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Volpe Bossa G, Hobbie E, May S. Counterion Release from Macroion Assemblies of Planar Geometry. J Phys Chem B 2024; 128:6966-6974. [PMID: 38958595 DOI: 10.1021/acs.jpcb.4c03222] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 07/04/2024]
Abstract
Macroions such as nanoparticles, polyelectrolytes, ionic gels, and amphiphiles can form condensed, often self-assembled, phases that are embedded in a solvent region. The condensed phase contains not only the partially or fully immobile charges of their macroions but also corresponding counterions that are mobile and thus free to migrate out of their confinement into the solvent region where they benefit from high translational entropy. Based on the nonlinear Poisson-Boltzmann model for monovalent ions, we quantify the corresponding fraction of released counterions for a planar slab geometry of the macroion phase. Slab thickness, extension of the solvent phase, fixed background charge density provided by the macroions, and dielectric constants inside slab and solvent combine into three dimensionless parameters that the fraction of released counterions depends on. We calculate that fraction and analyze the limits of a thin macroion phase, a large solvent phase, and linearized theory, where simple analytic results become available. Of particular interest is the presence of a single-planar interface that separates a bulk macroion phase from an extended solvent region. We calculate the apparent surface charge density that emerges due to the released counterions. Our model yields a comprehensive description of counterion partitioning between a planar macroion phase and a solvent region on the level of mean-field electrostatics in the absence of added salt ions.
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Affiliation(s)
- Guilherme Volpe Bossa
- Institute of Mathematical and Physical Sciences, Universidad Austral de Chile, Valdivia 5090000, Chile
| | - Erik Hobbie
- Department of Physics, North Dakota State University, Fargo, North Dakota 58108-6050, United States
| | - Sylvio May
- Department of Physics, North Dakota State University, Fargo, North Dakota 58108-6050, United States
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Mandalaparthy V, Tripathy M, van der Vegt NFA. Anions and Cations Affect Amino Acid Dissociation Equilibria via Distinct Mechanisms. J Phys Chem Lett 2023; 14:9250-9256. [PMID: 37812174 DOI: 10.1021/acs.jpclett.3c02062] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/10/2023]
Abstract
Salts reduce the pKa of weak acids by a mechanism sensitive to ion identity and concentration via charge screening of the deprotonated state. In this study, we utilize constant pH molecular dynamics simulations to understand the molecular mechanism behind the salt-dependent dissociation of aspartic acid (Asp). We calculate the pKa of Asp in the presence of a monovalent salt and investigate Hofmeister ion effects by systematically varying the ionic radii. We observe that increasing the anion size leads to a monotonic decrease in Asp pKa. Conversely, the cation size affects the pKa nonmonotonically, interpretable in the context of the law of matching water affinity. The net effect of salt on Asp acidity is governed by an interplay of solvation and competing ion interactions. The proposed mechanism is rather general and can be applicable to several problems in Hofmeister ion chemistry, such as pH effects on protein stability and soft matter interfaces.
