PNiPAM-grafted surface and BSA would be affected by changes in the arrangement of the bound water molecules around the PNiPAM chain and by changes in the conformation (i.e., in the chain mobility) of the PNiPAM chain during the phase transition.     

Molecular dynamics simulation of an electric field driven dipolar molecular rotor attached to a quartz glass surface

Molecular dynamics simulations of the response of a dipolar azimuthal 3-chloroprop-1-ynyl rotor mounted on the surface of quartz glass to a rotating electric field were performed. The rotor motion was classified as synchronous, asynchronous, random, or hindered, based on the value of the average lag of the rotor behind the field and a comparison of the intrinsic rotational barrier V(b) with kT. A phase diagram of rotor behavior was deduced at 10, 300, and 500 K as a function of field strength and frequency. A simple model for the rotor motion was developed, containing the driving force, the temperature, the height of the torsional barrier, and the friction constant of the rotor. Defining E(bo) to be the electric field strength necessary to get rotational response from the rotor ("breakoff field") and mu to be the rotor dipole moment component in the plane of rotation, we find that E(bo) is frequency independent when 2 microE(bo) is less than either V(b) or kT (the driving force needs to overcome the more important of the two, the intrinsic barrier or random thermal motion). At higher frequencies, E(bo) is a quadratic function of the frequency and the driving force fights friction, which is dictated by intramolecular vibrational redistribution (IVR) from the pumped rotational mode to all others. Fitting the simple model to simulation data, we derived a friction constant of 0.26 ps eV x (nu - 0.5)/THz between 500 and 1000 GHz.     

冠醚配位作用的热力学性质——水液溶中1,4,7,10,13,16-六氧杂环十八烷与若干钾盐配位作用的负离子效应

本文用量热滴定法测定了1,4,7,10,13,16-六氧杂环十八烷(以下称18-冠-6)与KCl,KBr,KI, KClO_4,KNO_3,KBrO_3及K_2S_2O_3共八种钾盐在水溶液中, 25 ℃时的配位作用, 直接计算了配合物稳定常数及配位反应热。实验结果表明, 18-冠-6与KNO_3,K_2S_2O_3形成的配位化合物, 其稳定常数最高。讨论了阴离子对配位稳定性的影响。     

Mechanism underlying bioinertness of self-assembled monolayers of oligo(ethyleneglycol)-terminated alkanethiols on gold: protein adsorption, platelet adhesion, and surface forces

The mechanism underlying the bioinertness of the self-assembled monolayers of oligo(ethylene glycol)-terminated alkanethiol (OEG-SAM) was investigated with protein adsorption experiments, platelet adhesion tests, and surface force measurements with an atomic force microscope (AFM). In this work, we performed systematic analysis with SAMs having various terminal groups (-OEG, -OH, -COOH, -NH(2), and -CH(3)). The results of the protein adsorption experiment by the quartz crystal microbalance (QCM) method suggested that having one EG unit and the neutrality of total charges of the terminal groups are essential for protein-resistance. In particular, QCM with energy dissipation analyses indicated that proteins absorb onto the OEG-SAM via a very weak interaction compared with other SAMs. Contrary to the protein resistance, at least three EG units as well as the charge neutrality of the SAM are found to be required for anti-platelet adhesion. When the identical SAMs were formed on both AFM probe and substrate, our force measurements revealed that only the OEG-SAMs possessing more than two EG units showed strong repulsion in the range of 4 to 6 nm. In addition, we found that the SAMs with other terminal groups did not exhibit such repulsion. The repulsion between OEG-SAMs was always observed independent of solution conditions [NaCl concentration (between 0 and 1 M) and pH (between 3 and 11)] and was not observed in solution mixed with ethanol, which disrupts the three-dimensional network of the water molecules. We therefore concluded that the repulsion originated from structured interfacial water molecules. Considering the correlation between the above results, we propose that the layer of the structured interfacial water with a thickness of 2 to 3 nm (half of the range of the repulsion observed in the surface force measurements) plays an important role in deterring proteins and platelets from adsorption or adhesion.     

