Sub-Femtoseconds proton dynamics in confined geometry and near proteins
Collaborative research among scientists from Italy - the Nast Centre for Nanoscience Nanotechnology and Innovative Instrumentation, UK - ISIS Spallation Neutron Source - and US, - University of Huston - have used Deep Inelastic Neutron Scattering (DINS) to shows how the proton momentum distribution in liquid water monitors the changing occurring …
…in hydrogen bond network due to disorder due to confinement (or impurities). DINS is the only experimental technique that provides direct access to the single particle atomic momentum distribution. Thus, DINS provides a powerful probe of the local environment and single particle properties, particularly in quantum systems such as helium and hydrogen. Liquid water is a topic which continues to attract intense interest and motivate a large number of experimental and theoretical investigations. Great attention has been devoted to the study of water confined in nanoporous systems of different geometries, in both solid or gel phases, or in proximity of macromolecules and surfaces, because of its biological and technological importance. The microscopic properties of water molecules interacting with the confining surfaces differ from those in the bulk phase. The competition between water-water and water-confining medium interactions leads to the appearance of new interesting physical properties. In addition changes in both structural and dynamical properties of water occur. These changes are introduced both by the confining geometry and by the enhanced surface interaction due to the large surface to volume ratio of porous materials and/or due to an interruption of the bulk correlation length. The behavior of water layers near surfaces, particularly hydrophilic or hydrophobic surfaces, is of great interest for biological processes. This study has shown evidence that the momentum distribution of the protons is strongly affected by the proximity to such biological surfaces.
Figure 1. – Water on the surface of lysozime at 180 K (blue) and at 290 K (red). The oscillation is evidence that the proton is moving coherently between two sites separated by about 0.3 Å in the room temperature measurement.
Figure 1 shows a recent measurement of the momentum distribution of water on the surface of lysozime, a largely spherical protein. No attempt has been made to subtract the signal from the proteins in the lysozime itself from those in the water, which constitute less than a monolayer of coverage of the protein. It is believed, however, that the protein hydrogen momentum distribution doesn’t change significantly with temperature (Physical Review Letters). It can be seen, that there is a great difference in the proton response above and below what is believed to be a glass transition for the water on the surface, and that the higher temperature response indicates coherent motion of the proton.
The development of coherence is also seen in water confined in the pores of xerogel, a hydophilic material. Recent Deep inelastic neutron scattering (DINS) measurements have been performed on water confined in nanoporous Xerogel powders, with average pore diameters of 24 Å and 82 Å, have been carried out for pore fillings ranging from 76% to nearly full coverage. DINS measurements provide direct information on the momentum distribution, n(p), of protons, probing the local structure of the molecular system. The results show that the proton momentum distribution is highly non Gaussian. A bimodal distribution appears in the 24 Å pore, indicating coherent motion of the proton over distances of approximately 0.3 Å. The proton mean kinetic energy of the confined water molecule is determined from the second moment of n(p).The values, higher than in bulk water, are interpreted in terms of the breakup of the hydrogen bonds network of water molecules occurring within layers close to pore surface. Figure 2 shows the comparison of the signal from the confined water, with the background xerogel signal subtracted. The blue is bulk water, the black water in 24 Å pores, the red 82 Å pores. Evidently, the coherence is developing in the surface layers. Again, the proposed instrument would make a systematic study with coverage and degree of hydrophilicity feasible.
Figure 2 - The momentum distribution of bulk water (blue), water in the in the 82 Å. A pores of xerogel, (red) and 24 Å pores in xerogel. (black)
The narrowing of the momentum distribution is shown in figure 3. The results suggest that the hydrogen bond network in water responds to disorder due to confinement (or impurities) by developing effective double well (or nearly so) potentials leading to coherent motion of the protons in the two wells. This is an entirely unexpected result. It simplications for biology and chemistry are potentially far reaching and unexplored.
Figure 3 - The best fit model wave function and potential (in mev) along the hydrogen bond direction corresponding to the momentum distribution of the water in the 24 Å pores. The narrowing of the momentum distribution is consistent with coherent delocalization in a flat bottom potential.
References
C. Andreani, D. Colognesi, J. Mayers, G. F. Reiter, and R. Senesi, Advances in Physics 54, 377 (2005).
R. Senesi, A. Pietropaolo, A. Bocedi, S. E. Pagnotta, and F. Bruni, Phys. Rev. Lett. 98, 138102 (2007)
Notes to editors
Contact details: For more information, please contact Dr. Roberto Senesi at tel: +39 06 72594549, E-mail