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Affiliation(s)
- Varun Mandalaparthy
- Department of Chemistry, Technical University of Darmstadt, 64287 Darmstadt, Germany
| | - Madhusmita Tripathy
- Department of Chemistry, Technical University of Darmstadt, 64287 Darmstadt, Germany
| | - Nico F A van der Vegt
- Department of Chemistry, Technical University of Darmstadt, 64287 Darmstadt, Germany
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Matsarskaia O, Roosen‐Runge F, Schreiber F. Multivalent ions and biomolecules: Attempting a comprehensive perspective. Chemphyschem 2020; 21:1742-1767. [PMID: 32406605 PMCID: PMC7496725 DOI: 10.1002/cphc.202000162] [Citation(s) in RCA: 36] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/02/2020] [Revised: 05/13/2020] [Indexed: 12/13/2022]
Abstract
Ions are ubiquitous in nature. They play a key role for many biological processes on the molecular scale, from molecular interactions, to mechanical properties, to folding, to self-organisation and assembly, to reaction equilibria, to signalling, to energy and material transport, to recognition etc. Going beyond monovalent ions to multivalent ions, the effects of the ions are frequently not only stronger (due to the obviously higher charge), but qualitatively different. A typical example is the process of binding of multivalent ions, such as Ca2+ , to a macromolecule and the consequences of this ion binding such as compaction, collapse, potential charge inversion and precipitation of the macromolecule. Here we review these effects and phenomena induced by multivalent ions for biological (macro)molecules, from the "atomistic/molecular" local picture of (potentially specific) interactions to the more global picture of phase behaviour including, e. g., crystallisation, phase separation, oligomerisation etc. Rather than attempting an encyclopedic list of systems, we rather aim for an embracing discussion using typical case studies. We try to cover predominantly three main classes: proteins, nucleic acids, and amphiphilic molecules including interface effects. We do not cover in detail, but make some comparisons to, ion channels, colloidal systems, and synthetic polymers. While there are obvious differences in the behaviour of, and the relevance of multivalent ions for, the three main classes of systems, we also point out analogies. Our attempt of a comprehensive discussion is guided by the idea that there are not only important differences and specific phenomena with regard to the effects of multivalent ions on the main systems, but also important similarities. We hope to bridge physico-chemical mechanisms, concepts of soft matter, and biological observations and connect the different communities further.
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Affiliation(s)
| | - Felix Roosen‐Runge
- Department of Biomedical Sciences and Biofilms-Research Center for Biointerfaces (BRCB), Faculty of Health and SocietyMalmö UniversitySweden
- Division of Physical ChemistryLund UniversitySweden
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Muralidharan A, Pratt L, Chaudhari M, Rempe S. Quasi-chemical theory for anion hydration and specific ion effects: Cl-(aq) vs. F-(aq). ACTA ACUST UNITED AC 2019. [DOI: 10.1016/j.cpletx.2019.100037] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
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Kiyohara K, Kawai Y. Hydration of monovalent and divalent cations near a cathode surface. J Chem Phys 2019; 151:104704. [PMID: 31521095 DOI: 10.1063/1.5113738] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Hydration of monovalent (Li+, Na+, K+, and Cs+) and divalent (Mg2+, Ca2+, Sr2+, and Ba2+) cations on a cathode surface was studied by a classical molecular dynamics simulation. The potential of mean force (PMF) for each cation species was calculated as a function of the distance from the cathode surface, and the potential barriers for dehydrating the first and second hydration shells near the cathode surface were estimated. The positions of the minimum of the PMF closest to the cathode surface were found to be in the order Li+ < Na+ < Mg2+ < Ca2+ < Sr2+ < Ba2+ < K+ < Cs+. It was found that Li+, Mg2+, Ca2+, Sr2+, and Ba2+ ions are most likely doubly hydrated when they are adsorbed on the cathode surface without an applied voltage, whereas Na+, K+, and Cs+ ions are most likely singly hydrated at room temperature. On the other hand, when a voltage of 1 V was applied to the electrodes, all the cation species that we studied appeared most likely to be singly hydrated on the cathode surface. The depths of the potential well closest to the cathode surface under an applied voltage of 1 V were found to be in the order Ba2+ < Sr2+ < Ca2+ < Mg2+ for the divalent cations and Li+ < Na+ < K+ < Cs+ for the monovalent cations in the set of models that we used. These orders coincide with the Hofmeister series from the kosmotropic to the chaotropic.
