In order to establish the general comparison of the behaviour of R110 and R123 rhodamines in the presence of surfactants and phospholipids, the relative intensity F/F-0 and lambda(Fluo,max) of fluorescence are used as sensors of the dye-lipid interactions and dye microenvironments. Dye materials have absorption and fluorescence properties which differ distinctly in media of contrasting polarity and organization such as aqueous solutions of oppositively charged surfactant monomers and micelles of CTAB(+) and SDS- and membrane vesicles of DPPC+, DPPS- and DPPG(-) phospholipids. Associated changes in the dye absorbance and F/F-0 on increasing the anionic [S-] surfactant or lipid concentration are encountered. This is due to the formation in S-0 of two types of self-aggregates initiated by surfactant monomers and phospholipid vesicles respectively and depending in both systems on the balance between electrostatic and hydrophobic competitive forces. Colloidal species of D+S- type formed by interaction of D+ dyes with S- surfactant monomers (which are neither salts nor micelles of conventional structure) are constituted at [S-]<cmc (critical micelle concentration) for the surfactant homomicelles, then continued in dye-surfactant aggregates (D+S-)(n), all species being associated with lower absorbance and fluorescence. At larger [SDS], induced premicelles with monomeric dye content, then S- micelles bind the D+ species on the Stem layer. Self-aggregation of R123 and R110 is maximum at [S]/[D]=1700 and 2125 respectively, which gives 27 and 34 induced premicelles of SDS per dye molecule. R123 thus appears to self-aggregate with SDS more than R110 does. Progressive disaggregation of the multimer (D+S-)(n) complexes into a homogeneous distribution of D+ in micelles results in an increase in F/F-0 and lambda(Fluo,max) red shift. Similarly, the cationic species of both dyes and the zwitterionic form of R110 self-associate in the phospholipid vesicle network of the negatively charged DPPS and DPPG at much lower S/D ratios; spectral changes caused by dye stacking are shifts of lambda(max) and decreases in absorbance and fluorescence, the maxima of which being obtained at S/D=21 and 26 (R110 with DPPG and DPPS) and S/D=30 and 37 (R123 with DPPG and DPPS). This means that between 100 and 140 molecules of R110 (80-115 for R123) are respectively aggregated on one vesicle, R110 appearing to self-aggregate more than R123 on anionic vesicles. The changes in lambda(Fluo,max) of the dye monomers by partitioning into membrane thus obtained were interpreted in terms of the effective local polarities of the dye surroundings. By comparison of the alcohol solvent effect on lambda(Fluo,max) with values measured in singly occupied (S-) micelles and vesicles, equivalent dielectric constants of 18 and 11 are respectively obtained for SDS micelles and DPPS and DPPG vesicles, showing that R110 and R123 monomers are embedded in regions of intermediate polarity behind the polar head groups of the lipids. Complex self-aggregates (D+S-)(n) with S- surfactant monomers and dye stacking on the external layer of S- vesicles are thus proposed as the mechanisms underlying the spectral changes observed in the S-0 state, such ligand self-association being of potential interest for studying the interaction of amphipathic molecules with vesicles at low S/D values.
