Fluorescence and absorption detected magnetic resonance of chlorosomes from green bacteria Chlorobium tepidum and Chloroflexus aurantiacus. A comparative study

A comparative study on the isolated chlorosomes from Chloriflexus aurantiac us, a green filamentous photosynthetic bacterium and Chlorobium tepidum, a green sulfur photosynthetic bacterium, was done by ODMR (optically detected magnetic resonance). Correlation between the results obtained by fluoresce nce and absorption detection is shown to be a sourer of information about t he functional interactions among pigments. Analogies and differences are po inted out between the light-harvesting systems of the two species. Triplet states are easily detected in both bacteria at 1.8 K under steady-state ill umination and are assigned to BChl c, BChl a, and carotenoid molecules. Car otenoids are found to be able to quench BChl a triplet states, but no evide nce of BChl c triplet states quenching by this triplet-triplet transfer mec hanism is found in both systems. Then from the data it appears that some ca rotenoids are in close contact with BChl a molecules. The relevance of this finding to the localization of carotenoids in the chlorosomes is discussed . in Cb, tepidum three different pools of BChl c oligomers connected to BCh l a were found by detection of their triplet state, while only one pool of BChl c was evidenced in Cf. aurantiacus. The latter appears to be unconnect ed, at least at 1.8 K, to BChl a. On the other hand, heterogeneity in the B Chl a triplet population was detected in Cf: aurantiacus. Even though the t wo bacteria show common features in the way the light excitation induces tr iplet formation at low temperature, the detected triplet states show spectr oscopic properties that strongly depend on the system. The results clearly indicate that differences in pigment organization exist both in the fore an d in the baseplate of the chlorosomes from the two different bacteria.