carbocyanines and 1-2-distearoyllecithin

A non-ideal lipid binary mixture (dilauroylphosphatidylcholine/distearoylphosphatidylcholine), which exhibits gel/fluid phase coexistence for wide temperature and composition ranges, was studied using photophysical techniques, namely fluorescence anisotropy, lifetime and resonance energy transfer (FRET) measurements. The FRET donor, N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-dilauroylphosphatidylethanol amine, and a short-tailed FRET acceptor, 1,1'-didodecil-3,3,3',3'-tetramethylindocarbocyanine (DiIC12(3)), were shown to prefer the fluid phase by both intrinsic anisotropy, lifetime and FRET measurements, in agreement with published reports. The other studied FRET acceptor, long-tailed probe 1,1'-dioctadecil-3,3,3',3'-tetramethylindocarbocyanine (DiIC18(3)), is usually reported in the literature as partitioning mainly to the gel. While intrinsic lifetime studies indeed indicated preferential partition of DiIC18(3) into a rigidified environment, FRET analysis pointed to an increased donor-acceptor proximity as a consequence of phase separation. These apparently conflicting results were rationalized on the basis of segregation of DiIC18(3) to the gel/fluid interphase. In order to fluid-located donors sense these interphase-located acceptors, fluid domains should be small (not exceed approximately 10-15 nm). It is concluded that membrane probes which apparently prefer the gel phase may indeed show a non-random distribution in this medium, and tend to locate in an environment which simultaneously leads to less strict packing constraints and to favorable hydrophobic matching interactions.

Topics: 4-Chloro-7-nitrobenzofurazan; Carbocyanines; Fluorescent Dyes; Phosphatidylcholines; Phosphatidylethanolamines; Spectrometry, Fluorescence; Temperature

Application of the regular solution model to thermal transition data obtained by differential scanning calorimetry has allowed the determination of partition coefficients, Kp, and the thermodynamics of transfer of a series of indocarbocyanine solutes between the gel and fluid phases of phospholipid bilayers. The indocarbocyanines with alkyl chain lengths of 12 to 22 carbons were partitioned between dipalmitoyl- and distearoylphosphatidylcholine phases at the transition temperatures of the gel-liquid-crystal phase transition. The results indicate that as the alkyl chain length of the solute nears that of the acyl chains of the bilayer lipid, the free energy of transfer from gel to fluid is least negative, and the enthalpy of transfer is most positive. There is almost complete entropy-enthalpy compensation in the transfer process. Comparison of the partition coefficients with published values determined by a fluorescence method show good agreement when the differences in temperature of the measurements are accounted for.

Topics: 1,2-Dipalmitoylphosphatidylcholine; Calorimetry, Differential Scanning; Carbocyanines; Gels; Lipid Bilayers; Models, Chemical; Phosphatidylcholines; Thermodynamics

A theory describing the shapes of polarized fluorescence photobleaching recovery (PFPR) curves for a population of fluorophores undergoing restricted rotational diffusion in two-dimensional systems such as planar membranes has been developed. In this model, restricted rotational diffusion of the fluorophores is described by using reflective boundary conditions, in which the fluorophores are assumed to diffuse freely but only within an angular space of width 2 omega. The magnitude and apparent rate of the PFPR postbleach fluorescence curves are a function of both omega and the angle between the bleaching and observation beam polarizations psi. It is shown that estimates of the degree of rotational restriction omega may be obtained from changes in the psi-dependent postbleach fluorescence intensities. Using angle-dependent PFPR, slow rotational reorientations of the fluorescent lipid analogue 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine in distearoylphosphatidylcholine Langmuir-Blodgett monolayers deposited on octadecyltrichlorosilane-treated fused quartz were measured. As theoretically predicted for a rotationally restricted fluorophore population, both the initial F psi (0) and final F psi (infinity) postbleach fluorescence intensities varied as a function of psi, and no measurable change in the postbleach fluorescence intensities was observed for psi = 45 degrees. Using the theory for restricted rotational motion, the psi-dependent variations of the final fluorescence intensities F psi (infinity) obtained at two bleaching intensities gave an average apparent omega approximately 52 degrees. However, to adequately fit the F psi (0) data, inclusion of the theoretical effects of rapid (faster than the duration of the photobleaching pulse) fluorophore dynamics was also required. Best fits of the F psi (0) and F psi (infinity) data were obtained when the fluorophores were assumed to rapidly wobble within a cone of semiangle delta approximately 30 degrees-50 degrees while slowly rotating within an angular space defined by semiangle omega approximately 35 degrees-60 degrees. Subsequent analysis of the time- and psi-dependent changes in the post-bleach fluorescence curves F psi (t) gave apparent diffusion coefficients ranging from D approximately 10(-3) s-1 to 4 x 10(-2) s-1.

Topics: Carbocyanines; Diffusion; Fluorescent Dyes; Liposomes; Mathematics; Models, Theoretical; Molecular Conformation; Phosphatidylcholines; Spectrometry, Fluorescence