COLOR DISCRIMINATION BY FORWARD AND REVERSE PHOTOCURRENTS IN BACTERIORHODOPSIN-BASED PHOTOSENSOR

The photocurrent response of bacteriorhodopsin is measured for purple membrane solid films at an electrode/electrolyte interface upon revers ible transformation of the native bR(570) pigment into a structurally and spectrally modified bR(480) form, in the presence of the halogenat ed general anaesthetic Enflurane. The reversible pigment transformatio n is accompanied by a reversible inversion of the photocurrent signal. Action spectra of these forward and reverse photocurrent patterns hav e their maxima at 570 and 480 nm, respectively, and indicate that the two bacteriorhodopsin spectral forms have their own characteristic pho toresponse. The time-resolved photoresponse curves of bR(570) and bR(4 80) both show the characteristic sequence of capacitive currents which accompany the photochemical activity of bacteriorhodopsin. However, l ike their respective differential photoresponse, the algebraic sums of their respective photocurrent components have opposite signs. The for ward and reverse photoresponse patterns thus have their origin in the charge displacement currents particular to each spectral species. This indicates that purple membranes readily self-orient upon deposition o nto an electrode. This orientation is maintained upon pigment transfor mation by the anaesthetic, even if it leads to an inverted photorespon se, suggesting a capacitive, rather than rectifying, character of the pigment/electrode junction.