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This chapter describes the preparation of two bacterial reaction centers (RCs) for crystallization. An essential innovation in preparing type II crystals from membrane bound proteins such as bacteriorhodopsin and photosynthetic RCs was introduced when small amphiphilic molecules were added to the proteindetergent complex. These amphiphiles are too small to form micelles themselves, inserted into existing detergent micelles however, they facilitate the nucleation process by several effects: (1) they create mixed detergent-amphiphile micelles with a reduced size due to a changed critical micellar concentration. (2) The amphiphiles allow a rearrangement of the detergent molecules in such a way that the detergent-amphiphile complex accommodates unevenesses of the protein surface better than the detergent alone. (3) The polar headgroups of the amphiphilic molecules are relatively small and allow more of the protein's polar groups to interact with neighboring protein molecules. The procedures developed for the crystallization of soluble proteins can also be applied to membrane proteins, for example, vapor diffusion and dialysis with salts or polymers as precipitating agents.
