The Wavelength-Dependent Refractive Index Change Associated with the Blue to Pink Membrane Photochemical Conversion in Bacteriorhodopsin

Abstract

Bacteriorhodopsin (BR) is the photosynthetic protein found in the cell membrane of the bacterium Halobacterium salinarium.^1,2 This microorganism thrives in salt brines where the concentration of sodium chloride is roughly four times greater than ocean water. BR’s biological function derives from its unique light-transducing properties whereby the absorption of a photon results in the pumping of a proton from the bacterium’s cytoplasm to its surrounding environment. This action establishes an electrochemical potential which the bacterium harnesses to synthesize metabolic adenosine triphosphate. During the past several decades much has been learned about bacteriorhodopsin’s photochemical and biophysical properties, and the molecular events associated with proton pumping.^3–5 More recently, several groups have demonstrated BR’s efficacy as a new type of optical recording material useful in photonic applications that include real-time holography,^{6, 7} interferometry,⁸ spatial light modulation,^{9, 10} and long term optical data storage.^11–13 All of the latter exploit BR’s inherent property of photochromism whereby a material under proper illumination reversibly changes its optical transmission properties.¹⁴ Although numerous synthetic photochromes have been reported over the last century, few offer the intrinsic advantages found in BR. These include high light sensitivity in both the forward and reverse directions (large quantum yield), freedom from photodegradation, and the ability to tailor the photochromic properties of the protein through genetic manipulation of the native protein^{15, 2}, or through the action of external agents such as chemical compounds,^{16, 17} humidity, pH,¹¹ or ionic strength.