Long-lived fluorescence probes for studying lipid dynamics: A review

Diffusion of α-Tocopherol in Membrane Models: Probing the Kinetics of Vitamin E Antioxidant Action by Fluorescence in Real Time

The new fluorescent membrane probe Fluorazophore-L, a lipophilic derivative of the azoalkane 2,3-diazabicyclo[2.2.2]oct-2-ene, is employed to study the quenching of alpha-tocopherol (alpha-Toc) by time-resolved fluorescence in the microheterogeneous environments of Triton XR-100 and SDS micelles, as well as POPC liposomes. Fluorazophore-L has a small nonaromatic fluorescent polar headgroup and an exceedingly long-lived fluorescence (e.g., 140 ns in aerated SDS micelles), which is efficiently quenched by alpha-Toc (3.9 x 10(9) M(-1) s(-1) in benzene). Based on solvatochromic effects and the accessibility by water-soluble quenchers, the reactive headgroup of Fluorazophore-L, along with the chromanol group of alpha-Toc, resides at the water-lipid interface, which allows for a diffusion-controlled quenching in the lipidic environments. The quenching experiments represent an immobile or stationary case; that is, interparticle probe or quencher exchange during the excited-state lifetime is insignificant. Different quenching models are used to characterize the dynamics and antioxidant action of alpha-Toc in terms of diffusion coefficients or, where applicable, rate constants. The ideal micellar quenching model is suitable to describe the fluorescence quenching in SDS micelles and affords a pseudo-unimolecular quenching rate constant of 2.4 (+/- 0.4) x 10(7) s(-1) for a single quencher per micelle along with a mean aggregation number of 63 +/- 3. In Triton micelles as well as in unilamellar POPC liposomes, a two-dimensional (lateral) diffusion model is most appropriate. The mutual lateral diffusion coefficient D(L) for alpha-Toc and Fluorazophore-L in POPC liposomes is found to be 1.8 (+/- 0.1) x 10(-7) cm(2) s(-1), about a factor of 2 larger than for mutual diffusion of POPC, but more than 1 order of magnitude lower than a previously reported value. The comparison of the different environments suggests a quenching efficiency in the order benzene >> SDS micelles > Triton micelles > POPC liposomes, in line with expectations from microviscosity. The kinetic measurements provide important benchmark values for the modeling of oxidative stress in membranes and other lipidic assemblies. The special case of small lipidic assemblies (SDS micelles), for which the net antioxidant efficacy of alpha-Toc may be lower than expected on the grounds of its diffusional behavior, is discussed.

A novel fluorescent coronenyl-phospholipid analogue for investigations of submicrosecond lipid fluctuations

A fluorescent phospholipid derivative, the 2'-(4-coronenylbutyric) ester of lyso-egg phosphatidylcholine, has been synthesized for use in studies of submicrosecond lipid dynamics. Synthesis of the phospholipid derivative involves Friedel-Crafts acylation of free coronene, followed by a Huang-Minlon reduction to yield the fatty-acyl derivative, 4-coronenylbutyric acid. Esterification of the carboxylic acid with lyso-phosphatidylcholine is achieved through a mixed anhydride intermediate. The resultant coronenyl-phospholipid adduct (Cor-PC) has been incorporated into sonicated unilamellar vesicles of dimyristoylphosphatidylcholine (DMPC) for dynamic lipid studies. Fluorescence quenching studies using potassium iodide, together with steady-state emission anisotropy (EA) measurements, confirm that the coronene moiety of the phospholipid adduct resides towards the head group interfacial region of the lipid bilayer. Unique properties of this new fluorescent phospholipid adduct are its long mean fluorescence lifetime (tau av approximately 112 ns at 14 degrees C), the planar symmetry of the fluorophore and its defined bilayer location. As a consequence, depolarizing motions of the coronene moiety target submicrosecond 'gel-fluid' lipid dynamics arising from a relatively narrow bilayer distribution. Our data suggest that the sensitivity of this new long-lived fluorescent phospholipid analogue to localized transverse submicrosecond lipid dynamics can provide important biological insights into varied processes including lipid-peptide interactions, bilayer fluidity gradients and passive ion transport.

Two-photon fluorescence microscopy observation of shape changes at the phase transition in phospholipid giant unilamellar vesicles

Using the sectioning effect of the two-photon fluorescence microscope, we studied the behavior of phospholipid giant unilamellar vesicles (GUVs) composed of pure diacylphosphatidylcholine phospholipids during the gel-to-liquid crystalline phase transition. We used the well-characterized excitation generalized polarization function (GP(ex)) of 6-dodecanoyl-2-dimethylamine-naphthalene (LAURDAN), which is sensitive to the changes in water content in the lipid vesicles, to monitor the phase transition in the GUVs. Even though the vesicles do not show temperature hysteresis at the main phase transition, we observed different behaviors of the vesicle shape, depending on how the GUV sample reaches the main phase transition. During the cooling cycles, we observed an increase in the vesicle diameter at the phase transition ( approximately 0.5-1%), followed by a decrease in the diameter when the vesicle reached the gel phase. During the heating cycles and close to the phase transition temperature, a surprising behavior is observed, showing a sequence of different vesicle shapes as follows: spherical-polygonal-ellipsoidal. We attribute these changes to the effect of lipid domain coexistence on the macroscopic structure of the GUVs. The "shape hysteresis" in the GUVs is reversible and largely independent of the temperature scan rate. In the presence of 30 mol% of cholesterol the events observed at the phase transition in the GUVs formed by pure phospholipids were absent.

A long-lifetime Ru(II) metal-ligand complex as a membrane probe

A luminescent metal-ligand complex, [Ru(bpy)2(dppz)]2+, (where dppz is dipyrido[3,2-a:2',3'-c] phenazine), was used as a photoluminescence probe for investigating submicrosecond lipid dynamics in a dipalmitoyl-L-alpha-phosphotidylglycerol (DPPG) model bilayer system. The luminescence of [Ru(bpy)2(dppz)]2+ in buffer is completely quenched but becomes luminescent when intercalated into DPPG vesicles. The experimental results show that the emission intensity of [Ru(bpy)2(dppz)]2+ intercalated into DPPG vesicles increases dramatically as temperature is increased towards the lipid phase transition temperature. This effect is abolished in bilayers containing a high concentration (> 30 mol%) of cholesterol, suggesting this probe is sensitive to the membrane composition. Frequency-domain emission intensity decays, measured as a function of increasing temperature towards the lipid phase transition temperature (2 to 57 degrees C), display two major lifetime components. The short lifetime disappears at temperatures well above the phase transition temperature. A comparison of oxygen quenching with iodide quenching suggests the heterogeneity of probe location at temperatures well below the lipid phase transition temperature and the homogeneity of probe location at temperature well above the lipid phase transition temperature. [Ru(bpy)2(dppz)]2+ displays polarized emission, enabling the study of membrane dynamics. The long decay time displayed by this probe allows measurement of the overall rotational correlation time of lipid vesicles on the microsecond time-scale. Because of the long lifetime, polarized emission, and background free nature of the photoluminescence measurements, [Ru(bpy)2(dppz)]2+ has numerous applications in the biophysical studies of membranes.