'North Pole' of the molecular world

RESEARCHERS HAVE devised a method to determine the alignment of a molecule's axis, the `poles' that govern how a molecule will interact with others. The advancement will help scientists and engineers predict the ways that atoms and molecules exchange energy, possibly enhancing solar energy devices or helping biochemists better understand proteins.

The research, appearing in the Physical Review Letters, shows how a tightly-focused laser employing a new kind of polarisation can produce valuable images of individual molecules in three dimensions.

The new method takes a snapshot of a phenomenon called the `molecular dipole moment.' This `moment' is an axis that runs through the molecule like a north and south pole, along which energy is emitted and absorbed. If two molecules are positioned so that their respective poles align, they are more likely to exchange energy. If they are completely misaligned, then an interaction is more difficult. Someday, researchers hope to control the alignment to direct chemical reactions at the atomic level.

"By imaging the dipole movement of certain molecules we can see exactly how certain chemical reactions happen," says Lukas Novotny, assistant professor of optics at the University of Rochester. "We are working now with biochemists to understand how various proteins in the body form."

Proteins fold when they form, but monitoring their folding is a tricky business that the Rochester team's method can help clarify. To watch the folding process, researchers place two marker molecules at each end of the protein - one marker emits green light when stimulated, and the other emits red. One marker (the green one) is charged with energy so that it emits its light.

When the protein folds itself and brings these two markers together like a gymnast touching her toes, the green marker gives some of its energy to the other, which then glows, causing a change in the overall color of light emitted from the protein. Exactly when this energy exchange takes place, however, depends on how far apart the marker molecules are and how their `poles' are oriented.

Biochemists will now be able to know exactly what the markers' orientations are, and so know exactly how far the protein has folded when the emitted light changes. This puts a powerful new tool in the hands of scientists investigating cellular processes.

Determining the north pole of an atom required a new class of light polarisation, the development of which was pioneered by Thomas Brown, associate professor of optics at the University. Regular light has linear polarisation, which means it essentially vibrates within a plane.

The molecule-imaging method, however, uses radial polarisation, where the vibration moves in several planes radiating outward from the light beam. By converting regular laser light to radial- polarised light and tightly focusing the laser beam, the team can create a tiny electric field that is of equal strength in all three dimensions, thanks to the radial polarisation.

The team then scans the beam along the molecule in all directions until one of the radial planes lines up with the north or south pole, and the atom absorbs the energy. A slight burst of fluorescence tells the team when they've hit their mark, and they can determine at exactly what angle the pole is oriented.

Novotny sees other applications for molecular dipole moments. "Cells in the body communicate through proteins located in their membranes. During an exchange of information, the shape of the proteins changes. By attaching molecular markers to the protein and monitoring their orientation and position, we should be able to better understand communication between cells."

Tracking the dipole moments might also shed light on how cancer cells grow in colonies. Novotny predicts that someday molecular dipole moments may be not just ascertained but controlled, allowing for quick, custom-made molecule alignment, or even data storage, since the orientation of the dipole moment could stand for a one or zero.

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