Ultrafast Structural Evolution of Photoactive Yellow Protein Chromophore Revealed by Ultraviolet Resonance Femtosecond Stimulated Raman Spectroscopy

We studied ultrafast structural dynamics of the chromophore of photoactive yellow protein, trans-p-coumaric acid (pCA), using newly developed ultraviolet resonance femtosecond stimulated Raman spectroscopy (UV-FSRS). The UV-FSRS data of the anionic form (pCA(-)) in a buffer solution showed clear spectral changes within 1 PS, followed by a spectrally uniform decay with a time constant of 2.4 ps. The observed spectral change indicates that the structural change occurs in excited pCA(-) from the Franck-Condon state to the S-1 potential minimum in the femtosecond time region. The S-1 Raman spectra exhibit spectral patterns that are similar to the ground-state spectrum, suggesting that pCA(-) yet retains a planar-trans conformation throughout the S-1 lifetime. We concluded that S-1 pCA(-) undergoes a femtosecond in-plane deformation, rather than a substantial C-et=C-et twist. With these femtosecond vibrational data, we discuss possible roles of the initial structural evolution of pCA in triggering the photoreceptive function when embedded in the protein.