Ternary phase diagram of dipalmitoyl-PC/dilauroyl-PC/cholesterol: nanoscopic domain formation driven by cholesterol.

A ternary phase diagram is proposed for the hydrated lamellar lipid mixture dipalmitoylphosphatidylcholine/dilauroylphosphatidylcholine/cholesterol (DPPC/DLPC/cholesterol) at room temperature. The entire composition space has been thoroughly mapped by complementary experimental techniques, revealing interesting phase behavior that has not been previously described. Confocal fluorescence microscopy shows a regime of coexisting DPPC-rich ordered and DLPC-rich fluid lamellar phases, having an upper boundary at apparently constant cholesterol mole fraction chi(chol) approximately 0.16. Fluorescence resonance energy transfer experiments confirm the identification and extent of this two-phase regime and, furthermore, reveal a 1-phase regime between chi(chol) approximately 0.16 and 0.25, consisting of ordered and fluid nanoscopic domains. Dipyrene-PC excimer/monomer measurements confirm the new regime between chi(chol) approximately 0.16 and 0.25 and also show that rigidly ordered phases seem to disappear around chi(chol) approximately 0.25. This study should be considered as a step toward a more complete understanding of lateral heterogeneity within biomembranes. Cholesterol may play a role in domain separation on the nanometer scale

Late-onset bleb-associated endophthalmitis and continuous positive airway pressure

Purpose: To raise awareness of a possible association between continuous positive airway pressure (CPAP) devices and postoperative bleb-related infection. Observations: A 57-year old patient on CPAP presented with unilateral bleb-associated endophthalmitis 32 months after routine ExPress Trabeculectomy with mitomycin C. The offending organism, Streptococcus mitis, is a nonmotile and generally non-virulent pathogen which predominates in the normal human respiratory flora. Conclusions and importance: This conceptual report underscores a potential relationship between CPAP use and bleb-associated endophthalmitis. Streptococcal species are the most commonly reported causative organisms in bleb-associated endophthalmitis, and S. mitis is of particular concern as the most abundant microbe among all human oral flora. A logical risk factor for infection, the CPAP device may inadvertently deliver such organisms to the vulnerable conjunctival filtering bleb. Keywords: Endophthalmitis, Bleb, Mitomycin C, Express trabeculectomy, Continuous positive airway pressure, Streptococcus miti

Fluorescence methods to detect phase boundaries in lipid bilayer mixtures

AbstractPhase diagrams of lipid mixtures can show several different regions of phase coexistence, which include liquid-disordered, liquid-ordered, and gel phases. Some phase regions are small, and some have sharp boundaries. The identity of the phases, their location in composition space, and the nature of the transitions between the phases are important for understanding the behavior of lipid mixtures. High fidelity phase boundary detection requires high compositional resolution, on the order of 2% compositional increments. Sample artifacts, especially the precipitation of crystals of anhydrous cholesterol, can occur at higher cholesterol concentrations unless precautions are taken. Fluorescence resonance energy transfer (FRET) can be used quantitatively to find the phase boundaries and even partition coefficients of the dyes between coexisting phases, but only if data are properly corrected for non-FRET contributions. Self-quenching of the dye fluorescence can be significant, distorting the data at dye concentrations that intuitively might be considered acceptable. Even more simple than FRET experiments, measurements of single-dye fluorescence can be used to find phase boundaries. Both FRET and single-dye fluorescence readily detect the formation of phase domains that are much smaller than the wavelength of light, i.e. “nanoscopic” domains