Abstract

The photosynthetic apparatus of heliobacteria is located in a single pigment- protein complex that contains approximately 50 bacteriochlorophyll g pigments, including those constituing the primary electron donor dimer P798. It constitutes a unique model in which light harvesting and photochemistry takes place within the same protein. The heterogeneously broadened, ~40 nm fwhm Q_Y band of the complex is located around 790 nm (Fig. 1). Predominant excitation of P798 is precluded by the overlapping absorption of the other pigments. Therefore, energy transfer and electron transfer processes cannot be studied seperately. Here we report the first transient absorption measurements with femtosecond resolution of the spectral evolution within the Q_Y band upon direct excitation in the Q_Y band, at the red (812 nm, 60 fs) or blue (770 nm, 30 fs) edge. Polarized measurements were performed at room temperature and at cryogenic temperature. Low excitation energies were employed to avoid singlet-singlet annihilation effects (~0.3 exciations/RC). The ensemble of our data is consistent with previous experiments using excitation at higher-energy transitions [1] and with experiments using much longer pulses [2], but they resolve the energy transfer between pools of pigments and furthermore they reveal unexpected competition between energy transfer and electron transfer.

© 1996 Optical Society of America