Oligomerization of light-harvesting I antenna peptides of Rhodospirillum rubrum

We investigated the oligomerization of the core light-harvesting complex (L H1) of Rhodospirillum rubrum from the separated alpha beta BChl(2) subunits (B820) and the oligomerization of the B820 subunit from its monomeric pept ides. 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 det ergent concentration in the buffer from 0.36 to 0.52% n-octyl-beta -D-gluco pyranoside. Temperature-induced transition measurements showed hysteresis: raising the temperature induced dissociation of B873 directly into B820 sub units whereas upon recooling an intermediate spectral form was observed wit h an absorption maximum located around 850 nm. This intermediate form was a lso observed in detergent-induced transitions. It is speculated that the B8 50 form is a small aggregate of B820, for instance a dimer. Additionally, d uring a temperature-mediated transition at low detergent concentration, a s et of spectral forms with maxima slightly blueshifted from 873 mn were obse rved, possibly due to opened rings with one or only a few a BChl2 units mis sing. The temperature-induced transition of LH1 is discussed in terms of a simple assembly model. It is concluded that a moderately cooperative assemb ly explains the formation of small aggregates of B820 as well as of incompl ete rings. Furthermore, the B820 subunits were reversibly dissociated into the monomeric B777 form by increasing either the temperature or the deterge nt concentration. Estimations of the enthalpy and entropy changes for the d imeric 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 deter gent concentration of 0.8% n-octyl-p-D-glucopyranoside.