Localized Excitations onthe B850a and B850b Bacteriochlorophylls in the LH2 Antenna Complex from Rhodospirillum molischianum As Probed by the Shifts of the Carotenoid Absorption

摘要

Changes in the carotenoid (Car) 1B_u~+ absorption upon excitation (using a photon density of 10~(15) photons cm~(-2)) of bacteriochlorophyll (BChl) in the LH2 complex from Rhodospirillum molischianum were traced with subpicosecond time resolution. Th absorption changes could be empirically fit by two components showing a blue shift and a red shift in a ratio of 1:3.6. The shifts of Car absorption were explained in terms of interaction between change in dipole moment upon the 1B_u~+ <- ground transition of Car and that upon the Q_y <- ground transition of BChl. Calculations of the shifts for three different types of excitation in the B850 ring of the LH2 complex, i.e., (i) localized excitation on B850a or B850b, (ii) partially delocalized excitation over four B850s, and (iii) completely delocalized excitation over the B850 ring predicted the Car shifts with direction (relative magnitude), (i) and blue shift (1) or a red shift (3.3), (ii) a red shift (0.8), and (iii) a small red shift (0.2), respectively. Thus, the blue shift and the red shift components were attributed to localized excitations on B850a and B850b, respectively. The B850a excitation decayed rapidly (approx 0.1 ps), whereas the B850b excitation decayed more slowly (approx 0.4 ps) and then stayed; the former decay was ascribed to singlet-singlet annihilation between B850a and B850b, whereas the latter decay to singlet-singlet annihilation between B850b's. The unique feature of excitation with high photon density, which preferentially generates the localized excitations through multiple excitation, and enhances subsequent singlet-singlet annihilation reactions, is discussed.