ENERGY-TRANSFER AND PRIMARY CHARGE SEPARATION IN HELIOBACTERIUM-CHLORUM STUDIED BY PICOSECOND TIME-RESOLVED TRANSIENT ABSORPTION-SPECTROSCOPY

Transfer of excitation energy to the reaction center and photo-oxidation of the primary electron donor, P798, were studied in membranes of Heliobacterium chlorum with a time-resolution of 10 ps or less. The absorption difference spectrum of excited antenna pigments was dominated by a bleaching around 805 nm and analysis of the kinetics showed that the life-time of excited antenna BChl g was essentially equal (about 20 ps) for all three spectral BChl g species, BChl g 778, BChl g 793 and BChl g 808. This indicates that the excitation energy rapidly equilibrates within the antenna and that most of the excitations have accumulated on BChl g 808 within a few ps after excitation. The 20-ps decay is attributed to energy transfer to the reaction center. After 100 ps, the excited BChl g had dissappeared and the remaining absorption difference signal showed a bleaching maximum at 798 nm, caused by the oxidation of the primary electron donor, P798. The rate of decay of antenna excited states was somewhat faster in the presence of P798+ than with reduced reaction centers, which is in agreement with earlier measurements that showed that P798+ is an efficient quencher of fluorescence (Deinum, G., Kramer, H., Kleinherenbrink, F.A.M., Aartsma, T.J. and Amesz, J. (1991) Biochim. Biophys. Acta 1058, 339-344). The time-resolved absorption difference spectrum also showed a bleaching around 670 nm, due to reduction of the primary electron acceptor, A(o), which was induced upon excitation at 585 nm, as well as in the Q(y)-band of antenna BChl g. From the kinetics at 670 nm, we conclude that the reduction of A(o) occurs at essentially the same rate as the decay of antenna excitations, indicating that charge-separation is a very rapid process.