Antenna organization in green photosynthetic bacteria. 2. Excitation transfer in detached and membrane-bound chlorosomes from Chloroflexus aurantiacus

Abstract

The photosynthetic antenna of Chloroflexus aurantiacus includes bacteriochlorophyll (BChl) c740 and BChl a792, both of which occur in chlorosomes, and B808-866 (containing BChl a808 and BChl a866), which is membrane-located (subscripts refer to near-infrared absorption maxima in vivo). BChl a792 is thought to mediate excitation transfer from BChl c740 to BChl a808. Lifetimes of fluorescence from BChl c740 and BChl a792 were measured in isolated and membrane-bound chlorosomes in order to study energy transfer from these pigments. In both preparations, the lifetime of BChl c740 fluorescence was at or below the instrumental limit of temporal resolution (about 30-50 ps), implying extremely fast excitation transfer from this pigment. Attempts to disrupt excitation transfer from BChl c740, either by conversion of part of this pigment to a monomeric form absorbing at 671 nm or by partial destruction of BChl a792 by oxidation with K3Fe(CN)6, had no discernible effects on the lifetime of BChl c740 fluorescence. Most (usually > 90%) of the fluorescence from BChl a792 decayed with a lifetime of 93 .+-. 21 ps in membrane-attached chlorosomes and 155 .+-. 22 ps in isolated chlorosomes at room temperature. Assuming that the only difference between these preparations is the occurrence of excitation transfer from BChl a792 to B808-866, a 41% efficiency was calculated for this process. This value is lower than the 60% efficiency of excitation transfer from BChl c740 to B808-866 determined by comparison of fluorescence excitation and absorption spectra of membranes with attached chlorosomes and compares even less favorably with the 100% efficiency of excitation transfer found in whole cells by the same method. Furthermore, measurements at 77 K (on different samples) did not show an increased lifetime of BChl a792 fluorescence when isolated chlorosomes were compared with membrane-bound chlorosomes. These results imply either that BChl a792 is not an obligatory intermediate in energy transfer from BChl c740 to B808-866 or (more probably) that chlorosome isolation introduces new processes for quenching fluorescence from BChl a792.