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Water Dynamics: Fluctuation, Phase Transition and Reactions
Water is the most ubiquitous substance on earth and known to have anomalous properties, which arise from the nature of hydrogen bonds. Professor Ohmine has made a distinguished contribution to the understanding of physical origin of water anormalities, especially of its dynamics in molecular scale. Water molecules are mutually strongly bounded with their hydrogen bonds (HB) and form disordered three-dimensional HB network structures. Liquid water is thus expected to exhibit collective molecular motions associated with hydrogen-bond-network rearrangement, and its fluidity is very characteristic in the molecular scale. In the following, some of his works on water are briefly described.
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Prof. Iwao Ohmine
Nagoya University
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1 Hydrogen-bond-network rearrangement (HBNR) dynamics in water
Ohminefs group succeeded in extracting collective motions from omplex dynamics of liquid water. Water dynamics is found to yield intermittent collective motions, and its spectrum exhibits so-called 1/f-fluctuation. This means that there exist multi-scale spatial and temporal relaxations in water hydrogen-bond-network rearrangement dynamics. Landscape of liquid water potential energy surface was intensively analyzed with developing various theoretical methods. Theoretical methods newly developed here are now used to deal with dynamics of various complex molecular systems, such as glass dynamics and biomolecular reactions.
2. Experimental Observation of HBNR dynamics
Ohminefs group investigated how intermittent collective motions associated with hydrogen-bond-network rearrangement are observed by experimental methods, such as Far IR, Raman, X-ray and Neutron scattering. It was found that different methods detect different aspects of molecular motions; for example, Raman spectrum yields 1/f-fluctuations in its low frequency profile, while far IR spectrum yields a simple Debye-relaxation. Ohminefs group have then developed a new theory of multi-dimensional spectroscopy, which can deal with phase-space dynamics of a system and extract the important motions causing large rearrangement in complex dynamics. This highly nonlinear laser photolysis, called off-resonant fifth-order two-dimensional (2D-) Raman spectroscopy, has the two-dimensional time axes and detects nonlinear couplings among intermolecular motions. 2D-Raman spectroscopy is found to be susceptible to the nonlinear anharmonic dynamics and intermediate-range structural order, and thus very sensitive to anisotropy in ice HB structure and to the difference of hydrogen-bond-network structure in various water amorphous phases. It is also a useful tool to observe molecular mechanisms of nucleation processes in water freezing and ice melting processes
3 Mechanisms of water freezing
Upon cooling, water freezes into ice. This process is a most familiar phase-transition, occurring in many places in nature, but had never been successfully simulated by a computer simulation. Water molecules possess strong directionality of hydrogen bonds and form a disordered three-dimensional HB network (HBN) and are hard to freeze into ice. Ohminefs group have made the first successful simulation for pure water freezing process, which gives a molecular level picture, particularly of how an initial nucleus is created and grows. It revealed that intermittent collective fluctuation associated with HBN rearrangement plays an important role in the initial nucleus formation. New graphical analysis, defining basic three-dimensional units of the network (called fragments), is introduced in order to characterize HBN structure in water freezing process. It was found that the initial nucleus in water freezing process consists not of crystalline-structure fragments with 6 member-rings, but of fragments with mixture of 5-, 6-, and 7-member-rings. These non-crystalline fragments, being stable by making good structure matching with surrounding supercooled water, persist until almost the end of the nucleation process when their transition to a crystalline structure takes place. Fragment analysis developed here is also a very useful theoretical tool to elucidate properties of other network forming molecular systems.
4 ransfer of excess proton in water and ice
Excess proton transfer in water, related with Redox reaction, is a most basic reaction in chemistry and an energy source of many biological systems. This transfer is known to be very fast, and to understand molecular mechanism of this fast proton transfer had been one of the most challenging problems in chemistry. After many investigations including Ohminefs groupfs, it is now well-established that proton transfer in liquid water is promoted by the structure fluctuation creating three-coordinated water molecules in hydrogen-bond-network rearrangement. This kind of large structural rearrangement, however, does not take place in ice. Nevertheless, proton transfer in ice is very fast. Strong constraint on the molecular geometry in ice itself is found to be the source of fast proton transfer. As this geometrical constraint reduces stabilization of the excess-proton, the excess-proton is not trapped in a deep energy minimum and makes facile transfer on small energy barrier surface. Long-range hydration from distant water molecular shells makes a significant contribution to constructing this near flat potential surface.
5. pH of water at various temperatures
The pH, with its well-known value of 7 at ambient condition, is a most basic property of water, with wide implications in chemistry and biology. The pH of water exhibits complex, non-monotonic temperature dependence whose mechanism has been not understood even qualitatively. When temperature rises from normal to supercritical temperature, pH of water decreases from 7 at first and then increases rapidly in the experiment. Accurate theoretical evaluation of pH was a very difficult task because the hydration of these ions, especially for OH-, was very difficult to reproduce. Ohminefs group challenged this problem and succeeded in obtaining the theoretical pH curve, which almost completely reproduces the curve obtained in the experiment. It was found that the imbalance between ion-water and water-water molecular interaction strengths serves to put a subtle balance to produce this temperature dependence of pH value. They have also explained the mechanism of high reactivity of super critical water by employing formic-acid decomposition reaction as an example.
6. Biomolecular reactions involving water molecules
The study of water was extended to deal with its role in functions of biological systems such as ion-channel, Photoactive Yellow Protein (PYP) and Bacteriorhodopsin. The rate of the ion permeation process was found to be determined mainly by the step of the ions entering the channel, and this ion penetration is entropy bottlenecked: pathways of exchanging water molecules around ions, needed for ion dehydoration, are very limited in vicinity of the entrance. Explanation of the fast ion permeation by so-called gNewton's ballsh mechanism, commonly used in past, should be thus modified. Photocycle of PYP is initiated by visible light-absorption, which causes isomerization of chromophore. Local proton transfer then takes place around the chromophore, followed by large structural rearrangement with the migration and coordination of water molecules, and finally results in the unfolding of N-terminal region of the protein. Namely, a local change (proton transfer) induced by photo-excitation is amplified to large protein structural rearrangement through water migration and HB reordering.
In addition to these studies of water, Professor Ohmine has also made a very important contribution to clarifying the mechanisms of gel phase transition and of photo-excitation and relaxation of polyenes. In summary, Professor Ohmine has opened a new horizon of the water study and established the novel theory of molecular origins of emany-body chemical processes.f
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