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 (H-v) 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 H-v 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 between different advanced ab initio methods imply that second-order H-v calculations can provide sufficiently accurate spectral data for biological chromophores in their native environments. This feature is significant because the second-order HU method is much easier and of lower computational cost to implement than other high level approaches, such as the MRCCSD, EOM-CCSD, and sa-CASMP2 methods. Additionally, we also discuss the hydrogen-bonding interactions between the phenolate anion and the PYP and analyze the charge distributions for the full chromophore in PYP.