Computer simulation of water in nanotubes

An oriented chain of water molecules inside a (6,6) carbon nanotube (oxygen = red; hydrogen = white; carbon = grey). A contour surface shows high and low water density in purple and blue respectively.

The well understood peculiarities of water make life possible, but this most common fluid continues to surprise scientists. In an article published in the November 8 issue of the journal Nature, two University of Maine chemists and a scientist at the National Institutes of Health (NIH) offer a new view of how water behaves at the scale of individual molecules, especially in tight spaces.

The authors are Gerhard Hummer of the NIH and Jayendran C. Rasaiah and Jerzy P. Noworyta of the UMaine Department of Chemistry. Their results have implications for medical research and may contribute to an understanding of how water behaves in the pores of cell membranes. The work at Maine was supported by a grant to Rasaiah from the National Science Foundation.

Using computer simulations, the research team found that water acts in unexpected ways. The causes, they suggest, are fluctuations in density and a kind of naturally occurring molecular dance that happens between the hydrogen bonded water molecules in bulk water and in pores.

The scientists began by generating on a computer a tiny tube of carbon atoms that they then placed into a virtual pool of water. Previous research led them to expect that water would not enter such tubes, known as carbon nanotubes. However, the scientists found that chains of hydrogen bonded water molecules that are only a single molecule wide move through the tube in short bursts. Small changes in the interaction between the carbon and water molecule that mimicked chemical modification of the nanotube can cause it to empty or fill up.

“Our work is important for understanding how water is conducted in biological channels known as aquaporins,” says Rasaiah. Heart function, for example, depends on concentrations of calcium in water both inside and outside of cells, and water movement through cell membranes may play an important role in balancing calcium. These simulations provide dynamic information that goes beyond static pictures of conventional structural biology, Rasaiah notes.

The conditions leading to this behavior, the authors suggest, might also be significant for the development of some types of sensors.

Rasaiah worked at NIH during the summer of 2001 as a visiting scientist, and Noworyta is a post-doctoral researcher at UMaine. Hummer conducts research in the Laboratory of Chemical Physics in the National Institute of Diabetes and Digestive and Kidney Diseases at NIH.

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