Bilayered mixed micelles or bicelles are magnetically anisotropic, self-assembling model membrane structures comprised of long-chain phospholipids and short-chain detergent molecules. In the most widely accepted model of this system, the bicelle is discoid in shape, with the short-chain DHPC molecules aggregating to form rims around long-chain DMPC bilayers. While this model is consistent with most NMR and scattering data (X-ray and neutron), it inadequately describes the liquid-crystalline behavior of bicelle solutions at temperatures where magnetic alignment occurs. Temperature plays a central role in the structure of lipid aggregates, and the impact of temperature on bicelles has not been studied as extensively as composition and concentration. Therefore, a series of fluorescence probe and resonance energy transfer (FRET) measurements of labeled bicelle solutions as a function of temperature were conducted to monitor lipid mixing as an indication of bicelle structure and aggregation. The results of these measurements are not consistent with the large-scale changes in lipid mixing with temperature that have been attributed to bicelle solutions in other studies. The spectral data indicate that there is reorganization within mixed lipid aggregates as a function of temperature and bilayer fusion. In an attempt to reconcile these data with physical data and the theory of liquid-crystalline behavior, the authors speculate that the structure of bicelles is an interconnected network of DMPC bilayers interrupted by DHPC rimmed pores at elevated temperatures.

This paper describes a multivariate analysis of the fluorescence emission of 6-propionyl-2-dimethylaminonaphthalene (PRODAN) in a series of isotropic solvents of differing polarity and hydrogen-bonding ability. Multivariate methods distill the essential features from spectral data matrices so that the structural details that are embedded within the data are revealed to the analyst. In the aprotic solvents investigated, the analysis reveals a pair of emission components that have emission maxima that scale with the orientational polarizability. In the alcohols, short-lived, polarity-independent blue bands tentatively attributed to neutral hydrogen-bonded solute-solvent complexes form and relax prior to emission from paired bands that have Stokes shifts that scale with the solvent hydrogen-bonding ability rather than the polarity. In water, the short-lived blue bands were not observed, but the shift in the paired bands did scale with the solvent hydrogen-bonding ability.

This paper describes a multivariate photokinetic analysis of the membrane phase dependence of PRODAN and LAURDAN photokinetics in DMPC vesicles. Decay data, arranged in the form of Fourier transformed emission-decay matrices (FT-EDMs), were collected as a function of temperature around the gel phase transition temperature. Each matrix was partitioned into the emission spectra and decay profiles of the underlying emission components using methods based on principal components analysis. The analysis revealed that both probes typically emit at least three spectral components, which vary in intensity as the membrane undergoes gel to liquid-crystalline phase transitions: a locally excited species (lambda max approximately 415 nm), a charge-transfer species (lambda max approximately 435 nm), and a solvent relaxed species (lambda max approximately 490 nm). In contrast to previous reports, the most red-shifted species is not photoexcited, but evolves from the locally excited species and does not exhibit the dynamic Stokes' shifts associated with conventional solvent relaxation. The primary difference in the emission of the two probes is the prominence of the charge-transfer species in the LAURDAN emission.