Evaluating the sensitivity of lipid headgroup-bound chromophores to their local environment

We report on the steady state and time-resolved fluorescence behavior of the chromophore 1,2-dimiryristoyl-sn-glycero-3-phosphoethanolamine-N-lissamine rhodamine B sulfonyl ammonium salt (SR-DMPE) in a series of solution phase and lipid bilayer environments. The issue of interest is whether or not the lipid headgroup-bound chromophore is sensitive to its local environment under conditions where vesicles have not been formed in lipid-containing mixtures and where unilamellar vesicles have been formed by extrusion. Our data point to the strong interaction of SR-DMPE with 1,2-dimirystoyl-sn-glycero-phosphocholine (DMPC) in solution whether or not the solution has undergone extrusion. The amount of SR-DMPE in the lipid-containing systems affects both the steady state and time-resolved spectroscopic response. Excitation of the chromophore to the S(2) state deposits sufficient excess energy into the system to influence its rotational diffusion dynamics, demonstrating significant interactions between SR-DMPE and DMPC. Comparison of SR-DMPE reorientation dynamics in DMPC-containing solutions with corresponding data on SR-DMPE in aqueous solution indicates that the lipids impose a restrictive local environment on the chromophore that is a factor of ca. 10 more viscous than an aqueous environment. The similarity of the reorientation data in all DMPC-containing solutions suggests that SR-DMPE is a local probe that is not sensitive to longer range organization.