NYU/European scientists solve the mystery of proton transport in aqueous solution

June 26, 2002

Research by an international team of chemists has shed new light on the process of transporting protons through aqueous basic solutions, invalidating some long-held misconceptions regarding proton transport and revealing that the process is more complex than ever before imagined. This research, published in the June 27th issue of Nature, builds on previous results from the same team on proton conduction through aqueous acidic solutions, which also appeared in Nature in February 1999.

Proton transport in acidic solution is an electrochemical process that is important for the generation of power in fuel cells. It is plays a fundamental role in numerous biochemical reactions and has important industrial applications. In basic solution, similar processes govern phenomena such as fermentation and the production of soaps (saponification), and several important enzymatic and glucose processing reactions occur in a basic environment.

Members of the research team include Mark Tuckerman, assistant professor of chemistry at New York University; Dominik Marx, Professor at of Chemistry at Ruhr-Universität in Bochum, Germany; and Michele Parrinello, Professor of Chemistry at Eidgenosse Technische Hochschule, in Zurich, Switzerland.

Led by Tuckerman, the team used groundbreaking computer simulation methods to track proton transport in an aqueous solution on a microscopic level. The researchers found that proton conduction depends critically on whether the environment is acidic or basic. Previously, it had long been thought that the mechanisms of these two environments were very similar and chemically analogous. The illumination of different mobility mechanisms operational in acidic and basic environments may help to clarify why nature might prefer acidic or basic conditions in different situations involving proton transport, and ultimately to exploit the different mechanisms in the design of processes or materials that utilize proton conduction phenomena.

"We were drawn to this problem because proton conduction is a fundamental process that arises in numerous biologically and technologically important phenomena," said Tuckerman. "Proton conduction is a complex process, and our research provides a clearer understanding of how it occurs in different media at the microscopic level. This knowledge could be used to aid in the design of materials or processes that rely upon or possibly enhance proton conduction in different applications."

In liquid water, H2O molecules form a complex network in which they are connected by hydrogen bonds, a hydrogen atom between two oxygen atoms in a roughly linear arrangement. Each water molecule is surrounded on average by four other water molecules. Adding or removing a proton (H+) in liquid water creates a defect in the water network that is transported by making and breaking of bonds within the system. H3O+, or hydronium ions, the fundamental component in acids, are formed when protons are added and latch on to water molecules. Hydronium ions are surrounded, on average, by three water molecules.

Water molecule A in diagram 1 , panel a (see diagram 1, http://www.nyu.edu/publicaffairs/science/proton.html) is connected to H3O+ by a hydrogen bond and water molecule B is connected to A by a hydrogen bond. (Red and grey spheres denote oxygen and hydrogen atoms, respectively, and the yellow sphere denotes the oxygen at the center of the H3O+ ion.) Proton conduction in acids occurs when the hydrogen bond between A and B breaks leaving A surrounded by only two other water molecules and the H3O+ (panel b - note that the two yellow oxygen molecules share the proton between them equally). This is followed by a transfer of the proton from H3O+ to water molecule A (panel c - note that a new H3O+ ion has been created where the new yellow oxygen resides). The whole process takes only one millionth of one millionth of a second.

Where H3O+ corresponds to an excess proton, hydroxide, or OH- ions, the fundamental component in basic solutions, were long thought to constitute "proton holes" and, therefore, to have chemical properties analogous to those of hydronium. In particular it was thought that proton conduction in basic solutions could be viewed as a kind of chemical "mirror image'' of its acidic counterpart. The team's study demonstrated that, in fact, no such simple chemical analogy exists between H3O+ and OH-. For example, the team showed that OH- is surrounded, on average, by 4-5 water molecules (see diagram 2, http://www.nyu.edu/publicaffairs/science/proton.html), quite unlike the hydronium case. Moreover, proton conduction in bases requires more complicated rearrangements of water molecules than in acids (panels b and c). Finally, the process is strongly influenced by a phenomenon known as quantum tunneling, a phenomenon that can occur at the microscopic level, which allows particles to traverse spatial regions they normally should not, provided they do it quickly enough.

The team carried out the study by solving the fundamental equations that govern how the system develops in time on a supercomputer. Each step of the calculation generates a single snapshot or "frame" in a long "movie" that can then be analyzed in order to extract the proton conduction mechanism. These findings are reported in a letter to Nature entitled "The Nature and Transport Mechanism of Hydrated Hydroxide Ions in Aqueous Solution."
Mark Tuckerman, who also serves as assistant professor of mathematics at New York University's Courant Institute of Mathematical Sciences, received his Ph.D. in 1993 from Columbia University. His fellowships and honors include NYU's Golden Dozen Teaching Award, 2000; A Whitehead Fellowship in Biological and Medical Sciences, 2001; The Camille Dreyfus Teacher Scholar award, 2002; the Computer and Information Science and Engineering (CISE) Postdoctoral Research Associateship from the National Science Foundation, 1995; a Postdoctoral Fellowship at the University of Pennsylvania, 1994; and a Postdoctoral Fellowship with the Computational Physics Group at IBM, Switzerland, 1993. His research interests include proton conduction phenomena, protein folding and drug binding, surface chemistry, statistical mechanics, algorithms for molecular dynamics and electronic structure, and computational chemistry.

This research was funded by grants from the National Science Foundation and the Research Corporation.

New York University

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