Protein-bound chromophores astaxanthin and phytochromobilin: excited state quantum chemical studies

We present an overview of excited state quantum chemical calculations aimed at elucidating controversial issues regarding the photochemistry of the protein-bound chromophores astaxanthin and phytochromobilin. In particular, we show how the application of time-dependent density functional theory and other single-reference quantum chemical excited state methods have contributed to shed new light on the origin of the >0.5 eV bathochromic shift of the electronic absorption by the carotenoid astaxanthin in the protein macromolecular complex crustacyanin, and the mechanism for C15-Z,syn --> C15-E,anti isomerization of the tetrapyrrole phytochromobilin that underlies the photoactivation of the plant photoreceptor phytochrome. Within the approximation that exciton coupling is neglected, the calculations on astaxanthin provide support for the notion that the bathochromic shift, which is responsible for the slate-blue coloration of lobster shell, is due to polarization rather than a conformational change of the chromophore in the protein-bound state. Furthermore, the polarization is attributed to a hydrogen-bonded protonated histidine residue. The calculations on phytochromobilin, in turn, suggest that a stepwise C15-Z,syn --> C15-E,syn (photochemical), C15-E,syn --> C15-E,anti (thermal) mechanism is much more favorable than a concerted, fully photochemical mechanism, and that neutral forms of the chromophore are much less likely to photoisomerize than the parent, protonated form. Accordingly, the calculations indirectly support the view that the photoactivation of phytochrome does not involve a proton transfer from the chromophore to the surrounding protein.