Physicists at Argonne National Laboratory have demonstrated the world's first X-ray microprobe of laser-aligned molecules. The technique may be used to better understand human proteins when studying drug interaction.

Linda Young, Ph.D.
Argonne National Laboratory

"We have used X-rays to examine an ensemble of individual molecules that have been aligned in free space using a laser," said Linda Young, Ph.D., group leader in Atomic Molecular and Optical Physics and a distinguished fellow at Argonne, located in Argonne, Ill.

Alignment, in this context, means that a unique molecular axis—the most polarizable one—is forced to be parallel to the laser's polarization axis, the direction of which can be easily controlled by the researcher. The linearly polarized electric field of the laser interacts with the polarizability of the molecule, producing an induced dipole which then settles into its minimum energy configuration along the laser polarization axis.

"While many others have used lasers to align molecules before, we are the first research group to use X-rays to probe such aligned molecules," said Dr. Young. "X-rays are a unique probe of molecules, indeed matter, because the extremely short wavelengths—sub-Ångstrom—provide information with atomic resolution."

LASER is an acronym for light amplification by stimulated emission of radiation. Laser light has the special property of being coherent, so its electric field can be linearly polarized in a direction perpendicular to its propagation.

Laser Alignment Offers Benefits over Protein Crystallization

Currently in protein imaging, crystals are used to create an X-ray diffraction pattern, often using X-rays from a synchrotron source, from which a real space image can be reconstructed. When proteins are not crystallized, however, X-rays scatter weakly and make a diffraction image unattainable.

"A major bottleneck now in the quest to determine protein structure is being able to grow crystals of sufficient size where X-ray crystallography becomes feasible," said Dr. Young. "The forefront in protein crystallography is to examine very small, say 20 micron length-scale, protein crystals through microdiffraction methods. The use of individual proteins, without the need for any crystal growth, would be a significant technological step forward."

Implications for Drug Development, Radiology Envisioned

Researchers at Argonne National Laboratory have used X-rays to examine an ensemble of individual molecules that have been aligned in free space using a laser. The diagram (a) describes the process, which involves the apparatus shown (b).

Finding a different way to view molecules involved in drug interactions than the one offered by crystallization is imperative because some of the molecules involved with drug interaction cannot be crystallized. The laser technique aligns millions of molecules so that they will scatter in the same way when bombarded with X-rays. The atomic-level-resolution images so produced do not require crystallization.

Understanding the protein structure-function relationship on a molecular level will be key to designing new drugs, said Dr. Young.

Dr. Young noted that the concept of laser alignment is simply a subset of laser control—laser alignment is a method to constrain the rotational degree of freedom in a molecule. She said she envisions arrays of aligned molecules located in a 2D or 3D lattice formed by crossing laser beams. These advanced optical methods to control molecules may enable better structure determination, she said.

Extending laser alignment to molecules in the liquid phase would also be a significant breakthrough, she said. "Beyond rotation, one can control other degrees of freedom, vibrational and electronic, as well," she said. "On a more global scale, laser control of chemical reactions—through bond softening and hardening, bond making and breaking, may be possible."

Laser alignment may have applications in radiology, said Dr. Young. "The field of X-ray coherent diffractive imaging, whereby an X-ray diffraction pattern of a non-periodic sample can be inverted to produce a real space structure with nanometer resolution, is in its infancy," she said. "Combining this powerful technique with laser control of molecular motions at the atomic level would be very exciting."

Improvements to X-ray Flux Sought

So far, researchers have used X-ray absorption to probe aligned molecules, said Dr. Young. However, she said, the gold standard for structure determination is X-ray diffraction, or coherent elastic scattering. "Because the scattering cross-section is much smaller, the realization is significantly more challenging," she said.

Dr. Young and colleagues are currently working to improve the X-ray flux onto the laser-aligned molecular sample by more efficiently using the flux produced by Argonne's Advanced Photon Source (APS), a premier source of X-rays in the U.S. Since the team's initial experiments used relatively low repetition rate lasers (1 kHz), they seek a statistical gain up to a factor of 6500x by using lasers where the repetition rate is matched to that of the APS.

A large international collaboration is working on the problem with an eye toward implementation at the Linac Coherent Light Source (LCLS), an X-ray free-electron laser located at the SLAC National Accelerator Laboratory that is scheduled for first use in the summer of 2009. The LCLS produces the same average flux as the APS, but the flux comes bunched in ultrashort packets with a repetition rate of 120 Hz. Shorter pulse lengths and larger photon number per pulse will spur developments in single shot imaging methods, said Dr. Young.