Preferential incorporation of coloured-carotenoids occurs in the LH2 complexes from non-sulphur purple bacteria under carotenoid-limiting conditions.

The effect of growing Rhodopseudomonas (Rps.) acidophila and Rps. palustris in the presence of different concentrations of the carotenoid (Car) biosynthetic inhibitor diphenylamine (DPA) has been investigated. Growth with sub-maximal concentrations of DPA induces Car limitation. The exact response to DPA is species dependent. However, both Rps. acidophila and Rps. palustris respond by preferentially incorporating the limiting amount of coloured Cars into their LH2 complexes at the expense of the RC-LH1 complexes. As inhibition by DPA becomes more severe there is an increase in the percentage of Cars with reduced numbers of conjugated C=C bonds. The effect of this changed Car composition on the structure and function of the antenna complexes has been investigated using absorption, fluorescence, CD and Raman spectroscopies. The results show that although the presence of Car molecules is important for the stability of the LH2 complexes that the overall native structure can be maintained by the presence of many different Cars.