Excited-state dynamics of the PYP chromophore in solution. Environment and structure effects

摘要

The Photoactive Yellow Protein (PYP) is the photoreceptor responsible for the negative phototaxis of the bacterium Halorhodospira halophila. Its chromophore is the deprotonated trans-4-hydroxycinnamic acid (Fig. 1).covalently linked to the side chain of the unique cysteine (Cys69) residue by a thioester bond 1,2. Under blue-light irradiation, PYP undergoes a photocycle that involves trans-cis isomerization of the chromophore as the first overall step. Although this step includes still debated femto-picosecond processes. the formation in a few nanoseconds of a relaxed cis isomer of the chromophore has been clearly established 3,4. The cis isomer is however not observed in steady-state photolysis of solutions containing isolated derivatives of the chromophore with a thioester-phenyl moiety at the a position of the carbonyl end group (Fig. 1, SC6H5 (pCT-)) 5-7 whereas it is well characterized in the case of analogues with 0- (pCA2-) or NH2 (pCM ) as the substituent 5-8. Our goal is to discriminate the role of the protein environment on the primary molecular steps of the PYP photocycle from that of the chemical structure of the chromophore. For that purpose, we carried out a comparative study of the photoinduced processes in five analogues of the chromophore in solution in several polar solvents by subpicosecond UV-Vis transient absorption-;spectroscopy.