Spectroscopic implications of the electron donor-acceptor effect in the photoactive yellow protein chromophore

The importance of the donor-acceptor push-pull system in the photoabsorption of the trans p-coumaric acid, the cofactor within the photoactive yellow protein and other xanthopsins, has been investigated. We recorded gas-phase absorption spectra and performed high-level quantum chemical calculations of three chromophore models, namely, the deprotonated trans ortho-, meta- and para-methyl coumarates. The ortho and para isomers, which have the electron-donating phenoxy oxygen and the electron-withdrawing acyl group in conjugation, present absorptions in the high-energy region of the visible spectrum, that is, in the interval of wavelengths in which the photoactivity of the xanthopsins is observed. On the other hand, the meta isomer, in which the conjugation between the phenoxy and acyl groups is disrupted, exhibits a significantly shifted maximum and presents no absorption in the region from blue to ultraviolet A. It is found that the push-pull system in the trans p-coumaric acid is critical for the wavelength and the intensity of its photoabsorption. Absorption spectra were also measured in methanol and showed an appreciable hypsochromic effect. Linear response calculations within the formalism of the approximate coupled cluster singles and doubles CC2 model and time-dependent DFT using the functional CAM-B3LYP provided insights into the relevant processes of excitation and aided to the interpretation of the experimental results. There is good agreement between theory and experiment in the description of the gas-phase absorption spectra of the considered chromophore models. Differential density plots were used to predict the effect of hydrogen-bonded amino acids to the trans p-coumaric acid on the protein tuning of this chromophore.