Probing the photochemical mechanism in photoactive yellow protein.

Selectively bridged model compounds related to the chromophore in photoactive yellow protein have been synthesized where the single bond adjacent to the benzene ring (bond 1) and where both bond 1 and the adjacent double bond (bond 2) are bridged. They were compared to the nonbridged reference compound regarding their photophysical properties using steady-state and time-resolved fluorescence at various temperatures. Quantum chemical calculations were additionally performed and showed that several conformers are populated in the ground state. The neutral model compounds show that the nonradiative deactivation channel is linked to both single- and double-bond twisting. The relative importance of single-bond twisting is increased for the corresponding deprotonated hydroxy compounds with an enhanced donor character. The simultaneous photochemical activity of both single and double bonds explains the ease of photochemical isomerization in the confined environment of the natural PYP protein and also of the primary step in the vision process in rhodopsin.