Reaction-coordinate tracking in the excited-state deactivation of the photoactive yellow protein chromophore in solution

The earliest steps of the photoactive yellow protein (PYP) photocycle are known to involve cis–trans photoisomerization of its chromophore, the deprotonated trans-p-coumaric acid. In aqueous solution PYP chromophore analogues bearing the same thioester function as in the protein however do not isomerize and restore the initial trans configuration via a short-lived charge-transfer intermediate. In order to gain further insight into the nature of this non-radiative process, we report the first study by femtosecond transient absorption spectroscopy of a ketone derivative of the trans-p-coumaric acid, pCK−, which has been used as a PYP chromophore model in recent theoretical studies. While the transient spectra of pCK− in basic aqueous solution are similar to those of the thioester derivatives, we identify in decanol an additional deactivation route. It involves the formation of photoproduct with a lifetime of a few seconds, which we attribute to the cis isomer. These results are discussed in terms of both static and dynamic solvent effects on the excited-state deactivation.