Laser Induces Conformational Changes
Since Louis Pasteur's investigations into the optical activity of fermentation compounds in 1848, scientists have understood that the chemical properties of a substance depend on both the type and the physical arrangement of its components. Unfortunately, a reliable, flexible technique for conformationally modifying a sample has remained elusive. Now a team at Purdue University in West Lafayette, Ind., has used laser radiation to induce changes in a molecule.
N-acetyl-tryptophan methyl amide has more than 150 conformational states. When prepared in a supersonic gas jet, rapid expansion and collisional cooling leave the molecular population in three of the lowest energy conformations. In the experiments, the researchers used a frequency-doubled dye laser to probe the electronic structure of the molecules downstream, where collisions no longer occur. They identified three conformations, which they labeled A, B and C and which feature two N-H bonds with slightly different vibrational frequencies.
An optical parametric converter, pumped by a seeded Nd:YAG, irradiated other samples of the molecules with an approximately 5-mJ pulse of infrared radiation tuned to the resonant frequency for a specific bond to influence the population of conformational states. For example, exciting configuration A with 2.89-µm radiation left A nearly completely depleted and C in abundance. If, however, the same N-H bond in the B state were excited at 2.90 µm, conformation C would form inefficiently and A abundantly.
The laser can be regarded as heating or energizing the molecules to reach more local conformational minima, but this does not adequately describe the situation. "The laser is different than a conventional 'heating source,' such as a Bunsen burner," said Timothy S. Zwier, who performed the work with Brian C. Dian and Asier Longarte. "The conformational distributions it creates depend on where the energy was initially placed in the molecule."
With the external control of molecular conformation possible, investigations into the effect of physical configuration on chemical interactions can be more easily pursued. Zwier hopes that scientists will extend the work to drive similar processes for molecules in solution or on a surface.
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