Feb. 18th Nature Article Offers Explanation for Why Protons In Water Are So Much More Mobile Than Other Ions
Findings could impact understanding of biological processes that rely on aqueous proton diffusion
A team of researchers from New York University and the Max Planck Institute for Solid State Research in Stuttgart, Germany is proposing an explanation to a 200-year-old riddle: why are protons in water so much more mobile than other ions?
As one learns in high-school chemistry, a proton in water does not remain on its own, but rather attaches itself to a water molecule to form a hydronium ion, H3O+. It is widely agreed that the high mobility results from a series of proton transfer or "relay" steps that result in an overall migration of positive charge with very little motion on the part of individual molecules.
However, there has not been scientific consensus on the microscopic details of this process, which is known as the Grotthuss mechanism after C.J.T. de Grotthuss, who originally proposed it in 1806. (See the end of this press release for a longer explanation of the Grotthuss mechanism.)
Now the team of researchers from NYU/Max Planck Institute has unraveled these microscopic details and proposed an explanatory model derived from computer simulations that fully represent the quantum-mechanical nature of the proton, as reported in the February 18th issue of Nature.
According to the researchers, their model captures the rich and complex behavior of the hydrated proton, where previous models have failed. Prevalent among these are the Eigen cation model, which identifies an H9O4+ complex (hydronium hydrogen-bonded to three nearby water molecules) as the primary structure for the shuttling of charges or the Zundel cation model, which identifies an H5O2+ complex (a proton sandwiched between two water molecules) as the structure.
In contrast, the researchers found that the hydrated proton is of an inherently fluxional, protean character. The Eigen and Zundel complexes are roughly equally likely to occur, as are a great number of other structures.
NYU Chemistry Professor Mark E. Tuckerman said, "The most important aspect of this research is that it leads to a greater understanding of one of the most fundamental, ubiquitous and important processes in chemistry and biology, which had remained a mystery for nearly two centuries. Already authors of chemistry textbooks are beginning to revise their treatment of this topic based on our previously published work which lead up to the present study. Our findings will help us understand how many biological processes, that rely on aqueous proton diffusion, work at a microscopic level. As our understanding of the microscopic details of such processes increases, we might be able to learn how to control them. In addition, our findings will help us better understand certain reactions that occur in the upper atmosphere, such as lead to the formation of acid rain and the seasonal ozone depletion."
Entitled "The nature of the hydrated excess proton in water," the Nature article was written by Dominik Marx, Mark E. Tuckerman, Jurg Hutter and Michele Parrinello. It was accompanied by a news and views piece by James T. Hynes entitled "The protean proton in water."
This project was supported by the American Chemical Society, the Max Planck Institute and New York University.
NYU Chemistry Professor Mark E. Tuckerman received his BS with honors in 1986 from the University of California at Berkeley, and his Ph.D. in 1993 from Columbia University. He was a postdoctoral fellow at the IBM Forschungslaboratorium, Zurich, Switzerland in 1994 and an NSF ASC postdoctoral fellow at the University of Pennsylvania.
THE GROTTHUSS MECHANISM
To understand how the Grotthuss mechanism works, it is helpful to consider an analogy. Imagine a crowd of moviegoers gathered in front of the box office, each in possession of a ticket. A new moviegoer approaches the crowd, wishing to obtain the ticket, which he has already ordered and paid for. Rather than trying to squeeze through the crowd, he calls to the cashier to pass his ticket through the crowd to him. The cashier hands the ticket to the person closest to the window. This person, now in possession of two tickets, a new one held in the hand closest to the cashier and his own held in the hand closest to the next person in the crowd. Rather than hand the next person the ticket just passed down from the cashier, out of convenience, he simply hands his original ticket to the next person in the crowd. This person, now holding two tickets, does exactly the same, and the relay process continues until the person who called for the ticket has one. In the end, everybody is in possession of a ticket, although nobody is holding their original ticket. This is essentially how proton diffusion in water occurs.
New York University. February 1999.