Giant vesicles from 72-membered macrocyclic archaeal phospholipid analogues: Initiation of vesicle formation by molecular recognition between membrane components

Stereochemically pure archaeal acyclic bola-amphiphilic diphosphates 4 and 5, with the basic structure of the phospholipids found in Sulfolobus, have been synthesized for the first time. The self-assembly properties have been compared with those of the nearly identical 72-membered macrocyclic tetrae ther phosphates 3a and 3b, analogues of the major phospholipid components o f Sulfolobus, Thermoplasma, and methanogenic Archaea, which were also synth esized. Phase contrast and fluorescence microscopies have shown that the di polar lipids 1 and 2 spontaneously formed vesicles. Whereas the macrocyclic dipolar phosphates 3 spontaneously formed vesicles (phase contrast and flu orescence microscopies), the bolaform phosphate 4 gave only a lamellar stru cture (synchrotron diffraction pattern: repeat distance of about 4.25 nm bu t with only a few layers). However, upon addition of the unphosphorylated p recursors phytanol, phytol, or geranylgeraniol to the acyclic lipids 4 and 5, giant vesicles were rapidly formed. Addition of n-hexadecanol or cholest erol did not lead to vesicle formation. Therefore it was concluded that thi s vesicle formation occurs only when the added molecule is closely compatib le with the constituents of the lipid layer and can be inserted into the do uble layer. A slight mismatch (cholesterol or n-hexadecanol/polyprenyl chai ns) is therefore enough to block the insertion process presumably required for vesicle formation.