Lasers coax large molecules to change their shape

May 23, 2002

WEST LAFAYETTE, Ind. -- Lasers are used to carry out functions ranging from reading a bar code label at the grocery store to shooting down enemy missiles in space. Now, chemists at Purdue University are using lasers to coax individual molecules to change their shape, a step that may someday enable scientists to direct molecules to perform specific functions.

A research team led by chemistry Professor Timothy Zwier has demonstrated how laser light can be used to prompt a large molecule to make alterations in its three-dimensional structure.

Large molecules, such as proteins, consist of polypeptide chains that twist and fold upon themselves to form a unique three-dimensional shape, or conformation. The molecules also can reconfigure into many different shapes. Such conformational changes can alter how a molecule reacts with other molecules.

Using laser light to excite a single chemical bond in a molecule that contained two peptide groups, Zwier's team showed that they could change the preferred shape of the molecule simply by choice of laser wavelength. Working with Zwier were graduate student Brian Dian, and postdoctoral research associate Asier Longarte.

The results suggest that scientists may someday be able to use laser light as a "switch" to change a molecule's structure, prompting it into action or altering its activity, Zwier says.

"Clearly, we are still a long way from applying these methods directly to proteins or DNA or the like," he says. "Nevertheless, this approach may someday allow us to use a laser to directly manipulate the structure of molecules, and thereby turn on and off their functionality with light."

The findings -- currently on the Science Express Web site, http://www.scienceexpress.org, and soon to be published in a print issue of the journal Science -- may provide fundamental information on the folding process used by proteins and other large molecules. The study also points to potential new avenues for developing applications for use in molecular electronic devices.

"Scientifically, the study is interesting because not only is laser excitation efficient at driving conformational change, but it also achieves this change in a unique way," Zwier says.

Since the invention of the laser in 1960, chemists have been interested in using its light as a reagent to drive chemical reactions. The hope was that lasers could be used to infuse energy selectively into a specific chemical bond, thereby causing the bond to break. Such an initiation step would enable chemists to form product molecules with a desired set of chemical properties.

The difficulty with this approach -- called mode-selective chemistry -- is that, although the laser initially excites a single bond or group of atoms, under most circumstances that energy is scrambled throughout the other parts of the molecule.

"In a sense, the molecule loses its memory of how the energy was initially deposited into it," Zwier says. "The result is that the laser ends up 'heating' the molecule in much the same way as would a traditional heating source, such as a Bunsen burner or hot plate."

Though mode-selective chemistry has been accomplished on a few small molecules, such as water, the difficulty in selectively breaking a single chemical bond has been thought to increase as the size of the molecule increases.

"Since larger molecules have many more vibrational motions available to them, they have many more ways to scramble the energy, and do so on a faster time scale than small molecules," Zwier says.

However, because of their size and complexity, large molecules, such as proteins, present an opportunity for a different kind of chemistry that does not involve breaking chemical bonds, Zwier says.

"Large molecules can reconfigure into many different shapes, or conformations," he says. "Such conformational changes are reactions in their own right, but they also can serve to change a molecule's reactivity with other molecules."

Zwier says making conformational changes in a molecule involves less energy than that required to break a chemical bond, and can be done through multiple pathways. His findings show that, by choosing different infrared wavelengths, the laser could be used to selectively choose the molecule's new shape.

In their study, the Purdue team used an infrared laser to excite specific bonds within a molecule that has three distinct conformations. The researchers applied the laser to a set of molecules found in one of the three conformations, giving them the energy they needed to undergo a conformational change. A second laser, using ultraviolet light, was then used to detect the changes in each of the conformations induced by the infrared laser.

The researchers were surprised to find that when a particular bond was vibrationally excited by the laser, it would cause most of the molecules to reconfigure into a specific conformation, while exciting another bond would prompt the majority of molecules into a different conformation.

"Though we have yet to produce a uniform population with all molecules in the same conformation, we have shown that this method can prompt the molecules to change shape, favoring the formation of one over another," Zwier says.

The results suggest that mode-selective chemistry may actually be easier to induce in large molecules than in small ones, if ways can be found to selectively excite single bonds, he says.

"We've been able to show the feasibility of this approach in our molecule, but larger molecules, such as proteins, have many more groups that may absorb at similar wavelengths, so it could get more difficult to selectively excite a single bond," he says.

Zwier and his research team now plan to pursue further studies to determine how the molecules move from one configuration to another. Understanding this process could allow scientists to fine-tune the process of producing molecules with a particular conformation.

"One of the major challenges of future studies is to take this work on isolated molecules in the gas phase and see if the laser can be used to drive similar processes in molecules found in solution or on a surface," Zwier says.
-end-
The study was funded by the National Science Foundation and the Petroleum Research Fund.

Writer: Susan Gaidos, (765) 494-2081; sgaidos@purdue.edu
Source: Timothy S. Zwier, (765)494-5278, zwier@purdue.edu

ABSTRACT

Conformational Dynamics in a Dipeptide After single-Mode Vibrational Excitation
Brian C. Dian, Asier Longarte, and Timothy Zwier

The dynamics of conformational isomerization are explored in a methyl-capped dipeptide, N-acetyl-tryptophan methyl amide (NATMA), using IR-UV hole-filling and IR-induced population transfer spectroscopies. IR radiation selectively excites individual NH stretch vibrational fundamentals of single conformations of the molecule in the early portions of a gas-phase expansion, and then this excited population is collisionally re-cooled into its conformational minima for subsequent conformation-specific detection. Efficient isomerization is induced by the IR excitation that redistributes population between the same conformations that have population in the absence of IR excitation. The quantum yields for transfer of the population into the various conformational minima depend uniquely on which conformation is excited and on which NH stretch vibration is excited within a given conformation.

Purdue University

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