Oligomerization of light-harvesting I antenna peptides of Rhodospirillum rubrum

We investigated the oligomerization of the core light-harvesting complex (LH1) of Rhodospirillum rubrum from the separated alpha beta BChl(2) subunits (B820) and the oligomerization of the B820 subunit from its monomeric peptides. The full LH1 complex was reversibly associated from B820 subunits by either varying the temperature in the range 277-300 K or by varying the detergent concentration in the buffer from 0.36 to 0.52% n-octyl-beta-D-glucopyranoside. Temperature-induced transition measurements showed hysteresis: raising the temperature induced dissociation of B873 directly into B820 subunits whereas upon recooling an intermediate spectral form was observed with an absorption maximum located around 850 nm. This intermediate form was also observed in detergent-induced transitions. It is speculated that the B850 form is a small aggregate of B820, for instance a dimer. Additionally, during a temperature-mediated transition at low detergent concentration, a set of spectral forms with maxima slightly blue-shifted from 873 nm were observed, possibly due to opened rings with one or only a few alpha beta BChl(2) units missing. The temperature-induced transition of LH1 is discussed in terms of a simple assembly model. It is concluded that a moderately cooperative assembly explains the formation of small aggregates of B820 as well as of incomplete rings. Furthermore, the B820 subunits were reversibly dissociated into the monomeric B777 form by increasing either the temperature or the detergent concentration. Estimations of the enthalpy and entropy changes for the dimeric association reaction of B777 into B820 yielded an enthalpy change of -216 kJ mol(-1) and an entropy change of -0.59 kJ mol(-1)K(-1), at a detergent concentration of 0.8% n-octyl-beta-D-glucopyranoside.