Investigations of intermediates appearing in the reassociation of the light-harvesting 1 complex of Rhodospirillum rubrum

We investigated the temperature-mediated reassociation of the B820 subunit of Rs. rubrum to form a light-harvesting 1 complex (LH 1). By combining several spectroscopic techniques with global spectral data analysis fitting, we present evidence for the occurrence of two spectral intermediates that appear during the reassociation process. At high temperatures, halfway the reassociation reaction, a prominent intermediate appears that has an absorption maximum around 850 nm, a fluorescence maximum around 860-867 nm, a high anisotropy (0.3 to 0.4) and a circular dichroism spectrum with three or four bands with alternating signs. At lower temperatures, more towards the end of the reassociation process, a second intermediate tends to appear that has an absorption maximum around 860 nm, a fluorescence maximum around 885 nm, a medium to high anisotropy (0.1 to 0.3) and a circular dichroism spectrum with two bands with alternating signs. The latter circular dichroism spectrum has a blueshifted zero-crossing compared to the spectrum of the LH 1 complex. Both intermediates have the spectroscopic features of a small oligomer. In the Q(y) region, the fluorescence anisotropy of both intermediates slightly increases at longer excitation wavelengths, indicative for energy transfer among the pigments within the intermediate oligomers.