Ultrafast photoisomerization of photoactive yellow protein chromophore analogues in solution: Influence of the protonation state

We investigate solvent viscosity and polarity effects on the photo-isomerization of the protonated and deprotonated forms of two analogues of the photoreactive yellow protein (PYP) chromophore. These are trans-p-hydroxybenzlidene acetone and trans-p-hydroxyphenyl cinnamate, studied in solutions of different polarity and viscosity at room temperature, by means of femtosecond fluorescence up-conversion. The fluorescence lifetimes of the protonated forms are found to be barely sensitive to solvent viscosity and to increase with increasing solvent polarity. In contrast, the fluorescence of the deprotonated forms are significantly slowed down in viscous media and accelerated in polar solvents. These results elucidate the dramatic influence of the protonation state of the PYP chromophore analogues on their photoinduced dynamics. The viscosity and polarity effects are, respectively, interpreted in terms of different isomerization coordinates and charge redistribution in S-1. A trans-to-cis isomerization mechanism involving mainly the ethylenic double-bond torsion and/or solvation is proposed for the anionic forms, whereas concerted intramolecular motions are proposed for the neutral forms.