Purple photosynthetic bacteria contain a highly efficient light-harvesting system of LH2 and LH1 complexes, consisting of peptides, bacteriochlorophyll, and carotenoids, which allow cells to grow photosynthetically under different environmental conditions.
Allochromatium (Alc.) vinosum is a sulphur purple photosynthetic bacterium that was found to produce several different LH2 complex types under different growth and nutritional conditions. These LH2 complexes have been identified as the B800-820, B800-840, and B800-850. All of the LH2 complex types of Alc. vinosum were confirmed as heterogeneous forming part of the basis of the unusual split B800 peak due to two potential B800 binding sites on the alpha peptides. This work produced CD data to suggest that excitonic coupling of the B800 BChl occurs and Monte Carlo simulations produced in conjunction with this work indicated that this would also contribute to the splitting of the band observed. The carotenoids of the spirilloxanthin pathway were identified within all of the LH2 complexes, however, the specific carotenoid composition varied depending on the LH2 complex type.
Unlike most purple photosynthetic bacteria, Alc. vinosum is able to produce LH2 complexes in the absence of carotenoid biosynthesis. Carotenoidless LH complexes are ideal for the process of reconstitution, often used to elucidate structure function relationships within complexes. Carotenoidless LH2 complexes were produced and used for proof of concept reversible dissociation studies. This work identified the B800-850 LH2 complex as a reconstitution candidate for further work
