Fluorescence Probing of the Temperature-Induced Phase Transition in a Glycolipid Self-Assembly: Hexagonal ↔ Micellar and Cubic ↔ Lamellar

Abstract

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.

Fluorescence characterization of water-driven self-assembled lipids and their temperature-induced phase transitions

“…This observation indicates that the glucose unit (a monosaccharide) of the head group is too short to show any polarity gradient as the tryptophan molecule gets closer. 34 We confirmed this finding by comparing the fluorescence spectra for the hexagonal phase of maltoside (βMalOC 12 ) 33 with that of glucoside (βGlcOC 8 ) 34 lipids when using Trp-C 8 as the probe (see Figure 2). It is clear that for the maltoside lipid, the tryptophan moiety of Trp-C 8 molecule experiences less polarity, approaching that of dioxane.…”

“…As shown in the graph, the fluorescence peak positions of Trp and its derivatives in lipid are blue-shifted compared to that of tryptophan in buffer (λ max = 355 nm) and approach those in methanol and ethanol (λ max = 340-342 nm). 34 This observation is an indication of the different environment experienced by the tryptophan moiety in the polar region of the head groups of the lipid (being less polar compared to pure water). The reduced polarity in the head group region compared to that in free water is due to the tendency of the water molecules in this region to be more ordered and less flexible.…”

“…It is clear that the head group region has a local polarity in the cubic phase which is slightly hydrophobic compared to that in the other phases. 34 In addition, we characterized a model inverse bicontinuous cubic phase with a gyroid Ia3d space group symmetry formed by the branched chain Guerbet glycoside (βGlcOC 10 C 6 ), 2 which closely resembles the natural products such as monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) from photosynthetic thylakoid membrane. 51 The asymmetric chain branching feature is thought to be responsible for the formation of a stable inverse bicontinuous cubic phase with a gyroid structure in excess water condition.…”

“…We quantify the polarity of different regions of the polar domain based on a parallel study in different solvents, and we discuss our results in relation to the flexibility of the lipid self-assembly. [33][34][35] Most of the lyotropic phases studied in this work are normal type unless it is stated the inverse kind such as the inverse cubic phase. 12 (98%), D-glucose (96%) and pyrene (99%) were purchased from Sigma-Aldrich.…”

Fluorescence lifetime study of intermolecular coupling between biphenyl–ester moieties of a thermotropic liquid crystal

“…Molecular orientation and intermolecular coupling primarily determine the physical properties of liquid crystalline materials [1][2][3][4]. Mesophase transitions and molecular orientation of liquid crystals (LCs) may be studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray diffraction (XRD), whereas dipole or electronic interactions between mesogen molecules may be examined by fluorescence spectroscopy [5][6][7][8].…”