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

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-d-glucopyranoside/ 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-C₄ and Trp-C₈) 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 (I₁/I₃). This is attributed to the compact interaction of the double alkyl chains of the lipid with the pyrene molecules. When adding Trp-C₈ to the lipid assembly, there was no change in the I₁/I₃ 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.