How do protein binding sites stay dry in water?

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ScienceDaily (Oct. 21, 2011) — In a report to be published soon in The European Physical Journal E, researchers from the National University of the South in Bahía Blanca, Argentina studied the condition for model cavity and tunnel structures resembling the binding sites of proteins to stay dry without losing their ability to react, a prerequisite for proteins to establish stable interactions with other proteins in water.

E.P. Schulz and colleagues used models of nanometric-scale hydrophobic cavities and tunnels to understand the influence of geometry on the ability of those structures to stay dry in solution.

The authors studied the filling tendency of cavities and tunnels carved in a system referred to as an alkane-like monolayer, chosen for its hydrophobic properties, to ensure that no factors other than geometrical constraints determine their ability to stay dry.

They determined that the minimum size of hydrophobic cavities and tunnels that can be filled with water is on the order of a nanometer. Below that scale, these structures stay dry because they provide a geometric shield; if a water molecule were to penetrate the cavity it would pay the excessive energy cost of giving up its hydrogen bonds. By comparison, water fills carbon nanotubes that are twice as small (but slightly less hydrophobic) than the alkane monolayer, making them less prone to stay dry.

The authors also showed that the filling of nanometric cavities and tunnels with water is a dynamic process that goes from dry to wet over time. They believe that water molecules inside the cavities or tunnels are arranged in a network of strong cooperative hydrogen bonds. Their disruption by means of thermal fluctuations results in the temporary drying of the holes until new bonds are re-established.

One of the many potential applications is in biophysics, to study water-exclusion sites of proteins, and understand the physical phenomenon linked to the geometry of those sites, underpinning the widespread biological process of protein-protein associations.

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Journal Reference:

E. P. Schulz, L. M. Alarcón, G. A. Appignanesi. Behavior of water in contact with model hydrophobic cavities and tunnels and carbon nanotubes. The European Physical Journal E, 2011; 34 (10) DOI: 10.1140/epje/i2011-11114-8

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Nearby planet-forming disk holds water for thousands of oceans

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ScienceDaily (Oct. 20, 2011) — For the first time, astronomers have detected around a burgeoning solar system a sprawling cloud of water vapor that's cold enough to form comets, which could eventually deliver oceans to dry planets.

Water is an essential ingredient for life. Scientists have found thousands of Earth-oceans' worth of it within the planet-forming disk surrounding the star TW Hydrae. TW Hydrae is 176 light years away in the constellation Hydra and is the closest solar-system-to-be.

University of Michigan astronomy professor Ted Bergin is a co-author of a paper on the findings published in the Oct. 21 edition of Science.

The researchers used the Heterodyne Instrument for the Far-Infrared (HIFI) on the orbiting Hershel Space Observatory to detect the chemical signature of water.

"This tells us that the key materials that life needs are present in a system before planets are born," said Bergin, a HIFI co-investigator. "We expected this to be the case, but now we know it is because have directly detected it. We can see it."

Scientists had previously found warm water vapor in planet-forming disks close to the central star. But until now, evidence for vast quantities of water extending into the cooler, far reaches of disks where comets and giant planets take shape had not emerged. The more water available in disks for icy comets to form, the greater the chances that large amounts will eventually reach new planets through impacts.

"The detection of water sticking to dust grains throughout the planet-forming disk would be similar to events in our own solar system's evolution, where over millions of years, these dust grains would then coalesce to form comets. These would be a prime delivery mechanism for water on planetary bodies," said principal investigator Michiel Hogerheijde of Leiden University in the Netherlands.

Other recent findings from HIFI support the theory that comets delivered a significant portion of Earth's oceans. Researchers found that the ice on a comet called Hartley 2 has the same chemical composition as our oceans.

HIFI is helping astronomers gain a better understanding of how water comes to terrestrial planets -- Earth and beyond. If TW Hydrae and its icy disk are representative of many other young star systems, as researchers think they are, then the process for creating planets around numerous stars with abundant water throughout the universe appears to be in place, NASA officials say.

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Journal Reference:

M. R. Hogerheijde, E. A. Bergin, C. Brinch, L. I. Cleeves, J. K. J. Fogel, G. A. Blake, C. Dominik, D. C. Lis, G. Melnick, D. Neufeld, O. Panic, J. C. Pearson, L. Kristensen, U. A. Yildiz, E. F. van Dishoeck. Detection of the Water Reservoir in a Forming Planetary System. Science, 2011; 334 (6054): 338 DOI: 10.1126/science.1208931

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