The location and microenvironment of dimerizing cationic dyes in lipid membranes as studied by means of their absorption spectra.

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The absorption spectra of three cationic dyes, Methylene Blue (MB), 3,3′-dipropyl-2,2′-thiadicarbocyanine (diS-C<SUB>3</SUB>(5)), and 9-aminoacridine (9-AA), were measured in aqueous suspensions of a phospholipid membrane and a surfactant micelle. The dyes were intensely partitioned into the membrane phase and dimerized to exhibit a remarkably large reduction in absorbance. The increase in the amount of the membrane beyond the amount where all the dye molecules were dimerized in the membrane phase resulted in a gradual increase in the absorbance due to the increase in the available membrane volume and the dilution of dye in the membrane phase. The changes in the wave lengths of the absorption maxima of the dye monomers by the partitioning into membrane thus obtained were interpreted in terms of the effective polarities of the dye surroundings (microenvironments). It was found that the rather hydrophobic diS-C<SUB>3</SUB>(5) penetrates deeply into the membranes and the micelles and is located in considerably nonpolar microenvironments, while MB remains at the polar-surface regions of the membranes and the micelles. On the basis of these results, the structures of the micelles and membranes are discussed.
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