One-step purification of delipidated Bacteriorhodopsin by aqueous-three-phase system from purple membrane of Halobacterium

• Bacteriorhodopsin (BR) of Halobacterium was easily isolated by using PPG–PEG–phosphate aqueous three-phase system.
• Lipids content of isolated BR was significantly reduced by employing CHAPS detergent in the three-phase system.
• Delipidated BR of high purity was recovered from the interface between PEG and phosphate phase with high yield.
• Delipidated BR film formed on ITO electrode demonstrated an enhanced photocurrent.

Bacteriorhodopsin (BR), the only protein in the purple membrane (PM) of certain extreme halophilic microorganisms, functions as a light-driven proton pump using light energy to generate transmembrane proton gradient for ATP synthesis. BR naturally aggregates in a highly ordered two-dimensional hexagonal array of trimers in the PM of Halobacterium. The BR in the isolated PM can be employed to generate a photocurrent in a photocell. However, delipidated BR (deBR) has been reported be more efficient than BR for photocurrent generation. In the present work, detergent CHAPS was included in anaqueous three-phase system (A3PS) to remove the lipids in the out layer of the BR trimer during the preparation of deBR. A3PS that consisted of polypropyleneglycol (PPG), polyethyleneglycol (PEG), and phosphate buffer purified deBR directly from the cell lysate of Halobacterium salinarum with a recovery yield of 89.7%. CHAPS along with the contaminant bacterioruberin pigment were partitioned into the top PPG-rich phase while deBR was mainly located at the interface between PEG-rich phase and the lower phosphate phase. After further purification by using ultrafiltration to remove PEG, the purified deBR when immobilized on indium tin oxide (ITO) glass was able to generate 60% higher photocurrent density.