Impact of Photon Absorption on the Electronic Properties of p-Coumaric Acid Derivatives of the Photoactive Yellow Protein Chromophore

The changes in the electrostatic properties, between the ground and excited state, of thiomethyl p-coumaric acid (TMpCA) and its sterically hindered derivative, thiomethyl-7-hydroxy-coumarin-3-carboxylic acid (TM7HC), have been determined at 77 K using Stark spectroscopy, to better understand the origin of the photoinduced charge motion observed in these chromophores in the native photoactive yellow protein (PYP) environment. Excitation of the anionic chromophores produce changes in the permanent dipole moment (j¢Ì bj) of 25 (TMpCA-a) and 15 D (TM7HC-a), which are significantly larger than the j¢Ì bj's measured in the neutral species: 9 (TMpCA-n) and 6 D (TM7HC-n). However, the similarity of the j¢Ì bj's between the anions and the corresponding de-protonated cofactors in the native protein environment implicates the intrinsic electronic properties of the chromophore for the photoreactivity of the initially excited species in the PYP photocycle. Furthermore, the results for the neutral species suggest that, if the cofactor in the protein were to be protonated in the ground state, photon absorption would induce a much smaller degree of charge motion. The implications of these distinct differences in the measured electrostatic properties are discussed in the context of facilitating and/or preventing the twisting of the chromophore and its relevance to the PYP photocycle. Ab initio (time dependent density functional, TDDFT) calculations on these systems yield quite accurate values for the electronic transition energies, and the molecular orbitals that contribute to these transitions provide an insight into the reactivity of the excited-state species. However, the changes in the permanent dipole moments associated with these transitions are underestimated, particularly in the anions, both from Configuration Interactions - Singles and Restricted Open-shell Kohn-Sham calculations.