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"Similarities" between confined and supercooled water

2009-09-08T22:00:00Z

"Similarities" between confined and supercooled water Ricci, Maria Antonietta; Bruni, Fabio; Giuliani, Alessia 2009-09-09 Faraday discussions 141 347-358 Neutron diffraction and deep inelastic neutron scattering experimentsperformed on bulk stable and supercooled water compared with the sameexperiments performed on water confined in silica substrates.Similarities and differences between the two cases clearly show up, asfar as both microscopic structure and single proton dynamics areconcerned. In particular in both supercooled bulk water and water underconfinement we observe a closer average distance between firstneighbouring oxygen sites and shortening of the H-bonds. In contrastthe number of H-bonds per molecule and the number of interstitial watermolecules are severely reduced under confinement, and the second peakof the oxygen-oxygen radial distribution function is shifted to shorterdistances, compared to the bulk phase. Based on these results apossible scenario for understanding changes evidenced by deed inelasticneutron scattering when water is either confined or supercooled isproposed.

Solvation of KSCN in water

2009-01-06T23:00:00Z

Solvation of KSCN in water Botti, Alberto; Pagnotta, Sara E.; Bruni, Fabio; Ricci, Maria Antonietta 2009-01-07 Journal of Physical Chemistry B 113 10014-10021 The results of a neutron diffraction experiment performed on aqueous solutions of KSCN are analyzed and discussed in comparison with similar data for KCl and KOH solutions. The effect of the different ions on the structure of water and the differences and similarities among the ion solvation shells are discussed in detail. In particular it is shown that the effect of KSCN on the structure of water is visible as a shift and a broadening of the second peak of the oxygen-oxygen radial distribution function, which corresponds to a lower number of interstitial water molecules, compared to pure water. The hydration shell of the cation is similar to that found in the case of KCl solutions, and more interestingly the hydration shell of SCN- is asymmetric, with water molecules H-bonded to the N site and weakly correlated to the S site. These results provide a reasonable microscopic description of the mechanism for the high efficiency of thiocyanate in crystallizing basic proteins.

Multiscale Approach to the Structural Study of Water Confined in MCM41

2009-11-22T23:00:00Z

Multiscale Approach to the Structural Study of Water Confined in MCM41 Mancinelli, Rosaria; Imberti, Silvia; Soper, A. K.; Liu, K. H.; Mou, C.Y.; Bruni, Fabio; Ricci, Maria Antonietta 2009-11-23 Journal of Physical Chemistry B 113 16169-16177 We present a protocol for simultaneous structural characterization of aconfined fluid and the confining Substrate, along with the extractionof site-site pair correlation functions of the liquid of interest. Thisis based on neutron diffraction experiments, exploiting where feasiblethe isotopic substitution technique, analyzed through numerical coarsegraining calculations and atomistic simulations. All of the subtletiesof the experimental procedure, the needed ancillary measurements, andthe recipe for tailoring the numerical codes to the real experiment andsample are described in the case of water confined in MCM41-S-15. Inparticular the excluded volume effects and the relevance ofliquid-Substrate cross-correlation terms in the neutron cross sectionare quantitatively discussed. The results obtained for the microscopicstructure of water evidence a non-homogeneous distribution of moleculeswithin the pore, with the presence of water-substrate hydrogen bonds,and a strong distortion of the water-water radial distributionfunctions with respect to those of bulk water extending at least up tothree hydration layers.

Dielectric Relaxations in Confined Hydrated Myoglobin

2009-06-30T22:00:00Z

Dielectric Relaxations in Confined Hydrated Myoglobin Schiro, Giorgio; Cupane, Antonio; Vitrano, Eugenio; Bruni, Fabio 2009-07 Journal of physical chemistry B 113 9606–9613 In this work we report the results of a broadband dielectricspectroscopy study on the dynamics of a globular protein, myoglobin, inconfined geometry, i.e. encapsulated in a porous silica matrix, at lowhydration levels, where about only one or two water layers surround theproteins. In order to highlight the specific effect of confinement inthe silica host, we compared this system with hydrated myoglobinpowders at the same hydration levels. The comparison between the datarelative to the two different systems indicates that geometricalconfinement within the silica matrix plays a crucial role inprotein-water dielectric relaxations, the effect of sol-gelencapsulation being essentially a suppression of cooperativerelaxations that involve the coherence/ cooperativity of solventmotions and solvent-coupled protein dynamics. We also provide directevidence that protein relaxations inside the gel depend on thehydration level and are "slaved" to the solvent beta-relaxation.

Solvation of KSCN in water

2009-06-30T22:00:00Z

Solvation of KSCN in water Botti, Alberto; Pagnotta, Sara Emanuela; Bruni, Fabio; Ricci, Maria Antonietta 2009-07 Journal of physical chemistry B 113 10014-10021 The results of a neutron diffraction experiment performed on aqueous solutions of KSCN are analyzed and discussed in comparison with similar data for KCl and KOH solutions. The effect of the different ions on the structure of water and the differences and similarities among the ion solvation shells am discussed in detail. In particular it is shown that the effect of KSCN on the structure of water is visible as a shift and a broadening of the second peak of the oxygen-oxygen radial distribution function, which corresponds to a lower number of interstitial water molecules, compared to pure water. The hydration shell of the cation is similar to that found in the case of KCl solutions, and more interestingly the hydration shell of SCN- is asymmetric, with water molecules H-bonded to the N site and weakly correlated to the S site. These results provide a reasonable microscopic description of the mechanism for the high efficiency of thiocyanate in crystallizing basic proteins.

