Localized excitations on the B850a and B850b bacteriochlorophylls in the LH2 antenna complex from Rhodospirillum molischianum as probed by the shiftsof the carotenoid absorption

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 subpicos econd time resolution. The absorption changes could be empirically fit by t wo 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 ch ange in dipole moment upon the 1B(u)(+) <-- ground transition of Car and th at 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 delocali zed excitation over four B850s, and (iii) completely delocalized excitation over the B850 ring predicted the Car shifts with direction (relative magni tude), (i) a blue shift (1) or a red shift (3.3), (ii) a red shift (0.8), a nd (iii) a small red shift (0.2), respectively. Thus. the blue shift and th e red shift components were attributed to localized excitations on B850a an d B850b, respectively. The B850a excitation decayed rapidly (similar to 0.1 ps), whereas the B850b excitation decayed more slowly (similar to 0.4 ps) and then stayed; the former decay was ascribed to singlet-singlet annihilat ion between B850a and B850b, whereas the latter decay to singlet-singlet an nihilation between B850b's. The unique feature of excitation with high phot on density, which preferentially generates the localized excitations throug h multiple excitation, and enhances subsequent singlet-singlet annihilation reactions, is discussed.