Dynamics of bolaamphiphilic fluorescent polyenes in lipid bilayers from polarization emission spectroscopy

The rotational motions of the biamphiphilic polyenes (bolapolyenes) dimethyl all-(E)-octacosa-10,12,14,16,18-pentaenedioate (DE28:5) and dimethyl all-(E)-tetratriaconta-13,15,17,19,21-pentaenedioate (DE34:5), with head-to-head distances of 34 and 42, respectively, have been examined by fluorescence anisotropy methods. The membrane-spanning bolapolyenes, which contain a central emitting pentaene group tethered to two methoxycarbonyl opposite polar heads by symmetric C-8 (DE28:5) and C-11 (DE34:5) polymethylene chains, were dispersed in lipidbilayers of DPPC or DMPC, and the stationary and picosecond-resolved emission was recorded as a function of temperature. In fluid-phase DMPC bilayers, three relaxation times could be determined, assigned to fast (0.2 and 2ns) single-bond isomerization processes localized on the alkyl chains, and to whole-molecule oscillations (similar to 11 ns), respectively. The anisotropy decay parameters were further analyzed in terms of a diffusive model for wobbling in a Gaussian ordering potential, to assess the anchoring effect of the symmetric polar heads. In this way, the average rotational diffusion constant of the bolapolyenes, D-perpendicular to, could be estimated as 0.022-0.026 rad(2) ns(-1) (DMPC bilayers, 35 degrees C), a value that is only 1 / 3 of that corresponding to the related pentaene fatty acid spanning a single membrane monolayer. In contrast, the amplitude of the equilibrium orientational distribution (theta(half-cone) similar to 50 degrees) is very similar for both the transmembrane and the single-headed polyenes. The reorientational oscillations of the central emitting group in the bolapolyenes necessarily would produce large-amplitude (2-5 angstrom) and very fast (ns) translational motions of the polar heads. (c) 2006 Elsevier B.V. All rights reserved.