The core light-harvesting complex (LH1) of Rhodospirillum rubrum is constituted of multiple heterodimeric subunits, each containing two transmembrane polypeptides, α and β. The detergent octylglucoside induces the stepwise dissociation of LH1 into B820 (an αβ dimer) and B777 (monomeric polypeptides), both of which still retain their bound bacteriochlorophyll molecules. We have investigated the absorption properties of B820 as a function of temperature, whereby a spectral population called ‘B851’ has been characterised. We show evidence that it is a dimer of the B820 complex. This may represent an intermediate oligomeric form in the process of the LH1 ring formation, as its existence was predicted from global analysis of the absorption spectra of the LH1/B820 equilibrium [Pandit et al. (2001) Biochemistry 40, 12913–12924]. Stabilisation of this dissociated form of LH1 may help in understanding both the electronic properties and the association process of these integral membrane proteins.
