INTERACTION OF A CONGO RED-DYE PROBE WITH LECITHIN VESICLES PROBED BYFORCED RAYLEIGH-SCATTERING

Solutions of large unilamellar phospholipid vesicles were doped with t he water soluble azo dye congo red for probing of diffusional processe s by Forced Rayleigh Scattering (FRS) experiments. The FRS signals sho w an unusual decay/rise/decay shape, which is caused by an overlay of two gratings that are spatially out of phase by 180 degrees. Photoexci tation of congo red creates a population grating of cis isomers and, a t the same time, a depletion grating of the trans isomer. The two isom ers are found to have different hydrodynamic properties. Measurements at different fringe spacings yield an unlimited lifetime, as expected and a diffusion coefficient D = 4.1 x 10(-11) m(2) s(-1) for the trans ground state. For the thermally unstable cis isomer, a lifetime of 85 ms and a diffusion coefficient of 6.1 x 10(-11) m(2) s(-1) are obtain ed. The lifetime of the cis isomer is unexpectedly long for an aqueous solution, and the observation of the different hydrodynamic behaviour of the two isomers is unusual. For comparison, FRS experiments in pur e aqueous solutions of congo red were carried out, and showed the norm ally expected monoexponential decay of the light-induced grating; a di ffusion coefficient of 2.0 x 10(-10) m(2) s(-1) is obtained. This resu lt indicates that in the vesicular solution, the diffusion coefficient of congo red is reduced to a different extent for the cis and trans f orms. Previously, only highly viscous polymer samples had been believe d to exert such a strong discriminating effect on the diffusion of the isomers. For the present system, the observations are interpreted in terms of a stabilization of the cis isomer by the vesicle surface, res ulting in a prolonged lifetime. Interaction of the negatively charged sulphonate groups of the dye with the positive trimethylammonium group s of lecithin represents a possible mechanism. Due to differences in s tructure and size, the interaction is not the same for the two 4 isome rs, leading to different hydrodynamic behaviour. This interpretation i s confirmed by the observation of a negative zeta-potential of -50 mV for the vesicles in the stained sample, and by analysis of the absorpt ion spectra in terms of contributions due to free and vesicle - bound dye molecules.