Spontaneous emulsification of detergent solubilized reaction center: protein conformational changes precede droplet growth

We show that ionization of a pH-sensitive detergent, DDAO, bound to a bacterial photosynthetic reaction center (RC), induces reversible emulsification of the protein over a narrow acidic pH range, resulting in stable micrometric RC-surfactant droplets. Electrostatic interactions play a key role in the phase separation process, as shown by a systematic analysis of ionic strength effects and by the use of a cationic detergent (DTAB) that mimics, also at basic pH, the ionized form of DDAO. Under all the conditions we tested, phase segregation seems to be coupled to a 15 nm blue-shift of the low energy absorption band of the primary electron donor P of the RC. This spectral change strongly suggests that surfactant-protein interactions leading to phase separation also induce a conformational transition of the RC. Time-resolved visible-NIR spectra recorded during the emulsification process reveal that the conformational change probed by P spectral shift is always faster than droplets formation. In line with these observations, phase segregation affects charge recombination kinetics following RC photoexcitation, as well as electron transfer from soluble cytochrome c2 to the photoxidized primary donor P+.