Assessing Micellar Interaction and Growth in Detergent Solutions Used to Crystallize Integral Membrane Proteins

Integral membrane proteins are solubilized in aqueous solutions by the addition of detergent, creating protein−detergent complexes (PDCs), which can then be crystallized. Interactions between the detergent moieties of PDCs contribute significantly to their crystallization behavior. Interaction forces can be quantified using the second osmotic virial coefficient (B 22 ). The B 22 behavior of protein-free detergent micelles is a good predictor of the behavior of the corresponding PDCs under similar conditions, suggesting that detergent B 22 measurements can be used as a screening tool when crystallizing PDCs. However, if the micelle size varies, B 22 measurements will not accurately reflect micelle−micelle forces. We therefore examined micelle size in a model detergent system, using small-angle X-ray scattering and static and dynamic light scattering, assessing the effects of temperature, detergent concentration, and precipitant on B 22 and micelle size. In the absence of poly(ethylene glycol) (PEG), decreases in B 22 principally reflect increases in micelle−micelle attractive forces and do not reflect significant changes in micelle size. In the presence of PEG, the apparent hydrodynamic radius of detergent micelles shows a similar dependence upon micelle concentration as in the absence of PEG, suggesting that PEG does not effect significant changes in micelle size but rather acts by enhancing interaction forces between micelles.