New Tool For Surface Structures

When chemists want to know the structure of a molecule floating in solution, they have a battery of techniques to help them figure it out. But when a molecule—especially a complex biomolecule such as a protein—sits atop a surface, determining what it looks like isn’t so easy.

Researchers want to understand the structure of molecules, big or small, at surfaces to design efficient sensors and tissue-compatible medical implants. They have some tools capable of probing a molecule’s structure on a surface. But many of those methods either don’t provide a complete picture or aren’t sensitive enough to examine a single layer of molecules on a surface.

To acquire more detailed data, a research team led by Martin T. Zanni of the University of Wisconsin, Madison, has beefed up sum-frequency generation spectroscopy, a traditional tool for analyzing molecular structures on surfaces. The scientists demonstrated their new method, called two-dimensional sum-frequency generation spectroscopy (2D-SFG), by measuring the configuration of a short peptide coating on a gold surface (J. Am. Chem. Soc. 2013, DOI: 10.1021/ja408682s).

To develop 2D-SFG, Zanni and colleagues drew on their expertise in 2-D infrared spectroscopy. In 2D-IR, a sequence of infrared light pulses hits a molecule floating in a liquid and causes different portions of the molecule to vibrate. The spectra generated reveal how the different regions of the compound are oriented with respect to one another.

In 2D-SFG, a similar set of infrared pulses is directed onto a surface in a standard SFG spectrometer. The resulting spectra also yield detailed structure information but only from molecules on the sample surface.

Using the new technique, Zanni and his team determined that their peptide is an α-helix that sits somewhat upright on the gold surface. But they also found that its ends are slightly disordered, a feature that, Zanni says, is not observable with traditional SFG.

The workhorse tool chemists use to determine molecular structure in solution is NMR spectroscopy. With 2D-SFG, says Franz M. Geiger, a chemist at Northwestern University, “we are one step closer to realizing a surface analog of NMR.”