Molecular Beam and ab Initio Studies of Photoactive Yellow Protein Chromophores: Influence of the Thioester Functionality and Single Bond Rotation

We report on exptl. high-​resoln. spectroscopic studies in combination with ab initio computational studies that investigate the excited-​state dynamics of methyl-​4-​hydroxycinnamate thioester and (5-​hydroxyindan-​(1E)​-​ylidene)​acetic acid, derivs. of the photoactive yellow protein (PYP) chromophore. These studies aim to elucidate (a) how the thioester moiety influences the photophysics and photochem. of the p-​coumaric acid chromophore and (b) to what extent rotation of the single bond adjacent to the Ph ring is involved in the decay dynamics of the electronically excited states. The exptl. studies show that sulfur substitution leads to broad, unstructured excitation spectra that contrast sharply with the well-​resolved spectra of compds. with an oxygen-​based ester. Furthermore, internal conversion to the lower-​lying nπ* state is absent. The absence of this decay channel is rationalized by quantum-​chem. calcns. that reveal that in the nπ* state of the thio compds. the mol. exhibits a large out-​of-​plane "kink" at the sulfur atom. Franck-​Condon simulations of the excitation spectra of the V(ππ*) state highlight the activity of various vibrational modes in the neutral chromophore and indicate that upon sulfur substitution internal conversion to the ground state occurs at a significantly higher rate. The similarities obsd. in the excitation spectra and decay dynamics of the locked and unlocked compds. suggest that in the present expts. single-​bond torsion does not show up prominently. The conclusion that for the isolated mol. double-​bond torsion is dominating the excited-​state dynamics is tentatively confirmed by the quantum-​chemical calculations.