ENERGY-TRANSFER FROM LONG-WAVELENGTH ABSORBING ANTENNA BACTERIOCHLOROPHYLLS TO THE REACTION CENTER

Charge separation was studied in membranes of Rhodopseudomonas viridis and Heliobacterium chlorum upon flash excitation in the bacteriochlorophyll (BChl) Q(y) band. In the first organism the maximum of the Q(y) absorption of the antenna BChl is situated at significantly longer wavelength than that of the primary electron donor, whereas in H. chlorum the same applies to the long-wavelength absorbing form BChl g-808. In both species the photochemical efficiency was independent of the wavelength of excitation throughout the Q(y) absorption region, both at 300 K and 6 K. Upon cooling to 6 K the absolute efficiency decreased by 29 +/- 4% in H. chlorum and by 55 +/- 5% in Rps. viridis. The fluorescence excitation spectrum of Rps. viridis membranes at 6 K showed virtually no contribution by reaction center pigments. This indicates that the efficiency for back transfer of energy from the excited reaction center to the antenna was at most a few percent and may be explained by a strong competition by the much faster rate of charge separation. In the absence of back transfer our results allowed an estimation of the rate constants for energy transfer from neighboring antenna BChls to the reaction center at 6 K, which were found to be (440 ps)-1 for H. chlorum and (1.3 ns)-1 for Rps. viridis. It is shown that these rates are not incompatible with the Forster mechanism of energy transfer if it is assumed that the antenna and reaction center bands are essentially homogeneously broadened. Application of the Forster equation yielded an estimated value of 1.8 nm for the distance between the primary electron donor P-798 and BChl g-808 in H. chlorum.