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.