Photoionization and Electron Radical Recombination Dynamics in Photoactive Yellow Protein Investigated by Ultrafast Spectroscopy in the Visible and Near-Infrared Spectral Region

Photoinduced ionization of the chromophore inside photoactive yellow protein (PYP) was investigated by ultrafast spectroscopy in the visible and near-infrared spectral regions. An absorption band that extended from around 550 to 850 nm was observed and ascribed to solvated electrons, ejected from the p-hydroxycinnamic acid anion chromophore upon the absorption of two 400 nm photons. Global kinetic analysis showed that the solvated electron absorption decayed in two stages: a shorter phase of around 10 ps and a longer phase of more than 3 ns. From a simulation based on a diffusion model we conclude that the diffusion rate of the electron is about 0.8 A2/ps in wild type PYP, and that the electron is ejected to a short distance of only several angstroms away from the chromophore. The chromophore-protein pocket appears to provide a water-similar local environment for the electron. Because mutations at different places around the chromophore have different effect on the electron recombination dynamics, we suggest that solvated electrons could provide a new method to investigate the local dielectric environment inside PYP and thus help to understand the role of the protein in the photoisomerization process.