Stereochemical Purity Can Induce a New Crystalline Mesophase in Phytantriol Lipids

The impact of stereochemical purity of lipids on their self-assembly behavior is critical for establishing their true phase behavior from their commercial counterparts, which often contains stereoisomeric mixtures and other impurities. Here, stereochemically pure phytantriol (PT), (3,7,11,15-tetramethylhexadecane-1,2,3-triol) was synthesized from the natural trans-phytol and its thermotropic and lyotropic phase behavior in water investigated by small-angle X-ray scattering (SAXS), polarized optical microscopy (POM), and differential scanning calorimetry (DSC). These chemically pure lipids contain two chiral centers at the hydrophilic head group region and two chiral centers at the lipophilic tail region, allowing us to address the question of whether the molecular stereochemistry is related to the macroscopic phase behavior of phytantriol. In contrast to its commercial stereoisomeric mixtures, which form an isotropic micellar phase, neat (2S,3S,7R,11R)-3,7,11,15-tetramethylhexadecane-1,2,3-triol (S,S-PT) shows a smectic lamellar phase at room temperature, whereas (2R,3R,7R,11R)-3,7,11,15-tetramethylhexadecane-1,2,3-triol (R,R-PT) forms solid crystals. The lyotropic phase behavior of R,R-PT appears to be identical to that of the previously reported commercial stereoisomeric PT mixtures. In contrast, S,S-PT exhibits a different phase behavior. A lamellar crystalline phase (L_c) is formed instead of an isotropic micellar phase at a low water content, which also coexisted with other phases at low temperature. Subtle change in the shape of the diastereomers leads to variable steric interactions and subsequently affects the packing of the lipids at the molecular level, thereby influencing its self-assembling behavior. Finally, lipidic cubic phase crystallization of the membrane protein bacteriorhodopsin yielded a larger number of microcrystals with a higher average crystal length from S,S-PT than from commercial PT, suggesting faster nucleation.