Cyanine dye structural and voltage-induced variations in photo-voltages of bilayer membranes

Abstract

Flash illumination alters the voltage across bilayer lipid membranes in the presence of certain cyanine dyes. The waveforms of the photo-voltage vary systematically with dye structure and imposed transmembrane voltage. Experimental results are reported for 27 positively charged cyanine dyes, primarily oxazole derivatives, using lecithin/oxidized cholesterol bilayer membranes and 10-mM sodium chloride solutions. Several dyes do not induce any photo-voltages. Examples are 3,3' diethyl 9 ethyl 2,2' oxacarbocyanine iodide, 3,3' diethyl 2 oxa 2" thiacyanine iodide, and 3,3' dimethyl 2,2' indocarbocyanine iodide. Several dyes, when added to one side of the membranes, induce monophasic waveforms. Examples are 3,3' dimethyl 2,2' oxacarbocyanine chloride, and 3,4,3',4' tetramethyl 2,2' oxazalinocarbocyanine iodide. Other dyes induce a photo-voltage only if transmembrane voltages are imposed. These waveforms are biphasic with some dyes (3,3' diethyl 2,2' oxacarbocyanine iodide, for example) and monophasic with other dyes (3,3' dibutyl 2,2' oxacarbocyanine iodide, for example). The photo-voltage waveforms are explained by models that consider the movement of charged dye molecules within the membrane, following optical excitation. The dye movements are probably induced through charge rearrangements in the dye associated with long-lived triplet states, isomerization, or through excimer formation. These results provide information on the location and orientation of the dye molecules within bilayer membranes. The variations which occur in the waveforms with applied voltage indicate that these membranes are fluid in the direction perpendicular to the membrane plane.