Purple membrane for conversion of light into electric current

Abstract

Purple membranes synthesised by Halobacterium halobium contain bacteriorhodopsin, a small protein endowed with unusual properties: when illuminated with light in the visible spectrum it pumps protons from the cytoplasmic to the exterior side of the membrane as a result of a series of photocycle-associated conformational changes. We have incorporated this ion pump into a synthetic-membrane system to convert light into electric current. The membrane possesses acceptable mechanical stability and yields stable photocurrents much larger in magnitude than those hitherto reported. The method of membrane preparation essentially involves the fixation of an oriented purple membrane layer between two cation-selective films by means of acrylic acid and acrylamide gel. This composite structure retains photoresponsiveness for prolonged periods without any substantial impariment in functional activity of bacteriorhodopsin.

For the purpose of comparison of photoelectroactivity under different experimental conditions, the eletrical power outputs produced upon illumination by purple membranes non-randomly immobilized on sulphonated polysulphone cation-exchange films have been estimated. It appears that the observed activity of the bacteriorhodopsin proton pump depends on temperature, pH and nature of the salt solution. Immobilization of purple membranes under an electric field does not affect their photoelectroactivity.