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Affiliation(s)
- Kenji Kiyohara
- Inorganic Functional Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan
| | - Yusuke Kawai
- Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan
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Gregory KP, Webber GB, Wanless EJ, Page AJ. Lewis Strength Determines Specific-Ion Effects in Aqueous and Nonaqueous Solvents. J Phys Chem A 2019; 123:6420-6429. [DOI: 10.1021/acs.jpca.9b04004] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Kasimir P. Gregory
- School of Environmental and Life Sciences, The University of Newcastle, Callaghan, New South Wales 2308, Australia
| | - Grant B. Webber
- School of Engineering, The University of Newcastle, Callaghan, New South Wales 2308, Australia
- Priority Research Centre for Advanced Particle Processing and Transport, The University of Newcastle, Callaghan, New South Wales 2308, Australia
| | - Erica J. Wanless
- School of Environmental and Life Sciences, The University of Newcastle, Callaghan, New South Wales 2308, Australia
- Priority Research Centre for Advanced Particle Processing and Transport, The University of Newcastle, Callaghan, New South Wales 2308, Australia
| | - Alister J. Page
- School of Environmental and Life Sciences, The University of Newcastle, Callaghan, New South Wales 2308, Australia
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Abstract
Salts differ in their ability to stabilize protein conformations, thereby affecting the thermodynamics and kinetics of protein folding. We developed a coarse-grained protein model that can predict salt-induced changes in protein properties by using the transfer free-energy data of various chemical groups from water to salt solutions. Using this model and molecular dynamics simulations, we probed the effect of seven different salts on the folding thermodynamics of the DNA binding domain of lac repressor protein ( lac-DBD) and N-terminal domain of ribosomal protein (NTL9). We show that a salt can act as a protein stabilizing or destabilizing agent depending on the protein sequence and folded state topology. The computed thermodynamic properties, especially the m values for various salts, which reveal the relative ability of a salt to stabilize the protein folded state, are in quantitative agreement with the experimentally measured values. The computations show that the degree of protein compaction in the denatured ensemble strongly depends on the salt identity, and for the same variation in salt concentration, the compaction in the protein dimensions varies from ∼4% to ∼30% depending on the salt. The transition-state ensemble (TSE) of lac-DBD is homogeneous and polarized, while the TSE of NTL9 is heterogeneous and diffusive. Salts induce subtle structural changes in the TSE that are in agreement with Hammond's postulate. The barrier to protein folding tends to disappear in the presence of moderate concentrations (∼3-4 m) of strongly stabilizing salts.
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Affiliation(s)
- Hiranmay Maity
- Solid State and Structural Chemistry Unit , Indian Institute of Science , Bengaluru , Karnataka , India 560012
| | - Aswathy N Muttathukattil
- Solid State and Structural Chemistry Unit , Indian Institute of Science , Bengaluru , Karnataka , India 560012
| | - Govardhan Reddy
- Solid State and Structural Chemistry Unit , Indian Institute of Science , Bengaluru , Karnataka , India 560012
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Kiyohara K, Minami R. Hydration and dehydration of monovalent cations near an electrode surface. J Chem Phys 2018; 149:014705. [PMID: 29981539 DOI: 10.1063/1.5037679] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
The mechanism of hydration and dehydration of monovalent ions, Li+, Na+, K+, and Cs+, in a dilute solution near an electrode surface was studied by molecular dynamics simulations. The potentials of mean force for these ions were calculated as a function of the distance from the electrode surface and the potential barriers for dehydrating the first and the second hydration shell near the electrode surface and were estimated for each ion species. It was found that the mechanism of hydration for Li+ is distinct from those for Na+, K+, and Cs+. Penetration of ions into the first layer of water molecules on the electrode surface is unlikely to occur for the case of Li+, while that would occur with certain probabilities for the case of Na+, K+, or Cs+, whether or not voltage is applied to the electrode. Li+ ions would be adsorbed on the electrode surface in a doubly hydrated form with a significant probability, while Na+, K+, and Cs+ ions would be adsorbed most likely in a singly hydrated form. Furthermore, the theory of ionic radii, which has been successfully used in the analysis of bulk solutions, was applied to the electrode/electrolyte interface. It was found that the theory of ionic radii is also useful in explaining the structural behaviors of ions near an electrode surface. The distance between an ion and the layers of water molecules on the electrode surface showed almost linear dependence on the radius of the ion, as predicted by the theory of ionic radii. Analysis of the deviation from the linearity showed that Li+ ions are most likely adsorbed in the first layer of water molecules on the electrode surface, while Na+, K+, and Cs+ ions are adsorbed on the second layer of water molecules. These analyses indicate that Li+ is a structure maker, while Na+, K+, and Cs+ are structure breakers, which is consistent with the widely accepted idea in explaining the behaviors of the bulk solutions.
