1,32-Dihydroxy-dotriacontane-bis(Rhodamine) 101 ester A lipid membrane spanning bichromophoric molecule as revealed by intramolecular donor–donor energy migration (DDEM)

Abstract

The bichromophoric and monochromophoric probes, 1,32-dihydroxy-dotriacontane-bis(Rhodamine) 101 ester (Rh101C₃₂Rh101) and Rhodamine 101 octadecyl ester (Rh101C₁₈), respectively, were solubilised in unilamellar lipid vesicles of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). At molar ratios of less than 1/5000 for Rh101C₁₈/lipid and Rh101C₃₂Rh101/lipid, intermolecular electronic energy migration is negligible, contrary to the intramolecular donor–donor energy migration (DDEM) within the Rh101C₃₂Rh101 molecules. The time-resolved fluorescence anisotropy determined for Rh101C₁₈ and Rh101C₃₂Rh101 were globally analysed by using a recently developed model that accounts for reorientational motions, as well as the rate of DDEM(L. B-Å. Johansson, F. Bergström, P. Edman, I. V. Grechishnikova and J. G. Molotkovsky, J. Chem. Soc., FaradayTrans., 1996, 92,1563). It was found that the rate of intramolecular DDEM within Rh101C₃₂Rh101 is typically about 3×10⁹ s^-1. In vesicles formed by DOPC and POPC, the distance between the Rhodamine groups of Rh101C₃₂Rh101 is found to be about 35 Å, while it is about 32 Å in DPPC vesicles. These values are in excellent agreement with reported thicknesses of the lipid bilayers. Thus, the present work strongly suggests that the Rh101C₃₂Rh101 molecules are spanning across the lipid bilayer, that is, the Rhodamine residues are located on opposite sides of the lipid bilayer.