Chemists develop new way to monitor molecules

Taking a page from modern astronomy, where scientists are making a raft of new discoveries by sampling starlight across the electromagnetic spectrum, a group of chemists from UW–Madison has refined a powerful new way to probe the molecular universe using light.

Building on a technique known as infrared spectroscopy, the Wisconsin chemists use two infrared lasers, choreographed to focus on a small sample, to read the barely detectable but telltale vibrations of molecules that can provide access to previously hidden but critically important information.

Chemistry professor John Wright, left, and graduate student Kent Meyer work in Wright’s research lab with laser equipment to study molecules. Above: This view shows how lasers are choregraphed to detect vibrations that reveal the interaction of molecules. Photos: Jeff Miller

The new technique, to be reported in the Feb. 14 issue of the scientific journal Physical Review Letters, allows chemists to read the fine print of the molecular world, the connections and fleeting interactions within and between molecules. Such a perspective, according to UW–Madison chemist John C. Wright, could lead to leaps in our understanding of a host of scientific problems, from how some bacteria thwart antibiotics to the weathering of soil.

Molecules are groups of two or more atoms connected by bonds that act like springs. Through infrared spectroscopy, scientists can read the vibrations, which act like fingerprints to identify a molecule. Taking the technique an important step further, the Wisconsin team has developed a way to use two infrared lasers to pluck two different springlike bonds to get a picture of how all the different bonds in the molecule are connected.

“The use of two infrared lasers to drive two different vibrational modes to find the interconnections within and between molecules promises a powerful new way to study biological systems,” Wright says. He says the new technique, essentially, is the long-sought analog to two-dimensional nuclear magnetic resonance spectroscopy (NMR), a mature technology that uses magnetic fields and energy from certain radio frequencies to tease information from atoms and molecules.

Wright is a UW–Madison professor of chemistry and, along with UW–Madison colleague Wei Zhao, is an author of the PRL report.

The new technique is known as Doubly Vibrationally Enhanced (DOVE) Four Wave Mixing. It employs lasers — whose light is routed through a maze of mirrors and lenses and focused onto a sample — to stimulate molecules of interest and capture spectral signatures that can reveal the deft interplay of molecules at work.

“It’s a new process that gives us a new tool,” Wright says, “and the important thing is that it can give you molecular information about what’s connected to what.”

Like NMR, the method developed by Wright and Zhao tunes in to the frequencies at which molecules vibrate. But instead of the radio waves used in NMR, DOVE depends on the concentrated light pulses of lasers to stimulate molecules and provide a window to molecular connections.

At room temperature, molecules have natural oscillations whose frequencies changes as molecules dock with one another.

“The vibrational frequency is a direct reflection of the bonding that glues the atoms in a molecule or two different molecules together,” Wright explains. “For example, when an enzyme is going to facilitate a reaction, it needs to bond with the molecule it is going to change. When it bonds, new vibrational modes appear that are characteristic of the new bond.”

But those oscillations, Wright notes, can also be driven by the force of laser light, just as the motion of a child on a swing can be altered with a gentle push.

“What we hope to be able to see is not only what’s present in a sample, but how they interact. How does an antibiotic bind and what does it bind to?”

The answers to those kinds of questions are fundamental to understanding critical questions in science. To see the molecular details of how soil and rocks weather, for example, promise a better understanding of how soils in some parts of the world become toxic as aluminum accumulates through the process of weathering.

“It would be really nice to watch soil as it weathers and see what interactions cause the soil to become toxic to plants,” says Wright.

Another key project would be obtaining a more detailed molecular picture of water, nature’s most important solvent and a substance present in virtually every reaction associated with life.

“Many people have studied water, but they still don’t understand it,” Wright says. “The most important question focuses on whether the hydrogen bonds that link water together are strengthened if other hydrogen bonds are already present.”

The new technique, he says, may be able to answer that key question, but it promises to be especially useful for looking at complex materials such as proteins: “The structure of proteins can be determined by measuring the vibrations, but (the data) can be incredibly complex because each molecule is complicated.”

DOVE Four Wave Mixing, says Wright, is applicable to virtually any area of science or technology where vibrations can be measured, and promises to shed new light on a host of scientific problems.