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Affiliation(s)
- Kenji Kiyohara
- Inorganic Functional Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan
| | - Riho Minami
- Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan
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Salanne M, Tazi S, Vuilleumier R, Rotenberg B. Ca 2+ -Cl - Association in Water Revisited: the Role of Cation Hydration. Chemphyschem 2017; 18:2807-2811. [PMID: 28510283 DOI: 10.1002/cphc.201700286] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/18/2017] [Revised: 05/16/2017] [Indexed: 11/08/2022]
Abstract
We investigate the dissociation of a Ca2+ -Cl- pair in water using classical molecular dynamics simulations with a polarizable interaction potential, parameterized from ab initio calculations. By computing the potential of mean force as a function not only of the interionic distance but also of the coordination numbers by water molecules, we show that it is necessary to use a collective variable describing the cation hydration in order to capture the dissociation mechanism. In the contact ion pair, the Ca2+ cation has a first coordination sphere containing 5 or 6 water molecules. The minimum free-energy path for dissociation involves a two-step process: First one or two additional water molecules enter the cation coordination shell, increasing the coordination number up to 7 with an almost fixed interionic distance. Then the dissociation of the ionic pair occurs at this fixed coordination number.
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Affiliation(s)
- Mathieu Salanne
- Sorbonne Universités, UPMC Univ Paris 06, CNRS, UMR 8234 PHENIX, 4 Place Jussieu, 75005, Paris, France
| | - Sami Tazi
- Sorbonne Universités, UPMC Univ Paris 06, CNRS, UMR 8234 PHENIX, 4 Place Jussieu, 75005, Paris, France
| | - Rodolphe Vuilleumier
- Sorbonne Universités, UPMC Univ Paris 06, ENS, CNRS, PASTEUR, 75005, Paris, France
| | - Benjamin Rotenberg
- Sorbonne Universités, UPMC Univ Paris 06, CNRS, UMR 8234 PHENIX, 4 Place Jussieu, 75005, Paris, France
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Shi Y, Beck T. Deconstructing Free Energies in the Law of Matching Water Affinities. J Phys Chem B 2017; 121:2189-2201. [DOI: 10.1021/acs.jpcb.7b00104] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Yu Shi
- Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221, United States
| | - Thomas Beck
- Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221, United States
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Okur HI, Hladílková J, Rembert KB, Cho Y, Heyda J, Dzubiella J, Cremer PS, Jungwirth P. Beyond the Hofmeister Series: Ion-Specific Effects on Proteins and Their Biological Functions. J Phys Chem B 2017; 121:1997-2014. [PMID: 28094985 DOI: 10.1021/acs.jpcb.6b10797] [Citation(s) in RCA: 411] [Impact Index Per Article: 58.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
Ions differ in their ability to salt out proteins from solution as expressed in the lyotropic or Hofmeister series of cations and anions. Since its first formulation in 1888, this series has been invoked in a plethora of effects, going beyond the original salting out/salting in idea to include enzyme activities and the crystallization of proteins, as well as to processes not involving proteins like ion exchange, the surface tension of electrolytes, or bubble coalescence. Although it has been clear that the Hofmeister series is intimately connected to ion hydration in homogeneous and heterogeneous environments and to ion pairing, its molecular origin has not been fully understood. This situation could have been summarized as follows: Many chemists used the Hofmeister series as a mantra to put a label on ion-specific behavior in various environments, rather than to reach a molecular level understanding and, consequently, an ability to predict a particular effect of a given salt ion on proteins in solutions. In this Feature Article we show that the cationic and anionic Hofmeister series can now be rationalized primarily in terms of specific interactions of salt ions with the backbone and charged side chain groups at the protein surface in solution. At the same time, we demonstrate the limitations of separating Hofmeister effects into independent cationic and anionic contributions due to the electroneutrality condition, as well as specific ion pairing, leading to interactions of ions of opposite polarity. Finally, we outline the route beyond Hofmeister chemistry in the direction of understanding specific roles of ions in various biological functionalities, where generic Hofmeister-type interactions can be complemented or even overruled by particular steric arrangements in various ion binding sites.
