Isomerization process in the native and denatured photoactive yellow protein probed by subpicosecond absorption spectroscopy

The Photoactive Yellow Protein (PYP) is the blue-light photoreceptor that presumably mediates negative phototaxis of the purple bacterium Halorhodospira halophila. Its chromophore is the deprotonated trans-p-cowmaric acid covalently linked, via a thioester bond, to the unique cystein residue of the protein. Like for rhodopsins, the trans to cis isomerization of the chromophore is shown to be the first overall step of the PYP photo-cycle, but the reaction path that leads to the formation of the cis isomer is not clear. From time resolved spectroscopy measurements on native PYP in solution, it comes out that the excited state deactivation involves a series of fast events on the subpicosecond and picosecond timescales correlated to the chromophore reconfiguration. On the other hand, chromophore H-bonding to the nearest amino acids is shown to play a key role in the trans excited state decay kinetics. In an attempt to evaluate further the role of the mesoscopic environment in the photophysics of PYP, this chapter makes a comparative study of the native and denatured PYP. The excited-state relaxation path and kinetics are monitored by subpicosecond time-resolved absorption and gain spectroscopy.