Influence of hydrophobic micelle structure on crystallization of the photosynthetic RC-LH1-PufX complex from Rhodobacter blasticus.

Small angle X-ray scattering (SAXS) experiments are performed on two non-ionic surfactants, the dodecyl ß-maltoside (DDßM) and the propyl(bi)cyclohexyl α-maltoside (PCCαM), a maltoside derivative containing a rigid bicyclohexyl group as hydrophobic chain, in order to compare the influence of both hydrophobic moiety structure and anomeric form on micelle form factors and intermicellar interactions relevant for membrane protein crystallization. Density and refractive index measurements were performed in order to determine volumetric and optical properties of surfactants, essential for determination of micelle molar masses by both SAXS and SEC-MALLS. SAXS form factors were analyzed by Guinier approximation and inverse Fourier transformation, to obtain the radius of gyration (RG) and the pair distribution function (P(r)) of each surfactant. Form factor model fitting was also performed to describe the shape and the assembly of both surfactant micelles. Finally, second virial coefficients were measured at different percentages of polyethylene glycol 3350, in order to correlate surfactant intermicellar interactions and RC-LH1-PufX phase diagram. It is thus found that while size, shape, and dimensions of micelles are slightly similar for both surfactants, their molar mass and aggregation number differ significantly. PCCαM are more densely packed than DDßM, which reflects (1) an increase in van der Waals contacts between PCCαM hydrophobic chains in the micelle bulk and (2) a supplementary intermicellar attraction compared to DDßM. Finally addition of PEG, which induces a depletion attraction, decreases the solubility of the RC-LH1-PufX complex in PCCαM.