Excitation energy transfer in chlorosomes of green bacteria: theoretical and experimental studies

Abstract

A theory of excitation energy transfer within the chlorosomal antennae of green bacteria has been developed for an exciton model of aggregation of bacteriochlorophyll (BChl) c (d or e). This model of six exciton-coupled BChl chains with low packing density, approximating that in vivo, and interchain distances of approximately 2 nm was generated to yield the key spectral features found in natural antennae, i.e., the exciton level structure revealed by spectral hole burning experiments and polarization of all the levels parallel to the long axis of the chlorosome. With picosecond fluorescence spectroscopy it was demonstrated that the theory explains the antenna-size-dependent kinetics of fluorescence decay in chlorosomal antenna, measured for intact cells of different cultures of the green bacterium C. aurantiacus, with different chlorosomal antenna size determined by electron microscopic examination of the ultrathin sections of the cells. The data suggest a possible mechanism of excitation energy transfer within the chlorosome that implies the formation of a cylindrical exciton, delocalized over a tubular aggregate of BChl c chains, and Forster-type transfer of such a cylindrical exciton between the nearest tubular BChl c aggregates as well as to BChl a of the baseplate.

Functioning of oligomeric‐type light‐harvesting antenna

“…For our set of parameters we obtained (KAA) 4 = 17.5-18 ps; the time of energy transfer to BChl a, (q KA " )4 = 17.5-22 ps. Recently we have obtained similar linear dependence z(<p>) for the green bacterium C.aurantiacus [10], with rate constants being estimated as (KAA) 1 = 27.4 ps, and (q KA ~ )-1 = 13.7 ps.…”

“…The theory of excitation energy transfer within the chlorosome was developed for our tubular aggregate model [8] to describe antenna-size-dependent kinetics of fluorescence decay in chlorosomal antennae [10]. We used the three-dimensional model of the chlorosome shown in Fig.1.…”

“…Thus, the data suggest a possible mechanism of excitation transfer within the chlorosome, based on a new original exciton model of BChl c aggregation [8,10].…”

“…the exciton level structure with intensive higher levels [6] and polarization of all levels parallel to the long axis of the chlorosome [4,5]. The theory of excitation energy transfer within an oligomeric-type light-harvesting antenna was developed for the first time [10]. In this work, we demonstrated that the theory developed …”

“…The different size of chlorosomal antennae was adjusted by inhibiting the formation of BChl c by gabaculine [10]. Electron microscopy studies were made with a Hitachi-12 electron microscope.…”