Molecular imaging technique gets a quick reaction

An imaging technique that combines laser pulses and x-rays is opening new vistas for scientists studying how chemical reactions work. Philip Coppens, a professor of chemistry at the State University of New York at Buffalo (UB), and his colleagues used their technique, called photocrystallography, to discover that nitric oxide can bind to metals like iron in ways different than has commonly been believed. The dual wavelength allows researchers to excite a molecule using the laser, then use x-ray crystallography techniques to monitor structural changes in the sample, viewing microsecond-scale phenomena. "This measures the structure of molecules in their excited state, so that's a time dimension that hadn't existed before," Coppens says.

Their system incorporates a diode-pumped neodymium-doped yttrium aluminum garnet (Nd:YAG) laser operating at 355 nm. The laser generates 100 ns, 300 µJ pulses at a repetition rate of more than 5 kHz. A tapered optical fiber brings the beam to focus on a sample crystal that is cooled with liquid nitrogen or helium. The beam excites a chemical reaction in the sample. A fiber bundle surrounding the transmitting fiber captures the resultant fluorescence and delivers it to a photosensor module.

Simultaneous with the optical pulse, the team directs an x-ray pulse at the sample. A spinning steel disk causes the x-ray beam, from a synchrotron source, to strobe on the sample at a desired rate. A charge-coupled-device (CCD) detector collects the x-ray emissions. The UB team currently uses the National Synchrotron Light Source at Brookhaven National Laboratory (Upton, NY).

Coppens and his team discovered that when the laser light strikes the sample, very small, two-atom molecules rearrange to bind in a different way, attaching, for example, nitric oxide to iron or nickel through the oxygen rather than through the nitrogen atom. Iron, of course, is an important part of hemoglobin in human blood, and nitric oxide plays a crucial role in such bodily processes as blood pressure and the transmission of nerve signals. Coppens says scientists seeking to understand those processes will now have to take into account the new species his team discovered.

Photocrystallography can help researchers see reactions that happen on microsecond time scales. Coppens predicts that the technique will be useful for scientists trying to understand photosynthesis, which could in turn lead one day to the creation of artificial photosynthesis. That could give them a new way to store and release energy. "Photosynthesis has many steps, and it's an enormously complex molecular machine that's being studied in labs all over the world," he says. One advantage of the system is that it uses such small samples that they heat evenly under the laser beam, eliminating irregularities.

Coppens's lab also developed a prior version of the technique, which instead of measuring real-time reactions uses the laser to add energy to the cooled molecules for up to an hour, then uses x-ray diffraction to study the molecules in their higher-energy state.

A coming step will be to use the Advanced Photon Source at Argonne National Laboratory (Argonne, IL), which delivers more x-ray photons in a narrow angular range. That could cut the testing time from several hours to perhaps half an hour, reducing the possibility of damage to the crystal being studied by cutting its exposure to laser energy. It also could lead to resolutions at time scales of nanoseconds, or perhaps even shorter.

Another group at Argonne is working on a comparable project, Coppens says, but that setup is designed to study the reactions of larger molecules, including those of such biological interest as enzymes, proteins, and DNA. And a German group at the European Synchrotron Radiation Facility (Grenoble, France), is working on a way to sample powders rather than single crystals.

"We're really in the beginning in this time-resolved work," Coppens says. "There's an enormous amount to study."