Phytochrome models. 6. Conformation control by membrane of biliverdin dimethyl ester incorporated into lipid vesicles

Abstract

Biliverdin dimethyl ester has been incorporated into liposome membranes. When the incubation of the bilindione was carried out at room temperature without external perturbation, the helically coiled conformation initially predominated in all cases. It persisted in the gel-type liposomes, but changed on standing in the more fluid liquid-crystalline liposomes to stretched conformation(s). The same irreversible change could also be effected by heating to a temperature above the phase transition temperature, and by sonication of the bilindione-containing liposomes. Stretched species consequently were the only forms contained in liposomes which had been prepared by exhaustive cosonication of bilindione and lipid. A study of the influence of zinc ions on the bilindione-containing liposomes and of the zinc complex of bilverdin dimethyl ester incorporated into liposomes demonstrated that the pigment was incorporated into the lipophilic part of the liposome bilayer. The absorption spectra of freshly incorporated coiled bilindione were all similar. The spectral characteristics of incorporated stretched BVE were complex, e.g., a red-shifted vis band was selectively exhibited in the liposomes of the unsaturated lipids. The conformational analysis was carried out by fluorescence and membrane-induced circular dichroism. The determination of the final emission quantum yields showed that the conformationally flexible bilindione fluoresced more efficiently in the membranes than in ethanolic solution. The rigid zinc complex of biliverdin dimethyl ester had similar quantum yields in ethanol and in the membranes. The variations in the former case reflected a decrease in the rate of radiationless relaxation processes owing to conformational constraints in the highly ordered molecular environment of the membranes. The time dependence of the photoacoustic signal and fluorescence intensities indicated that a thermally reverting photoisomerization of BVE selectively occurred in the gel-type liposomes. The analysis of the changes in absorption coefficients and Stokes shifts with conformational changes revealed a similar geometry of ground and excited states of stretched BVE conformers, and a larger difference in the case of the coiled form.