No abstract

The light-harvesting chromoprotein of cyanobacteria, allophycocyanin (APC), is studied both experimentally and theoretically. We observe fast kinetics and quantum beats in the pump-probe time-resolved profiles. Fourier analysis of the quantum beats provides eight vibrational frequencies: 186-215, 273-303, 352, 400, 449-469, 508, 625-664, and 840-879 cm -1 . We also measure anisotropy profiles which exhibit two types of kinetics; one has two fast (<1 ps) decay components, and the other almost shows no decay within 2 ps. To make use of the observed data in terms of a molecular description, the crude Born-Oppenheimer adiabatic approximation is applied to define both the electronic and vibrational states of the APC trimer. On the basis of this approximation, we construct a vibronic model, which describes the six chromophores in the APC trimer. With this molecular description, it is possible to study the steady-state optical properties and transient dynamics of the system. We analyze the absorption and CD spectra of the APC trimer, as well as oscillatory behaviors appearing in the pump-probe profiles and kinetics observed in the femtosecond anisotropy profile. We obtain a coherent set of properties of the potential energy surfaces of the system. On the basis of the vibronic model and using these molecular properties, we have simultaneously calculated the absorption and CD spectra in the spectral region 600-700 nm of the APC trimer. We have also simulated IC rate constant as a function of the electronic energy gap, electronic coupling constant, and promoting mode. Photorelated biosystems like the APC trimer may achieve an efficient IC by adjusting the electronic energy gap, promoting mode and vibronic coupling constant due to the breakdown of adiabatic approximation.