The ternary system palmitoylsphingomyelin (PSM)/palmitoyloleoylphosphatidylcholine (POPC)/cholesterol is used to model lipid rafts. The phase behavior of the three binary systems PSM/POPC, PSM/cholesterol, and POPC/cholesterol is first experimentally determined. Phase coexistence boundaries are then determined for ternary mixtures at room temperature (23 degrees C) and the ternary phase diagram at that temperature is obtained. From the diagram at 23 degrees C and the binary phase diagrams, a reasonable expectation is drawn for the ternary phase diagram at 37 degrees C. Several photophysical methodologies are employed that do not involve detergent extraction, in addition to literature data (e.g., differential scanning calorimetry) and thermodynamic rules. For the ternary phase diagrams, some tie-lines are calculated, including the one that contains the PSM/POPC/ cholesterol 1:1:1 mixture, which is often used in model raft studies. The diagrams here described are used to rationalize literature results, some of them apparently discrepant, and to discuss lipid rafts within the framework of liquid-ordered/liquid-disordered phase coexistence.

Large unilamellar vesicles of dimyristoylphosphatidylcholine/cholesterol mixtures were studied using fluorescence techniques (steady-state fluorescence intensity and anisotropy, fluorescence lifetime, and fluorescence resonance energy transfer (FRET)). Three compositions (cholesterol mole fraction 0.15, 0.20, and 0.25) and two temperatures (30 and 40°C) inside the coexistence range of liquid-ordered (l o ) and liquid-disordered (l d ) phases were investigated. Two common membrane probes, N-, which form a FRET pair, were used. The l o /l d partition coefficients of the probes were determined by individual photophysical measurements and global analysis of timeresolved FRET decays. Although the acceptor, Rh-DMPE, prefers the l d phase, the opposite is observed for the donor, NBD-DMPE. Accordingly, FRET efficiency decreases as a consequence of phase separation. Comparing the independent measurements of partition coefficient, it was possible to detect very small domains (Ͻ20 nm) of l o in the cholesterol-poor end of the phase coexistence range. In contrast, domains of l d in the cholesterol-rich end of the coexistence range have comparatively large size. These observations are probably related to different processes of phase separation, nucleation being preferred in formation of l o phase from initially pure l d , and domain growth being faster in formation of l d phase from initially pure l o .

Lipid bilayers composed of two phospholipids with significant acyl-chain mismatch behave as nonideal mixtures. Although many of these systems are well characterized from the equilibrium point of view, studies concerning their nonequilibrium dynamics are still rare. The kinetics of lipid demixing (phase separation) was studied in model membranes (large unilamellar vesicles of 1:1 dilauroylphosphatidylcholine (C(12) acyl chain) and distearoylphosphatidylcholine (C(18) acyl chain)). For this purpose, photophysical techniques (fluorescence intensity, anisotropy, and fluorescence resonance energy transfer) were applied using suitable probes (gel phase probe trans-parinaric acid and fluid phase probe N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-dilauroylphosphatidylethanolamine). The nonequilibrium situation was induced by a sudden thermal quench from a one-fluid phase equilibrium situation (higher temperature) to the gel/fluid coexistence range (lower temperature). We verified that the attainment of equilibrium is a very slow process (occurs in a time scale of hours), leading to large domains at infinite time. The nonequilibrium structure stabilization is due essentially to temporarily rigidified C(12) chains in the interface between gel/fluid domains, which decrease the interfacial tension by acting as surfactants. The relaxation process becomes faster with the increase of the temperature drop. In addition, heterogeneity is already present in the supposed homogeneous fluid mixture at the higher temperature.

The hydroxyl group of 7-hydroxyquinoline in acidic media (i.e., of 7-hydroxyquinolinium cation, 7-HQ(C)) undergoes photoinduced deprotonation even in strongly acidic media. It occurs from dilute perchloric acid solutions up to 8 mol L -1 concentration, which is evidence of an outstanding photoacidity. Excited-state proton ejection was shown to be reversible at acid concentrations greater than 1 mol L -1 . Below this value, the proton ejection was not balanced by the proton recombination. Concerning the rates of the excited-state reactions, (i) the proton ejection rate constants k 1 increased upon acid dilution, attaining at high dilution the limiting value of (k 1 ) 0 ) 5.5 × 10 10 s -1 . k 1 was shown to vary with the water activity according to k 1 ) (k 1 ) 0 (a H 2 O ) 4 . The number 4 is then a key number for the ability of water to accept a proton from 7-HQ(C*). (ii) The proton recombination rate constant varies from 3 × 10 9 to 4 × 10 10 s -1 when the acid concentration ranges from 1 to 5 mol L -1 . This observation can tentatively be ascribed to a nondiffusional recombination process occurring with the ejected proton in concentrated acid solutions. The results as a whole are consistent with the coupling of an intramolecular electron transfer with the proton transfer, due to the withdrawing effect of the quinolinium function gNH + on the -O -group generated by the deprotonation. The photoproduct is then most likely to be the ketonic tautomeric form of 7-hydroxyquinoline rather than a zwitterion. Thus, the excited-state behavior is fully accounted for by the photoinduced synergy of the two functional groups.

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.

Excitation energy transport in several β-cyclodextrins containing seven appended chromophores was studied theoretically and experimentally by steady-state and time-resolved fluorescence anisotropy. The absorption spectra compared to those of reference chromophores did not reveal significant interactions between the chromophores in the ground state, thus allowing us to assume a very weak coupling regime for energy transfer. The measured long time anisotropies were found to be in all cases close to one-seventh of the fundamental anisotropy, showing that the chromophores are randomly oriented. A realistic model in which the chromophores are in fixed positions but randomly oriented was developed to interpret the steady-state and time-resolved emission anisotropy data. A Monte-Carlo simulation based on the appropriate master equation allowed the calculation of the theoretical anisotropy decay in terms of reduced variables and parameters. The decay contains a wide spectrum of rate constants. A good fit to the experimental decays was obtained. Moreover, the nearest-neighbor distance recovered from the anisotropy and the steady-state anisotropy for all cyclodextrins (5−7 Å in all cases) are compatible with the nearest-neighbor distances expected from molecular modeling, which confirms the validity of the theoretical model.

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The water-soluble β-cyclodextrin, CD-St, with seven steroidic naphthalene chromophores linked to the primary rim, can form inclusion complexes with a merocyanine dye (DCMOH) and an oxazine dye (Ox725); the stoichiometry is 2:1 (CD-St:dye). This system works as an antenna since the dye is surrounded by 14 chromophores. The efficiency of transfer from the antenna chromophores to the encased dye was found to be close to 100%. The dynamics of this heterotransfer and homotransfer (i.e., energy hopping among the antenna chromophores) was investigated by time-resolved fluorescence intensity and time-resolved fluorescence anisotropy experiments, respectively. The distribution of rate constants for homotransfer was recovered thanks to a previously described Monte Carlo simulation from which an average rate constant was calculated and found to be about 4 × 1011 s-1. This value is about 10 times faster than the rate constant for heterotransfer in the case of Ox725, and about three times faster than in the case of DCMOH. The results are discussed in terms of interchromophoric distances, mutual orientations and Förster radii.