Fluorescence Probing ofthe Temperature-Induced PhaseTransition in a Glycolipid Self-Assembly: Hexagonal ↔ Micellarand Cubic ↔ Lamellar.

Water-driven self-assembly of lipids displays a varietyof liquidcrystalline phases that are crucial for membrane functions. Herein,we characterize the temperature-induced phase transitions in two compositionsof an aqueous self-assembly system of the octyl β-d-glucoside (βGlcOC8) system, usingsteady-state and time-resolved fluorescence measurements. The phasetransitions hexagonal ↔ micellar and cubic ↔ lamellarwere investigated using tryptophan (Trp) and two of its ester derivatives(Trp-C4and Trp-C8) to probe the polar headgroupregion and pyrene to probe the hydrophobic tail region. The polarityof the headgroup region was estimated to be close to that of simplealcohols (methanol and ethanol) for all phases. The pyrene fluorescenceindicates that the pyrene molecules are dispersed among the tailsof the hydrophobic region, yet remain in close proximity to the polarhead groups. Comparing the present results with our previously reportedone for βMaltoOC12, increasing thetail length of the hexagonal phase from C8to C12leads to less interaction with pyrene, which is attributed to themore random and wobbling motion of the longer alkyl tail. We measureda reduction (more hydrophobic) in the ratio of the vibronic peak intensitiesof pyrene (I1/I3) for the lamellar phase compared to that of the cubic phase. Thehigher polarity in the cubic phase can be correlated to the natureof its interface, which curves toward the bulk water. This geometryalso explains the slight reduction in polarity of the headgroup regioncompared to the other phases. Upon the addition of Trp-C8, the fluorescence lifetime of pyrene is reduced by 28% in the lamellarand cubic phases, whereas the I1/I3value is only slightly reduced. The resultsreflect the dominant role of dynamic interaction mechanism betweenthe C8chain of Trp-C8and pyrene. This mechanismmay be important for these two phases since they participate in theprocess of membrane fusion. Both lipid compositions show completelyreversible temperature-induced phase transitions, reflecting the thermodynamicequilibrium structures of their mesophases. Probing both regions ofthe different lipid phases reveals a large degree of heterogeneityand flexibility of the lipid self-assembly. These properties are crucialfor carrying out different biological functions such as the abilityto accommodate various molecular sizes. [ABSTRACT FROM AUTHOR]