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Affiliation(s)
- Halil I Okur
- Laboratory for Fundamental BioPhotonics (LBP), Institute of Bioengineering (IBI), School of Engineering (STI), École Polytechnique Fédérale de Lausanne (EPFL) , CH-1015 Lausanne, Switzerland
| | - Jana Hladílková
- Institute of Organic Chemistry and Biochemistry, The Czech Academy of Sciences , Flemingovo nam. 2, 16610 Prague 6, Czech Republic.,Division of Theoretical Chemistry, Lund University , P.O.B. 124, SE-22100 Lund, Sweden
| | | | - Younhee Cho
- Department of Chemistry, Texas A&M University , College Station 77843, Texas, United States
| | - Jan Heyda
- Institut für Weiche Materie und Funktionale Materialien, Helmholtz-Zentrum Berlin für Materialien und Energie , Hahn-Meitner Platz 1, 14109 Berlin, Germany.,Department of Physical Chemistry, University of Chemistry and Technology, Prague , Technická 5, 16628 Prague 6, Czech Republic
| | - Joachim Dzubiella
- Institut für Weiche Materie und Funktionale Materialien, Helmholtz-Zentrum Berlin für Materialien und Energie , Hahn-Meitner Platz 1, 14109 Berlin, Germany.,Institut für Physik, Humboldt-Universität zu Berlin , Newtonstrasse 15, 12489 Berlin, Germany
| | | | - Pavel Jungwirth
- Institute of Organic Chemistry and Biochemistry, The Czech Academy of Sciences , Flemingovo nam. 2, 16610 Prague 6, Czech Republic
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Násztor Z, Bogár F, Dér A. The interfacial tension concept, as revealed by fluctuations. Curr Opin Colloid Interface Sci 2016. [DOI: 10.1016/j.cocis.2016.05.007] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
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13
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Machado MR, Pantano S. Exploring LacI-DNA dynamics by multiscale simulations using the SIRAH force field. J Chem Theory Comput 2015; 11:5012-23. [PMID: 26574286 DOI: 10.1021/acs.jctc.5b00575] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The lac repressor protein (LacI) together with its target regulatory sequence are a common model for studying DNA looping and its implications on transcriptional control in bacteria. Owing to the molecular size of this system, standard all-atom (AA) simulations are prohibitive for achieving relevant biological time scales. As an alternative, multiscale models, which combine AA descriptions at particular regions with coarse-grained (CG) representations of the remaining components, were used to address this computational challenge while preserving the relevant details of the system. In this work, we implement a new multiscale approach based on the SIRAH force field to gain deeper insights into the dynamics of the LacI-DNA system. Our methodology allows for a dual resolution treatment of the solute and solvent, explicitly representing the protein, DNA, and solvent environment without compromising the AA region. Starting from the P1 loop configuration in an undertwisted conformation, we were able to observe the transition to the more stable overtwisted state. Additionally, a detailed characterization of the conformational space sampled by the DNA loop was done. In agreement with experimental and theoretical evidence, we observed the transient formation of kinks at the loop, which were stabilized by the presence of counterions at the minor groove. We also show that the loop's intrinsic flexibility can account for reported FRET measurements and bent conformations required to bind the CAP transcription factor.
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Affiliation(s)
- Matias R Machado
- Biomolecular Simulations Group, Institut Pasteur de Montevideo , Montevideo, Uruguay , 11400
| | - Sergio Pantano
- Biomolecular Simulations Group, Institut Pasteur de Montevideo , Montevideo, Uruguay , 11400
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15
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Enami S, Colussi AJ. Long-range specific ion-ion interactions in hydrogen-bonded liquid films. J Chem Phys 2013; 138:184706. [DOI: 10.1063/1.4803652] [Citation(s) in RCA: 37] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Abstract
Ionic solutions are dominated by interactions because they must be electrically neutral, but classical theory assumes no interactions. Biological solutions are rather like seawater, concentrated enough so that the diameter of ions also produces important interactions. In my view, the theory of complex fluids is needed to deal with the interacting reality of biological solutions.
