Theoretical Studies on Excited States of a Phenolate Anion in the Environment of Photoactive Yellow Protein

In an effort to understand the dynamical process in the photocycle of photoactive yellow protein (PYP), we perform ab initio effective valence shell Hamiltonian (Hv) calculations for the low-lying excited states in two simple models of the PYP chromophore:  (i) a phenolate anion surrounded by seven charged amino acids, which are modeled as point charges and (ii) to consider the effect of hydrogen bonding of the Try 42 and Glu 46 residues, a phenolate anion with two hydrogen-bonded water molecules embedded in the same point-charge field as in the previous model. Second-order Hv calculations for the isolated phenolate anion are in good agreement with calculations using the EOM-CCSD and sa-CASMP2 methods, while the hydrogen bonds exert a minor influence for the lower excited states of the phenolate anion in the environment of the PYP chromophore. The electrostatic enviroment of PYP provides the dominant stablization to shift the lowest singlet excited state below the lowest ionization potential. Comparisons ...