DETERGENT MICELLE STRUCTURE AND MICELLE-MICELLE INTERACTIONS DETERMINED BY SMALL-ANGLE NEUTRON-SCATTERING UNDER SOLUTION CONDITIONS USED FOR MEMBRANE-PROTEIN CRYSTALLIZATION

We have characterized micelle structure and intermicelle interaction for the detergents lauryldimethylamine N-oxide, LDAO, and n-octyl-beta-D-glucoside, OG, under conditions used for protein crystallization using SANS. We found that LDAO and OG micelles differ significantly in size, sensitivity to heptanetriol, and nature of intermicelle interactions. Our results suggest that successful crystallization methods can be rationalized in terms of an optimization of micelle size, number density, flexibility of micelle radius of curvature, and suppression of intermicelle interactions. LDAO and OG micelles were found to differ significantly in size and shape. The LDAO micelle was found to be best fit as an ellipsoid with semiaxes of 30.6 and 19.4 Angstrom, while the OG micelle was found to be spherical with a radius of 22.9 Angstrom. The addition of heptanetriol to pure LDAO resulted in the formation of smaller, spherical, mixed micelles with radii in the range 17-21 Angstrom, depending upon conditions. The results suggest that both micelle size and curvature restrictions may contribute to the incompatibility of LDAO for protein crystallization in the absence of additional amphiphiles. The mixed OG-heptanetriol micelle was found to be significantly smaller than that with LDAO, having radii in the range 15-18 Angstrom, depending upon conditions, and exhibited a greater number density increase. Evidence was found for interaction between OG and polyethylene glycol, PEG, that prevents micelle aggregation at high ionic strength and likely contributes to the particular success of PEG as a protein precipitant when OG is used as the solubilizing detergent. These measurements suggest that the chemical constituents in membrane protein crystallization can be manipulated to optimize micelle size, number density, and interparticle interactions.