Trapped spheres of water make perfect protein prison

时间:2019-03-02 01:03:03166网络整理admin

By Phil McKenna (Image: ACS) (Image: ACS) A material with a love-hate relationship with water immobilises droplets so completely that small spheres of water remain pinned to its surface even when hung upside down. Not just a cool party trick, the new surface could help detect traces of pharmaceuticals in drinking water supplies, or advance understanding of protein folding. The surface is strongly hydrophobic – so droplets placed on its surface form near-perfect spheres with minimal surface contact. But an unexplained mechanism allows that tiny point of contact to firmly hold the drop so it cannot roll around as they usually do on water-repellent surfaces. Even when the surface is tilted or inverted, the 1-millimetre-wide drops refuse to budge, and remain largely spherical (see image, below right). They are held as if by an invisible container. The silicon material has a surface like a microscopic egg carton, pitted with circular depressions 10 microns across – about the size of a human red blood cell – with much smaller sharp projections (15 to 20 nm) sticking up in between (see image, right). Researchers at the Weizmann Institute of Science in Rehovot, Israel, made the surface using photolithography, the chemical process used to carve computer chips from silicon wafers. Exactly how the surface holds droplets so tightly is unknown. One possible explanation is that tiny pockets of air trapped in the depressions beneath the drop are responsible – when the droplet pulls away from the surface, the air pressure in the pockets drops, holding the water onto the surface. Immobilising the spherical drops in this way without a container may help chemists identify biologically active molecules such as pharmaceuticals by holding them in a closely confined space long enough for highly refined spectroscopy readings that use laser light to reveal molecular structures. “This opens the way to detect a single molecule in water,” says Ron Naaman, a co-author of the study. That could be useful for looking for small traces of compounds in very small samples of water – for example, molecules of drugs contaminating drinking water, he says. Royce Murray, of the University of North Carolina at Chapel Hill, US, is unsure whether there is much need to use such small samples when testing drinking water. However, such small immobilised droplets could serve as perfect prison cells for single protein molecules, he adds, trapping them in one small place long enough for single-molecule spectroscopy analysis to show how they unfold. Studying proteins in that way currently requires bonding the molecule to a fixed surface, or enclosing it inside a narrow tube. But these methods can alter the protein’s properties. Holding the protein in a spherical drop would avoid those effects, says Murray. “If you can do single-molecule spectroscopy of a protein in a single droplet and do that for a lot of proteins and see how different they are, that could be hugely significant,” he says. Journal Reference: Nanoletters (DOI: