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Ions at Aqueous Interfaces : From Water Surface to Hydrated Biomolecules Pavel Jungwirth

Ions at Aqueous Interfaces : From Water Surface to Hydrated Biomolecules Pavel Jungwirth Durham , January 11, 20 10. Limiting Laws for Ion Solvation : No Ion Specificity. Aqueous bulk: Debye-H ückel theory Air/water interface: Onsager-Samaras model. repulsion from the surface by

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Ions at Aqueous Interfaces : From Water Surface to Hydrated Biomolecules Pavel Jungwirth

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  1. Ions at AqueousInterfaces: FromWaterSurface to HydratedBiomolecules Pavel Jungwirth Durham, January 11, 2010

  2. Limiting Laws for Ion Solvation:No Ion Specificity Aqueous bulk: Debye-Hückel theory Air/water interface: Onsager-Samaras model repulsion from the surface by image forces Only the absolute value of the charge matters…

  3. Ionic Size Aqueous bulk: Born model of solvation ΔGsolv = -(1 - 1/ε) q2/4πε0R Air/water interface: (already in the Onsager- Samaras model) Free energy profile: point charge (dashed) vs. sodium (full line) Markin, V. S.; Volkov, A. G.: J. Phys. Chem. B 2002, 106, 11810.

  4. Can Ion Specificity Be Rationalized as a Purely Ion Size & |Charge| Effect? e.g., surface tension: No! 1st paper: A. Heydweiler Ann. Physik 1910, 33, 145. bases Weissenborn & Pugh JCIS 1996, 184, 550. acids

  5. Effect of Ion Polarity Aqueous bulk: Anions solvate slightly better than cations of the same size sodium- like and chloride- like ions with varying charge Asymmetry effect missing in continuum solvent models… R. M. Lynden-Bell: Pure Appl. Chem. 2001, 73, 1721. Air/water interface: Anions penetrate closer to the surface than cations Free energies: Na+ F- Cl- G. D. S. Still ions repelled from the surface (no polarization, etc.)… M. A. Wilson & A. Pohorille: J. Chem. Phys. 1991, 95, 6005.

  6. Aqueous sodium halides series Polarization Effects Molecular dynamics simulations with polarizable force fields in slabgeometry.   inter- mediate    Surface propensity increases when moving down the periodic table.

  7. “Chemical” Effects Inorganic molecular ions – H3O+, OH-, …, plus “straight-forward” hydrophobic effects for organicmolecular ions. weakly negative (water oxygen charge -0.8e) lousy hydrogen bond acceptor side hydrophobic region -0.4 e O H H H good hydrogen bond donor (water hydrogen charge +0.4 e) hydrophilic region +0.47e +0.47e +0.47e O -1.35e -good hydrogen bond acceptor, week donor, -disrupts the water hydrogen bonding network less than H3O+ H +0.35e vs. Petersen & Saykally JPCB 2005, 109, 7976.

  8. Hydration of ions at the solution/vapor interface • New view based on molecular dynamics simulations andsurface • selective spectroscopies: Ion specific behavior withlarge polarizable • (soft) ions (Cl-, Br-, I-, SCN-, …) and H3O+ exhibiting surface affinity. • Questions for discussion: • What forces cause and how strong is the • surface ion effect (precise quantification)? • What are the microscopic vs macroscopic • manifestations of this effect? • How transferable is the effect to other aqueous • interfaces (water/oil, water/non-polar solid, • water/polar or charged solid)? • 3. What is the surface behavior of the intrinsic water ions? • Consensus on H3O+ exhibiting surface propensity, but • OH- still controversial: MD and spectroscopy – no or • weak surface affinity  electrophoresis, titration – • dramatic (millionfold!) surface enhancement.

  9. From Water Surface to Hydrated Biomolecules: Ionic Behavior at Surfaces of Proteins, Membranes, DNA, …

  10. “Classical” effect of ions Salting-in & salting-out of proteins p. 113: Salting in – at low (milimolar) salt concentrations via screening electrostatic interactions between proteins. Salting out – at high (molar) salt concentrations via competion for water, where ions win over proteins. - at low concentrations cations screen anions and vice versa, - at high concentrations not enough “free” water. But…i) Proteins unlike salts are like-charge objects. Adopted from CRC Handbok (2006) ii) Even at (several) molar concetrations activity smaller than unity. iii) There are strong ion-specific effects (beyond Debye-Huckel model).

  11. Lyotropic (Hofmeister) series of ions Pharmacological Inst. Prague C4H4O62- > SO42- > HPO42- > C3H5O(CO2)33- > CH3CO2- > HCO3- > CrO42- > Cl- > NO3-> ClO3- “About regularities in the protein precipitating effects of salts and the relation of these effects with the physiological behaviour of salts.” (F. Hofmeister Arch. Exp. Pathol. Pharmakol. 1888, 24, 247) • ion specificity originally derived for salting-out/salting-in of proteins, • invoked in many other processes: • i) denaturation of proteins, • ii) enzymatic activities, • iii) crystallization of proteins, • iv)swelling of tissues, • v) ion exchange, • vi) bubble coalescence, • vii) salt solubilities, • viii)surface properties of electrolytes, etc.

  12. Traditional explanation od salting-out Kosmotropes vs Chaotropes e.g., F- or SO42-: organize water layers,„steel water“, efficient at salting-out e.g, I-: disorganize water, inefficient at salting-out …but modern spectroscopies, diffraction, and simulations show that ions do not impose longe-range ordering beyond the first solvent shell Search for alternative explanations! Tobias & Hemminger

  13. Cherche la…interface If not water “structure making” and “structure breaking”... Zhang & Cremer, Current Oppinion Chem. Biol., 2006, 10, 658. …then direct interactions of ions with surfaces of hydrated proteins may be decisive for specific ion effects. Exploring ion specificity at charged, polar, and non-polar regions of protein surface!

  14. Case study: Na+ vs K+ drawing by W. Fenn What? Cytosol poor in sodium & rich in potassium. How? Via ion pums and ion channels in the membrane. Costs about 30 % of all available energy. Why?Co-transport, nerve excitation & “usual” answer: enzymes in the cell function better at low Na+ and high K+ - hmm…but why…

  15. Ions at Aqueous Biomolecules Both bulk and interfacial behavior important for Hofmeister effects. Questions for discussion: 1.What forces drive ions to aqueous proteins? How well does reductionism work? 3. What causes ion specificity? What elementary interactions are behind the lyotropic (Hofmeister) series of ions? 2. Is there any connection between ionic behavior at the simplest air/water interface and more complex protein/water, membrane/water, DNA/water, … interfaces?

  16. See you in Holderness… …in summer!

  17. Acknowledgment Luboš Vrbka, Robert Vácha, Jan Heyda Dr. Barbara Jagoda-Cwiklik, Dr. Mikael Lund Dr. Jiří Vondrášek, Dr. Phil Mason (Cornell), Dr. Rainer Böckmann (Saarbrücken), Prof. Max Berkowitz (UNC), Prof. Doug Tobias (UCI) Discussions and experimental biochemical feedback: Prof. Werner Kunz (Uni Regensburg) Prof. Jan Konvalinka (IOCB Prague) Prof. Kim Collins (Uni Maryland) Czech Ministry of Education and Academy of Sciences National Science Foundation (US)

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