Amphiphilic block copolymers significantly influence functions of bacteriorhodopsin in water

Abstract

This paper investigates the effects of a macromolecular amphiphile poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO–PPO–PEO) on a photoresponsive membrane protein, bacteriorhodopsin (BR). After incubation of BR in EO₂₃–PO₆₅–EO₂₃ (P123) solution, BR maintained its function as a light-driven proton pump; however, the rate of proton uptake and lifetime of the M intermediate in the photocycle of BR upon illumination were, under appropriate conditions, prolonged by about three orders of magnitude compared with that of native BR, even at neutral pH. Measurements using circular dichroism spectroscopy and dynamic light scattering indicated that BR molecules were still in a trimer state after treatment with the copolymers. This is quite different to BR, which showed a much slower photoresponse during drying. The BR–P123 assemblies did not exhibit significantly different photoresponsive behavior with changes to the water content, which implied that in the case of dried BR films or dried BR–polymer films, the elongation of the M decay may be caused not by lack of water molecules necessary for proton transfer, but by protein immobilization. Determination of the critical micelle concentration of P123 with and without BR revealed that this prolongation effect is closely related to the formation of micelles. The above phenomenon was also observed with 6 other Pluronic copolymers. In solutions of small molecular detergents, such as Triton X-100, the photoresponse of BR was prolonged as well; the extent of prolongation was, however, much less than in solutions of macromolecular amphiphiles. The formation of a local polymer coating due to self assembly of the copolymer and protein molecules might be responsible for this very significant prolongation effect, which is beneficial for the potential application of BR as an information material.