Electronic structure of the two isomers of the anionic form of p-coumaric
D. Zuev, K.B. Bravaya, T.D. Crawford, R. Lindh, and A.I. Krylov
J. Chem. Phys. 134, 034310
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
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.
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Understanding photoactive proteins in gas phase and in realistic environments