Figure 2.

Solubility of membrane protein/amphipol complexes in aqueous solution. Aliquots from stock solutions of amphipols (5 g/liter in water) were added to purified membrane proteins in detergent solution and the mixtures diluted 10× with detergent-free buffer or with water. After 15 min incubation at 4°C, the solutions were centrifuged for 30 min at 4°C in the A-110 rotor of an Airfuge (Beckman) at 20 psi (1 psi = 6.89 kPa; ≈210,000 × g). The concentration of protein in the supernatant was determined from the absorbance at 564 nm (redox difference spectrum of b₆), 546 nm (BR), 278 nm (OmpF), or 802 nm (RC). (A) Cytochrome b₆f complex. Stock solution was ≈5 μM b₆f complex in 20 mM HG (cmc ≈19.5 mM), 0.1 g/liter EPC, 400 mM NaOH/AP buffer (pH 8.0). Final amphipol concentrations following 10× dilution with water were 0.5 g/liter (open bars) or 0.05 g/liter (solid bars). Control experiments included dilution with a 20 mM HG solution (Hecameg) or with water (buffer) in the absence of amphipols, and dilution with water in the presence of nonderivatized low MW polyacrylate (precursor). (B) Other proteins. Stock solutions: bacteriorhodopsin, ≈0.1 g/liter in 100 mM AP (pH 8.0), ≈10% sucrose, 10 mM OTG (cmc ≈9 mM); OmpF porin, ≈4 g/liter in 0.2% (wt/wt) octyl-POE (≈9.2 mM; cmc ≈7 mM) in the same buffer; reaction center, ≈3 g/liter in 20 mM HG in 20 mM NaOH·Tricine buffer (pH 8.0). Tenfold dilution with 100 mM AP (pH 8.0) to a final amphipol concentration of 0.5 g/liter.