Chlorosomes of green photosynthetic bacteria are the most amazing example of long-range ordered natural light-harvesting antennae. Chlorosomes are the largest among all known photosynthetic light-harvesting structures (~ 10(4)-10(5)pigments in the aggregated state). The chlorosomal bacteriochlorophyll (BChl)c/d/emolecules are organized via self-assembly and do not require proteins to provide a scaffold for efficient light harvesting. Despite numerous investigations, a consensus regarding the spatial structure of chlorosomal antennae has not yet been reached. In the present work, we studied hyperchromism/hypochromism in the chlorosomal BChlcQ/B absorption bands of the green photosynthetic bacteriumChloroflexus(Cfx.)aurantiacus. The chlorosomes were isolated from cells grown under different light intensities and therefore, as we discovered earlier, they had different sizes of both BChlcantennae and their unit building blocks. We have shown experimentally that the Q-/B-band hyperchromism/hypochromism is proportional to the size of the chlorosomal antenna. We explained theoretically these findings in terms of excitonic intensity borrowing between the Q and B bands for the J-/H-aggregates of the BChls. The theory developed by Gulen (Photosynth Res 87:205-214, 2006) showed the dependence of the Q-/B-band hyperchromism/hypochromism on the structure of the aggregates. For the model of exciton-coupled BChlclinear chains within a unit building block, the theory predicted an increase in the hyperchromism/hypochromism with the increase in the number of molecules per chain and a decrease in it with the increase in the number of chains. It was previously shown that this model ensured a good fit with spectroscopy experiments and approximated the BChlclow packing density in vivo. The presented experimental and theoretical studies of the Q-/B-band hyperchromism/hypochromism permitted us to conclude that the unit building block ofCfx.aurantiacuschlorosomes comprises of several short BChlcchains. This conclusion is in accordance with previous linear and nonlinear spectroscopy studies onCfx.aurantiacuschlorosomes.
