Electronic structure of the two isomers of the anionic form of p-coumaric acid chromophore
A theoretical study of the electronic structure of the photoactive yellow protein (PYP) model chromophore, p-coumaric acid (pCA), is presented. Electronically excited states of the phenolate and carboxylate isomers of the deprotonated pCA are characterized by high-level ab initio methods including CASPT2, EOM-CCSD and CC3. We found that the two isomers have distinctly different patterns of ionization and excitation energies. Their excitation energies differ by more than 1 eV, in contradiction to the experimental report [Rocha-Rinza et al, J. Phys. Chem. A v. 113, 9442 (2009)]. The calculations confirm metastable (autoionizing) character of the valence excited states of both phenolate and carboxylate isomers of pCA- in the gas phase. The type of resonance is found to differ between the two forms. In the phenolate, the excited state lies above the detachment continuum (a shape resonance). However, in the carboxylate the excited pi-pi* state lies below the pi-orbital ionization continuum, but is above the states arising from ionization from three other orbitals (Feshbach resonance). The computed oscillator strength of the bright electronic state in the phenolate is higher than in the carboxylate, in agreement with Huckel's model predictions. The analysis of photofragmentation channels shows that the most probable products for the methylated derivatives of the phenolate and carboxylate forms of pCA- are CH3, CH2O and CH3, CH2O, CO2, respectively, thus suggesting an experimental probe that may discriminate between the two isomers.