Antenna-size dependent hyperchromism of the Qy absorption band of chlorosomal oligomeric bacteriochlorophyll (BChl) c antennae of green bacteria

Abstract

We have previously shown that antenna pigment oligomerization being universal to lattices of any geometrical type is a biologically expedient strategy for light harvesting and have suggested several principles for designing optimal oligomeric antennae. In this contribution we examined the feasibility of some of the predicted design principles in chlorosomal oligomeric BChl c antennae of green bacteria, belonging to two different families Chloroflexaceae and Oscillochloridaceae. These species are able to grow in a wide range of light intensities significantly changing the BChl c antenna size. It thus offers remarkable model systems with varying antenna sizes in which even more stringent requirements for optimization of larger antenna can be studied [Mol. Biol., 18 (1984) 1354]. The linear optical spectra of chlorosomes isolated from the cell cultures with different BChl c contents were shown to be antenna-size dependent. Strong hyperchromism effects in the Qy absorption band manifesting the changes in the BChl c antennae structure of both families of green bacteria upon variations in the antenna size were observed. The antenna size dependence of hyperchromism has been rationalized by the exciton interactions between non-degenerate excited states in the neighboring BChl c pigments, in addition to interactions between their degenerate excited states in the tubular chlorosomal antenna models