The internal structure of vesicles formed solely by mannosylerythritol lipid-A (MEL-A), a glycolipid produced by basidiomycetous yeasts of the genus Pseudozyma, was visualized by liquid cell and cryogenic transmission electron microscopy. Moreover, it was shown for the first time that their morphology can be controlled by dicarboxylic acids acting as molecular signals, combined with the pH of the solution. MEL-A formed nano-sized unilamellar vesicles (200 nm diameter, 5 nm bilayer thickness) at pH 7 and pH 4 without dicarboxylic acids. Upon addition of dicarboxylic acids at pH 7, divalent dicarboxylate anions engage with the hydroxyl groups of the erythritol moiety in MEL-A, driving the growth of unilamellar vesicles with diameters of 250–300 nm. At pH 4, the addition of dicarboxylic acids (oxalic, maleic, fumaric or malonic acids), which exist predominantly as monoanions, induced the expansion to micro-sized vesicles (0.8–2.1 µm in diameter) and the transition into a multilamellar structure consisting of up to 18 layers. For oxalic acid, the vesicular membrane exhibited a thickness of 80 nm and consisted of 16 layers, of which both the outermost and the innermost layer thickness was 5 nm. Vesicle multilayering in MEL-A assemblies is driven by cooperative dissociated/undissociated carboxyl interactions in dicarboxylate monoanions.
