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

Abstract

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 S1 potential minimum in the femtosecond time region. The S1 Raman spectra exhibit spectral patterns that are similar to the ground-state spectrum, suggesting that pCA– yet retains a planar-trans conformation throughout the S1 lifetime. We concluded that S1 pCA– undergoes a femtosecond in-plane deformation, rather than a substantial Cet═Cet twist. With these femtosecond vibrational data, we discuss possible roles of the initial structural evolution of pCA in triggering the photoreceptive functio...