Solvent Effects on the Absorption Spectra of the <i>para</i>-Coumaric Acid Chromophore in Its Different Protonation Forms

The effects of the solvent and protonation state on the electronic absorption spectrum of the <i>para</i>-coumaric acid (pCA), a model of the photoactive yellow protein (PYP), have been studied using the ASEP/MD (averaged solvent electrostatic potential from molecular dynamics) method. Even though, in the protein, the chromophore is assumed to be in its phenolate monoanionic form, when it is found in water solution pH control can favor neutral, monoanionic, and dianionic species. As the pCA has two hydrogens susceptible of deprotonation, both carboxylate and phenolate monoanions are possible. Their relative stabilities are strongly dependent on the medium. In gas phase, the most stable isomer is the phenolate while in aqueous solution it is the carboxylate, although the population of the phenolate form is not negligible. The s-<i>cis</i>, s-<i>trans</i>, <i>syn</i>, and <i>anti</i> conformers have also been included in the study. Electronic excited states of the chromophore have been characterized by SA-CAS­(14,12)-PT2/cc-pVDZ level of theory. The bright state corresponds, in all the cases, to a π → π* transition involving a charge displacement in the system. The magnitude and direction of this displacement depends on the protonation state and on the environment (gas phase or solution). In the same way, the calculated solvatochromic shift of the absorption maximum depends on the studied form, being a red shift for the neutral, carboxylate monoanion, and dianionic chromophores and a blue shift for the phenolate monoanion. Finally, the contribution that the solvent electronic polarizability has on the solvent shift was analyzed. It represents a very important part of the total solvent shift in the neutral form, but its contribution is completly negligible in the mono- and dianionic forms