Evidence of Basic Medium in the Polar Nanochannels of the Inverse Bicontinuous Cubic Phase of a Guerbet Glycolipid: A Steady-State and Time-Resolved Fluorescence Study

Abstract

Characteristic structures and properties of nanochannels are thought to play important roles in biological activity and chemical and physical processes at the interfaces. In this work, we characterized the polar nanochannels of the inverse bicontinuous cubic phase of the 2-hexyl-decyl-β-Dglucopyranoside/water system, which has a gyroid Ia3d space group symmetry, by introducing fluorescent probes. The fluorescence signals of tryptophan (Trp) and two of its ester derivatives (Trp-C 4 and Trp-C 8 ) show a local basic environment inside the nanochannels that is equivalent to an aqueous solution of pH ≥ 10.0. This was confirmed by measuring the fluorescence spectra of aqueous tryptophan in different pH solutions. The basic effect is attributed to the restricted motion of water in the nanochannels (diameter of 2.3 nm) in which water molecules are situated very close to the OH groups of the sugar units. The very small channels also force the tryptophan moiety to be very close to the sugar units and the water molecules in which the local environment is mostly basic. We characterized the hydrophobic region of the lipid by measuring the fluorescence change of pyrene upon incorporation in the tail region. A larger hydrophobicity was reflected in the measured small ratio of the vibronic peak intensities of pyrene (I 1 /I 3 ). This is attributed to the compact interaction of the double alkyl chains of the lipid with the pyrene molecules. When adding Trp-C 8 to the lipid assembly, there was no change in the I 1 /I 3 ratio nor in the pyrene lifetimes which implies that there is no interaction between the pyrene and the C 8 -chain. This observation indicates that the pyrene molecules are well-shielded inside the tail region. The measured two lifetime components for each tryptophan and pyrene point to the presence of a degree of heterogeneity and flexibility in the lipid self-assembly. These properties are crucial for carrying out different biological functions such as the ability to accommodate various molecular sizes. The current results suggest the structural importance of very narrow polar nanochannels of the lipidic cubic phase which may facilitate the transfer of highly polar substances, such as ions, to the cells.

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

“…The possibility of the tryptophan molecules to self-assemble in a micellar-like structure, particularly in Trp-C 8 , is also ruled out because of the similar spectral positions for Trp and Trp-C 8 in the inverse cubic phase (tryptophan itself cannot form a micellar aggregation even at a high concentration). 35 The fluorescence results obtained here for tryptophan and its derivatives in the inverse cubic phase of the βGlcOC 10 C 6 lipid are also in stark contrast to those reported for the inverse cubic phase of monoolein. 49,53 The different space group symmetry and geometrical nature of the two lipidic cubic phases may have led to the different results.…”

“…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. 35 If we compare the peak positions of Trp and its derivatives in the inverse cubic phase of βGlcOC 10 C 6 35 with those in the normal cubic phase of the βGlcOC 8 34 lipid, it is clear that the tryptophan molecule experiences different local environment (see Table 1). In the normal cubic phase, the peak positions are located at 346-347 nm, close to those of methanol and ethanol (λ max = 340-342 nm).…”

“…Unlike in the other systems of wider nanochannels, the tryptophan molecules in our system will be forced to be very close to both the sugar units and the water molecules so that the effect of basicity is expected to dominate the tryptophan local environment. 35 It is important to note here that our buffer for the inverse cubic phase lipid was neutral (pH 7.2). On the other hand, the buffer used for the monoolein lipids was acidic (pH 5.6) which is expected to neutralize any basic effect that may result from the self-assembly of the lipid.…”

“…As a hydrophobic molecule, pyrene is expected to favor the tail region of the lipid assembly. [33][34][35] We first show the fluorescence spectra of pyrene measured in buffer and in cyclohexane ( Figure 6). The sharp and structured band is due to the monomer fluorescence, whereas the unstructured band centered at ~ 465 nm is fluorescence from the pyrene excimer.…”

“…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.…”