Anomalous pH dependent stability behavior of surfactant-free nonpolar oil drops in aqueous electrolyte solutions

Recent advances in atomic force microscopy (AFM) force measurement techniques have allowed the direct measurement and theoretical interpretation of the interaction between a liquid droplet and a solid surface or between two liquid droplets. In this study, we investigated the interaction across an aqueous thin film between fluorocarbon (perfluoropentane) droplets, hydrocarbon (tetradecane) droplets, and a droplet and a flat mica surface in the absence of stabilizers. It was found that even at a relatively elevated electrolyte concentration of 0.1 M NaNO3, depending on the solution pH, interactions between two identical droplets or a droplet and a mica surface could be repulsive. A simple theoretical analysis of the magnitude and range of these interactive forces suggests that the DLVO theory cannot explain the observed behavior. The measured force behavior is discussed in the context of ion adsorption, and the arising charging effects, at the bare oil-water interface.     

On the complexation of proteins and polyelectrolytes

Both natural and synthetic polyelectrolytes form strong complexes with a variety of proteins. One peculiar phenomenon is that association can take place even when the protein and the polyelectrolyte carry the same charge. This has been interpreted as if the ion-dipole interaction can overcome the repulsive ion-ion interaction. On the basis of Monte Carlo simulations and perturbation theory, we propose a different explanation for the association, namely, charge regulation. We have investigated three different protein-polymer complexes and found that the induced ionization of amino acid residues due to the polyelectrolyte leads to a surprisingly strong attractive interaction between the protein and the polymer. The extra attraction from this charge-induced charge interaction can be several kT and is for the three cases studied here, lysozyme, alpha-lactalbumin, and beta-lactoglobulin, of the same magnitude or stronger than the ion-dipole interaction. The magnitude of the induced charge is governed by a response function, the protein charge capacitance Z2-Z2. This fluctuation term can easily be calculated in a simulation or measured in a titration experiment.     

Force-momentum-based self-guided Langevin dynamics: a rapid sampling method that approaches the canonical ensemble

The self-guided Langevin dynamics (SGLD) is a method to accelerate conformational searching. This method is unique in the way that it selectively enhances and suppresses molecular motions based on their frequency to accelerate conformational searching without modifying energy surfaces or raising temperatures. It has been applied to studies of many long time scale events, such as protein folding. Recent progress in the understanding of the conformational distribution in SGLD simulations makes SGLD also an accurate method for quantitative studies. The SGLD partition function provides a way to convert the SGLD conformational distribution to the canonical ensemble distribution and to calculate ensemble average properties through reweighting. Based on the SGLD partition function, this work presents a force-momentum-based self-guided Langevin dynamics (SGLDfp) simulation method to directly sample the canonical ensemble. This method includes interaction forces in its guiding force to compensate the perturbation caused by the momentum-based guiding force so that it can approximately sample the canonical ensemble. Using several example systems, we demonstrate that SGLDfp simulations can approximately maintain the canonical ensemble distribution and significantly accelerate conformational searching. With optimal parameters, SGLDfp and SGLD simulations can cross energy barriers of more than 15 kT and 20 kT, respectively, at similar rates for LD simulations to cross energy barriers of 10 kT. The SGLDfp method is size extensive and works well for large systems. For studies where preserving accessible conformational space is critical, such as free energy calculations and protein folding studies, SGLDfp is an efficient approach to search and sample the conformational space.     

The inevitable accumulation of large ions and neutral molecules near hydrophobic surfaces and small ions near hydrophilic ones

The present contribution offers a unified explanation to three central phenomena in physical chemistry of interfaces in contact with aqueous solution: (1) Accumulation of large anions at the air/water interface. (2) Accumulation of neutral gas molecules near hydrophobic surfaces and the resulting hydrophobic interaction between two such surfaces, and (3) The Hofmeister effect, namely, the enhanced propensity of small ions to hydrophilic surfaces and large ions to hydrophobic surfaces. The common thread linking these phenomena is the free energy balance between ion or molecule hydration in solution and the cost of localizing these objects at the water-surface interface. Comparing the results of an abstract lattice-gas model to force spectroscopy data collected by AFM we reveal the underlying principles and demonstrate their universality.     