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Affiliation(s)
- Bob Eisenberg
- Department of Molecular Biophysics and Physiology, Rush University, Chicago, Illinois
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Imaging the membrane lytic activity of bioactive peptide latarcin 2a. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2012; 1818:3072-80. [DOI: 10.1016/j.bbamem.2012.07.030] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/01/2012] [Revised: 07/25/2012] [Accepted: 07/30/2012] [Indexed: 11/22/2022]
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Bresme F, Chacón E, Tarazona P, Wynveen A. The structure of ionic aqueous solutions at interfaces: An intrinsic structure analysis. J Chem Phys 2012; 137:114706. [DOI: 10.1063/1.4753986] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Hopkins P, Schmidt M. Radial distribution functions of non-additive hard sphere mixtures via Percus' test particle route. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2011; 23:325104. [PMID: 21775799 DOI: 10.1088/0953-8984/23/32/325104] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
Using fundamental density functional theory we calculate the partial radial distribution functions, g(ij)(r), of a binary non-additive hard sphere mixture using either Percus' test particle approach or inversion of the analytic structure factor obtained via the Ornstein-Zernike route. We find good agreement between the theoretical results and Monte Carlo simulation data for both positive and moderate negative non-additivities. We investigate the asymptotic, [Formula: see text], decay of the g(ij)(r) and show that this agrees with the analytic analysis of the contributions to the partial structure factors in the plane of complex wavevectors. We find the test particle density profiles to be free of unphysical artefacts, contrary to earlier reports.
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Affiliation(s)
- Paul Hopkins
- H H Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK
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Hopkins P, Schmidt M. First-order layering and critical wetting transitions in nonadditive hard-sphere mixtures. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2011; 83:050602. [PMID: 21728474 DOI: 10.1103/physreve.83.050602] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/17/2011] [Indexed: 05/31/2023]
Abstract
Using fundamental-measure density functional theory we investigate entropic wetting in an asymmetric binary mixture of hard spheres with positive nonadditivity. We consider a general planar hard wall, where preferential adsorption is induced by a difference in closest approach of the different species and the wall. Close to bulk fluid-fluid coexistence, the phase rich in the minority component adsorbs either through a series of first-order layering transitions, where an increasing number of liquid layers adsorbs sequentially, or via a critical wetting transition, where a thick film grows continuously.
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Affiliation(s)
- Paul Hopkins
- H H Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom
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21
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Déjugnat C, Dufrêche JF, Zemb T. Ion-specific weak adsorption of salts and water/octanol transfer free energy of a model amphiphilic hexapeptide. Phys Chem Chem Phys 2011; 13:6914-24. [PMID: 21412527 DOI: 10.1039/c0cp01750g] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
An amphiphilic hexapeptide has been used as a model to quantify how specific ion effects induced by addition of four salts tune the hydrophilic/hydrophobic balance and induce temperature-dependant coacervate formation from aqueous solution. The hexapeptide chosen is present as a dimer with low transfer energy from water to octanol. Taking sodium chloride as the reference state in the Hofmeister scale, we identify water activity effects and therefore measure the free energy of transfer from water to octanol and separately the free energy associated to the adsorption of chaotropic ions or the desorption of kosmotropic ions for the same amphiphilic peptide. These effects have the same order of magnitude: therefore, both energies of solvation as well as transfer into octanol strongly depend on the nature of the electrolytes used to formulate any buffer. Model peptides could be used on separation processes based on criteria linked to "Hofmeister" but different from volume and valency.
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Affiliation(s)
- Christophe Déjugnat
- Institut de Chimie Séparative de Marcoule, UMR 5257 CEA/CNRS/UMII/ENSCM, CEA Centre de Marcoule, Bât. 426, BP 17171, 30207 Bagnols-sur-Cèze cedex, France
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