In the past five years, Mathies and his colleagues have used femtosecond stimulated Raman spectroscopy to investigate similar atomic motions in large, light-absorbing molecules including rhodopsin, the visual pigment in the eye; bacteriorhodopsin, the light-capturing pigment in photosynthetic bacteria; and phytochrome, a light-sensing pigment found in plants and bacteria. This technique probes a range of vibrations tens of femtoseconds to 1 picosecond (one-trillionth of a second) that is important in chemical reactions, but until now largely inaccessible.
"This is something I've wanted to do for 40 years, ever since I came to Berkeley in 1976, but I didn't have the ability," Mathies said. "I had to develop the tools to get to tackle this challenging problem."
Femtosecond stimulated Raman spectroscopy on GFP involves hitting the protein molecule with an approximately 80 femtosecond pulse of ultraviolet light, which excites many vibrational modes in the molecule, and then a one-two punch of picosecond red and femtosecond white light to stimulate Raman emission. The spectrum of emitted signals tells researchers the vibrational modes of various parts of the molecule. If the molecule is in the middle of a reaction, the emitted light at different time delays tells the researcher the various steps the molecule goes through during the reaction.
"Now, we can get very, very high resolution structure down to 10-25 femtoseconds," Mathies said.
Mathies compares proton or electron transfer the key event in absorption or emission of light to a worker's trip from office to home, which can involve any number of routes. But previous techniques with mere picosecond resolution provided only a blurry picture, and often just the start and end points. With ultrafast femtosecond spectroscopy, he said, "y
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University of California - Berkeley