定标粒子理论计算非水溶液的盐效应常数

本文应用定标粒子理论计算了非电解质溶质在盐(NaI、或KI)和环丁砜组成的非水电解质溶液中溶解度的盐效应常数。硬球作用项采用Masterton-Lee的方法。软球作用项采用胡英等的径向分布函数处理方法, 并考虑进了偶极-偶极、偶极-诱导偶极、电荷-偶极和电荷-诱导偶极等相互作用。分子的硬球直径σ和能量参数∈/k由经验方程计算。由理论值和实验结果比较得出: 当σ_2取0.563 nm、离子半径取电子密度标度时, 理论值与实验值符合得较好。     

Optical observation of gas bridging between hydrophobic surfaces in water

The very strong and long-ranged interaction force between hydrophobic surfaces in water has been a debated issue for a long time. Recent studies suggest that the long-range attraction is attributable to the bridging of nanoscopic bubbles attached on the surfaces. However, it is still unclear at present whether such a bridging is able to exit stably or not. To clarify the existence of the gas bridge, we conducted the optical observation and the force measurement between the hydrophobic glass particle and plate in water simultaneously, using a combined apparatus of an atomic force microscope and an optical inverted microscope. It is found that (i) the image of the dark ring of ca. 1 mum in diameter appears in the region where the existence of the bridge is confirmed by force curves, but disappears when the separation between surfaces becomes shorter than the low limit of the wavelength of visible light, and (ii) the sudden disappearance of the image coincides well with the breakage of the bridge estimated from the separating force curve. The results obtained here are consistent with the above-mentioned mechanism for the long-range attraction between hydrophobic surfaces in water.     

Force Measurements between Semifluorinated Thiolate Self-Assembled Monolayers: Long-Range Hydrophobic Interactions and Surface Charge

Long-range interactions between self-assembled monolayers (SAMs) of semifluorinated alkanethiols have been studied by direct force measurements in water and aqueous NaCl solutions. SAMs prepared from three different thiols, with identical fluorinated head groups but varying hydrocarbon spacer lengths, were investigated: CF(3)(CF(2))(9)(CH(2))(x)SH, where x=2, 11, or 17. Force measurements show that the interactions in water and electrolyte solutions are composed of both double-layer interactions emerging from what appears to be charges adsorbed onto the surfaces and long-range "hydrophobic" attractions, in excess of the expected van der Waals forces. The three investigated thiols produce similar results in force measurements, though the contact angles with water are slightly different. The "hydrophobic" attraction has the form of step-like attractive discontinuities in the force profiles at separations ranging from 20 to 40 nm, caused by bridging of microscopic bubbles residing at the surfaces. The shape or range of these discontinuities are not significantly affected by replacement of the water with either 1 mM or 1 M NaCl solutions. The origin of the charges causing the electrostatic double-layer interaction is unclear, but some possible causes are discussed. Copyright 2001 Academic Press.     

Gemini表面活性剂与DNA在气/液界面上的相互作用

在气/液界面上, 阳离子表面活性剂可以通过静电作用与阴离子型的脱氧核糖核酸(DNA)分子形成复合膜, 并压缩沉积得到LB(Langmuir-Blodget)膜. 利用表面压-表面积(π-A)曲线、原子力显微镜(AFM)和石英晶体微天平(QCM)研究了阳离子Gemini表面活性剂([C₁₈H₃₇(CH₃)₂N⁺-(CH₂)_s-N⁺(CH₃)₂C₁₈H₃₇]·2Br^-, 简写为18-s-18, s=3, 4, 6, 8, 10, 12)与DNA(双链DNA(dsDNA), 单链DNA(ssDNA))之间的相互作用, 并对18-s-18在不同下相表面的分子面积进行了比较. 实验结果表明连接基团和下相的DNA对Gemini表面活性剂在气/液界面上的性质有很大影响. 此外, Gemini表面活性剂在界面上对DNA的吸附能力与它们之间的相互作用方式密切相关.     

