Ultrafast geminate electron-radical recombination dynamics in photoactive yellow protein

Photoinduced ionization of the chromophore inside photoactive yellow protein (PYP) was investigated by ultrafast spectroscopy in the visible-near infrared spectral region. A solvated electron absorption-like band was observed that extended from around 550nm to 850nm, centred at 700nm. Simulation of the decay traces of the band with a classic diffusion model indicated that ejected electrons were located an average distance of ~3Å away from the radical centre, and around 40% the solvated electrons were annihilated by geminate recombination. The remaining 60% escaped out of the protein pocket to be annihilated by bulk recombination. This result indicates that the chromophore is in a local environment inside PYP that is only slightly different from the bath solvent.