T. J. V. Prazeres

Accurate determination of the limiting anisotropy of rhodamine 101. Implications for its use as a fluorescence polarization standard.

The S1-S0 limiting anisotropy of a widely used fluorophore, rhodamine 101, is determined with unprecedented accuracy. From time-resolved and steady-state fluorescence measurements in several solvents, it is shown that the limiting anisotropy of rhodamine 101 is for all practical purposes equal to the theoretical one-photon fundamental anisotropy value of 2/5, both in rigid and in fluid media. This fact, along with the favorable chemical and photophysical properties of rhodamine 101, point to its use as a standard for fluorescence polarization measurements. It is also shown that if the excitation pulse can be considered a delta impulse with respect to the time scale of the anisotropy decay (but not necessarily to the time scale of the intensity decay), then no deconvolution procedure is needed for anisotropy decay analysis.

Thermo-responsiveness of poly(N,N-diethylacrylamide) polymers at the air–water interface: The effect of a hydrophobic block

The poly(N,N-diethylacrylamide) (h-PDEA) homopolymer and the poly(N-decylacrylamide)-b-PDEA (PDcA(11)-b-PDEA(231)) diblock copolymer were studied in the range of 10 to 40 degrees C, at the air-water interface. The pi-A isotherms of h-PDEA appear nearly invariant with temperature while the pi-A isotherms of PDcA(11)-b-PDEA(231) deviate significantly to lower areas with the temperature increase evidencing the thermo-responsiveness of this diblock copolymer at the interface. For the copolymer, the limiting area per segment versus temperature shows a break point around 29 degrees C, slightly lower than the lower critical solution temperature (LCST) of h-PDEA in water (31-33 degrees C). AFM images of LB monolayers transferred at 40 degrees C revealed for both polymers the presence of hydrophobic aggregates due to the conformational changes (collapse) of chains that occur at the LCST. Differences in the morphology of these aggregates, flat irregular structures for h-PDEA and round-shaped domains for PDcA(11)-b-PDEA(231), were related with the condensing effect of the hydrophobic block. The PDcA(11) block, anchoring the polymer to the interface, ensures a better stability and cohesion to the film and preserves the thermo-sensitivity of the h-PDEA at the interface.

Spontaneous vesicles, disks, threadlike and spherical micelles found in the solubilization of DMPC liposomes by the detergent DTAC.

The spontaneous colloidal nanostructures formed in water by the zwitterionic phospholipid DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) with the cationic detergent DTAC (n-dodecyltrimethylammonium chloride) were investigated at a fixed DMPC concentration and variable detergent:lipid total molar ratios (D:L). Apparent (neutral-sphere-equivalent) hydrodynamic diameters (Φ(e)) of liposomes and micelles were obtained by dynamic light scattering (DLS). Fluorescence lifetime imaging microscopy (FLIM), using chlorophyll-a as a probe, showed the morphology of giant vesicles and threadlike micelles. Micro-differential scanning calorimetry (micro-DSC) detected the presence of bilayers, in vesicles and discoidal micelles (disks). Pure DMPC liposomes are multilamellar and polydisperse (Φ(e)≈100-10,000 nm). As D:L increased, smaller vesicles were found, due to the bigger spontaneous curvature of the bilayer: at D:L=1, ULVs (unilamellar vesicles; Φ(e)≈100 nm) appeared and, at D:L=2-10, ULVs coexisted with disks (Φ(e)≈30 nm). Bilayers totally disappeared at D:L≥15, giving rise to spheroidal (Φ(e)≈2-16 nm) and threadlike (Φ(e)≈100-10,000 nm) micelles. A quasi-equilibrium structural diagram for the DMPC-DTAC-water system shows equivalent diameters of the scattering nanoparticles as a function of D:L. The results obtained herein for the system DMPC-DTAC show the role of electrostatic interactions in the formation of the mixed structures.