The association state of a detergent-solubilized membrane protein measured during crystal nucleation and growth by small-angle neutron scattering

Reaction centers (RCs) from Rhodobacter sphaeroides R-26 were used as a model for membrane protein macromolecular organization and crystallization. Small-angle neutron-scattering measurements were made during pre-saturation, nucleation, and crystal growth phases of crystallization, using polyethylene glycol, PEG, as the precipitant, and with fully deuterated reaction center protein. Solution 2H 2O/ 1H 2O ratios were adjusted to match the average scattering length density of the detergent, and allow the scattering from the deuterated reaction center to be selectively detected within the detergent and PEG containing mixtures. Neutron-scattering profiles for the detergent-solubilized reaction center in PEG -containing solutions were found not to deviate detectably from that expected for mono-dispersed, noninteracting particles. Analysis of the scattering in terms of discrete aggregates suggests that the reaction center exists predominately in the monomeric form throughout the crystallization process. The absence of detectable (