Characterization of the head group and the hydrophobic regions of a glycolipid lyotropic hexagonal phase using fluorescent probes

Lyotropic liquid crystal phases are related to biological entities such as cell membranes, as well as to technological applications, for example, emulsifier and drug-delivery systems. We characterized here one of the most stable lyotropic phases of the βMaltoOC₁₂ lipid, the hexagonal phase (comprised of 65% (w/w) aqueous formulation of n-dodecyl β-d-maltoside), using fluorescent probes. Probing different parts of the polar head group region using tryptophan (Trp) and two of its ester derivatives (Trp-C₄ and Trp-C₈) indicates a polarity gradient. Both Trp and Trp-C₄ reside slightly away from the maltoside sugar units, and the local polarity is similar to that of simple alcohols (methanol and ethanol). For Trp-C₈, the long chain length pulls the Trp moiety closer to the head groups and the local polarity approaches that of 1,4-dioxane. The reduction in polarity indicates a smooth transition from the polar domain to the hydrophobic domain, which is important for the stability of the lipid. Two fluorescence lifetimes were measured for tryptophan and its derivatives in lipid. The results point to a degree of flexibility of the lipid self-assembly that allows the Trp side chain to adapt two different rotamers. Using pyrene to probe the hydrophobic region of the lipid self-assembly indicates the tendency of the pyrene molecules to disperse among the hydrophobic tails and to avoid dimerization. By comparing the measured ratio of the pyrene vibronic peak intensities (I₁/I₃) in lipid to that in buffer and in cyclohexane, it was concluded that pyrene must be close to the head groups. Two lifetime components were measured for pyrene in lipid which indicates a degree of heterogeneity in the pyrene local environment. Interaction between the C ₈ chain of Trp-C₈ with pyrene is observed as a slight decrease in the I₁/I₃ ratio and the pyrene lifetime. The results presented here will be useful as a benchmark to utilize the present probes combination to characterize other biologically related lipid phases that are thought to play a crucial role in lipid-membrane interaction.