On the anatomy of the adsorption heat versus loading as a function of temperature and adsorbate for a graphitic surface

In this paper we review and classify the various patterns of isosteric heat versus loading for adsorption of gases on graphitised thermal carbon black at temperatures ranging from below the 3D triple point to temperatures above it, but less than the 3D critical point. We have identified the features of heat curve and highlighted the microscopic origin of these features. The patterns vary with temperature and with the relative strength of the fluid-fluid interaction and solid-fluid interaction. For simple adsorptives (by simple we meant there is no strong association between fluid particles), the heat curve is typified by fluid-fluid attraction and layering phenomena. For adsorptives showing strong association such as water, ammonia and methanol, the heat curve essentially begins below the condensation heat and then approaches it as loading is increased. This is mainly due to the strong hydrogen bonding in these fluids. A third group includes adsorptives such as benzene, where the heat curve is constant in the sub-monolayer coverage region (but is higher than the condensation heat) and then drops off to the condensation heat when higher layers are formed. The constant heat in the sub-monolayer region is due to the balance between the energy factor (from fluid-fluid interaction) and entropy factor (due to re-orientation of adsorbed molecules). Our proposed classification is supported by detailed GCMC simulations of various gases that have been reported in the literature, and we supplement these with new results for the adsorption of xenon on graphite to investigate in more detail the change in heat pattern with temperature. Xenon is chosen because of its high fluid-fluid interaction, allowing us to study the 2D-phase transition in the first as well as higher layers.     

On the molecular mechanism of stabilization of proteins by cosolvents: role of Lifshitz electrodynamic forces

Several ionic and nonionic additives are known to affect structural stability of proteins in aqueous solutions. At a fundamental level, the mechanism of stabilization or destabilization of proteins by cosolvents must be related to three-body interactions between the protein, additive, and the water medium. In this study, the role of the Lifshitz-van der Waals electrodynamic interaction between various additives (sucrose, glycerol, urea, poly(ethylene glycol)-200, betaine, taurine, proline, and valine) and bovine serum albumin (BSA) in water medium was examined. The electrodynamic interaction energy was attractive for all of the additives studied here when both far ultraviolet and infrared relaxations of the additives were included in their dielectric susceptibility representations. However, when only the infrared contribution was included for structure stabilizers and both far ultraviolet and infrared contributions for the structure destabilizers, the resulting electrodynamic interaction energy (E/kT) followed the structure stabilizing and/or destabilizing behavior of the additives; that is, the interaction was attractive for urea and PEG200 (structure destabilizers), whereas it was repulsive for sucrose, glycerol, betaine, taurine, alanine, valine, and proline (structure stabilizers). The electrodynamic interaction energy E/kT at any given surface-to-surface separation distance between the additives and BSA was positively correlated (r(2) = 0.92) with the experimental thermal denaturation temperature (T(d)) of BSA in 1 M solutions of the additives. These analyses provided a mechanistic basis for the experimental observations of exclusion of the structure-stabilizing additives from the protein-water interface and binding of the structure-destabilizing additives to the protein surface. The role of water structure in the three-body electrodynamic interaction is discussed. It is hypothesized that in the case of additives that enhance water structure the hydration shells formed around the additives effectively dampen the contribution of ultraviolet frequencies to the dielectric susceptibility of the additives and thus impart repulsive electrodyanamic interaction between the additive and the protein, whereas the opposite occurs in the case of additives that breakdown the hydrogen-bonded structure of water.     

查尔酮修饰β环糊精与过渡金属离子间相互作用

利用稳态荧光光谱和时间分辨荧光技术研究了新型环表衍生物(DMAC-CD)与过渡金属离子之间的相互作用。结果表明,由于该环糊精衍生物在水溶液是"自包结"构象存在,使其所连的二甲氢基查尔酮的羰基及氮杂烷氧链与过渡金属离子发生强烈配位作用,从而观察到主体分子DMAC-CD与金属离子间的诱导电子转移猝灭极大地偏离了Stern--volmer直线关系,符合典型的静态猝灭机制。测定了该主体分子与过渡金属离子间形成配合物的 稳定常数和荧光猝灭速率常。结果显示该查尔酮修饰的环糊精对过渡金属离子,特别是Cu^2+的很高的敏感性和选择性。