Effect of detergent alkyl chain length on crystallization of a detergent-solubilized membrane protein:: correlation of protein–detergent particle size and particle–particle interaction with crystallization of the photosynthetic reaction center from Rhodobacter sphaeroides

The effect of detergent alkyl chain length on the crystallization of detergent-solubilized membrane proteins was characterized using the reaction center (RC) from Rhodobacter sphaeroides as a model. Crystallization was performed in the presence of both n-alkyl-β-D-glucopyranosides, C n G, where the alkyl chain length, n, was varied from 8 to 10, and dodecyl-dimethylamine-N-oxide (C 12DAO). A pronounced effect of detergent chain length was found on the ability to crystallize the reaction center. Small-angle neutron scattering measurements (SANS) demonstrated that the variation in the ability to crystallize the reaction center is correlated with changes in the apparent radius of gyration, R g, of the reaction center–detergent particle, reflecting changes in the size of the reaction center and strength of the inter-particle interactions. Specifically, reaction center crystallization in the absence of additional amphiphiles was found to occur only with octylglucoside, C 8G. Crystallization in the presence of longer chain detergents, C 9G, C 10G, C 12DAO, required the use of an additional small amphiphile, heptanetriol (HT). SANS measurements showed that the apparent R g of the reaction center–detergent particle increased with increasing detergent chain length, and that the addition of HT reduces this parameter. The addition of HT was found to impede crystallization in the presence of C 8G. This inhibition could be reversed by increasing C 8G concentration with respect to HT, resulting in the formation of alternate crystal forms. Taken together, these results suggest that detergents and small molecule amphiphiles used in membrane protein crystallization must be chosen to optimize both the size and solubility of the protein–detergent particle. Data for the reaction center suggest that crystallization occurs within a restricted range of size of the protein–detergent complex.