Thermodynamics of the oligomerization of LH1 antenna peptides of Rhodospirillum rubrum

Abstract

The isolated LH1 antenna complex of the carotenoidless mutant of Rhodospirillum rubrum, called Rs. rubrum G9+, was reversibly associated from B820 subunits by either varying the temperature in the range of 277 K to 300 K, or by varying the buffer detergent concentration from 0.36% to 0.52% octylglucoside. During transitions that were induced by recooling of the B820 samples or by varying the detergent concentration, an intermediate spectral form was observed with an absorption maximum located around 850 nm. Additionally, during a temperature-mediated dissociation using buffers with low detergent concentrations, a set of spectral forms blueshifted from 873 nm were observed. Estimations for the thermodynamic parameters of the complete association reaction yielded DH = -2.4 103 kJ mol-1 and DS = -6 kJ mol-1K-1. Results are compared with simulated absorption spectra of a set of size-ranged aggregates and with a kinetic assembly model. We conclude that the reassociation of the LH1 ring complex from B820 subunits occurs via the formation of small B820 aggregates whereas dissociation may occur via stepwise dissociation of large uncomplete rings.