Fluorescence probing of the temperature-induced phase transition in a glycolipid self-assembly: hexagonal ↔ micellar...

Water-driven self-assembly of lipids displays a variety of liquid crystalline phases that are crucial for membrane functions. Herein, we characterize the temperature-induced phase transitions in two compositions of an aqueous self-assembly system of the octyl β-D-glucoside (βGlcOC(8)) system, using steady-state and time-resolved fluorescence measurements. The phase transitions hexagonal ↔ micellar and cubic ↔ lamellar were investigated using tryptophan (Trp) and two of its ester derivatives (Trp-C(4) and Trp-C(8)) to probe the polar headgroup region and pyrene to probe the hydrophobic tail region. The polarity of the headgroup region was estimated to be close to that of simple alcohols (methanol and ethanol) for all phases. The pyrene fluorescence indicates that the pyrene molecules are dispersed among the tails of the hydrophobic region, yet remain in close proximity to the polar head groups. Comparing the present results with our previously reported one for βMaltoOC(12), increasing the tail length of the hexagonal phase from C(8) to C(12) leads to less interaction with pyrene, which is attributed to the more random and wobbling motion of the longer alkyl tail. We measured a reduction (more hydrophobic) in the ratio of the vibronic peak intensities of pyrene (I(1)/I(3)) for the lamellar phase compared to that of the cubic phase. The higher polarity in the cubic phase can be correlated to the nature of its interface, which curves toward the bulk water. This geometry also explains the slight reduction in polarity of the headgroup region compared to the other phases. Upon the addition of Trp-C(8), the fluorescence lifetime of pyrene is reduced by 28% in the lamellar and cubic phases, whereas the I(1)/I(3) value is only slightly reduced. The results reflect the dominant role of dynamic interaction mechanism between the C(8) chain of Trp-C(8) and pyrene. This mechanism may be important for these two phases since they participate in the process of membrane fusion. Both lipid compositions show completely reversible temperature-induced phase transitions, reflecting the thermodynamic equilibrium structures of their mesophases. Probing both regions of the different lipid phases reveals a large degree of heterogeneity and flexibility of the lipid self-assembly. These properties are crucial for carrying out different biological functions such as the ability to accommodate various molecular sizes.