Protein Conformational Rearrangement induced by the modulation of vibrational modes

We succeeded to induce a specific conformational change in a protein by infrared laser excitation and proved that the reaction was a non-thermal effect. An alpha helical – beta sheet structural rearrangement of an intrinsically fluorescent protein, LSSmOrange was monitored through its fluorescence intensity change upon irradiation with a free electron IR laser. The picosecond timescale IR pulse caused a reversible structural rearrangement of LSSmOrange on the second timescale. We found that the structural rearrangement was induced by a single IR photon with a very low, less than 10-6 photon efficiency and the maximum effect was detected at 1033 cm-1 IR irradiation. The IR induced reaction is not a trivial thermal effect because 1. the IR pulse increases the fluorescence of LSSmOrange whereas temperature jump decreases it, 2. its magnitude depends on the wavelength of the IR pulse. Based on the Arrhenius plots of the IR induced conformational change and its relaxation, we found that the IR excitation increases the free energy of the alpha-helical structure dominated state by 12 kJ/mol. Since this energy shift is exactly equal to the energy of the IR photon, we concluded that there are highly efficient energy flow pathways between the vibrational modes in the protein as it is indicated by recent theoretical calculations. These results may open a new horizon towards the direct investigation of the functional role of vibrational modes in proteins.