"Similarities" between confined and supercooled water

2008-12-31T23:00:00Z

"Similarities" between confined and supercooled water Ricci, Maria Antonietta; Bruni, Fabio; Giuliani, Alessia 2009 Faraday Discussions 141 347-358 Neutron diffraction and deep inelastic neutron scattering experiments performed on bulk stable and supercooled water are compared with the same experiments performed on water confined in silica substrates.Similarities and differences between the two cases clearly show up, as far as both microscopic structure and single proton dynamics are concerned. In particular in both supercooled bulk water and water under confinement we observe a closer average distance between first neighboring oxygen sites and shortening of the H-bonds. In contrast the number of H-bonds per molecule and the number of interstitial water molecules are severely reduced under confinement, and the second peak of the oxygen-oxygen radial distribution function is shifted to shorter distances, compared to the bulk phase. Based on these results a possible scenario for understanding changes evidenced by deep inelastic neutron scattering when water is either confined or supercooled is proposed.

A new water anomaly: the temperature dependence of the proton meankinetic energy

2009-06-15T22:00:00Z

A new water anomaly: the temperature dependence of the proton meankinetic energy Flammini, Davide; Ricci, Maria Antonietta; Bruni, Fabio 2009-06-16 Journal of chemical physics 130 236101 In a recent article [Pietropaolo et al., Phys. Rev. Lett.100, 127802 (2008)] we discussed the results of a deep inelastic neutron scattering experiment aimed at the measure of mean kinetic energy, , and momentum distribution of protons in supercooled water. The observed excess of proton mean kinetic energy, with respect to theoretical predictions and measurements in water stable liquid and solid phases, suggested a possible link between the anomalous temperature dependence of water density and the temperature dependence of . Nevertheless, the limited number of data did not allow a more quantitative description. Here we report on new measurements, above and below the temperature of maximum density and in the supercooled phase. These show two maxima: The first one, in the supercooled phase in the range of 269-272 K, and a second one at 277 K, the temperature of the maximum density of water.

Influence of concentration and anion size on hydration of H+ Ions and water structure

2008-09-22T22:00:00Z

Influence of concentration and anion size on hydration of H+ Ions and water structure Mancinelli, Rosaria; Sodo, Armida; Bruni, Fabio; Ricci, Maria Antonietta; Soper, Aka Alan K 2008-09-23 Journal of physical chemistry B 113 4075-4081 Neutron diffraction experiments with hydrogen isotope substitution onaqueous solutions of HCl and HBr have been performed at concentrationsranging from 1:17 to 1:83 Solute per water molecules, at ambientconditions. Data are analyzed using the empirical potential structurerefinement technique in order to extract information on both the ionhydration shells and the microscopic structure of the solvent. It isfound that the influence of these solutes on the water structure isless concentration dependent than that of salts or hydroxides. Moreoverprotons readily form a strong H-bond with a water molecule uponsolvation, at all proportions. The majority of them is also bonded viaa longer bond to another water molecule, giving a prepeak in theg(OwOw). At high solute concentration, the second water molecule may beSubstituted by the counterion. In particular at solute concentrationsof the order of 1:17 or higher, all protons have an anion within adistance of 4.5 angstrom.

Quantum Behavior of Water Protons in Protein Hydration Shell

2009-02-28T23:00:00Z

Quantum Behavior of Water Protons in Protein Hydration Shell Pagnotta, S. E.; Bruni, Fabio; Senesi, R.; Pietropaolo, A. 2009-03 Biophysical Journal 96 1939-1943 Quantum effects on the water proton dynamics over the surface of ahydrated protein are measured by means of broadband dielectricspectroscopy and deep inelastic neutron scattering. Dielectricspectroscopy indicates a reduced energy barrier for a hydrogenatedprotein sample compared to a deuterated one, along with a large andtemperature-dependent isotopic ratio, in good agreement withtheoretical studies. Recent deep inelastic neutron scattering data havebeen reanalyzed, and now show that the momentum distribution of waterprotons reflects a characteristic delocalization at ambienttemperatures. These experimental findings might have far-reachingimplications for enzymatic catalysis involving proton transferprocesses, as in the case of the lysozyme protein studied in thisreport.

Comment on "Excess of Proton Mean Kinetic Energy in Supercooled Water'' Reply

2009-07-31T22:00:00Z

Comment on "Excess of Proton Mean Kinetic Energy in Supercooled Water'' Reply Pietropaolo, A.; Senesi, R.; Andreani, C.; Botti, Alberto; Ricci, Maria Antonietta; Bruni, Fabio 2009-08 Physical Review Letters 103 069802 Comment on "Excess of Proton Mean Kinetic Energy in Supercooled Water'' Reply. A Reply to the Comment by A. K. Soper.