Synthesis and thermotropic properties of macrocyclic lipids related to archaebacterial membranes

Macrocyclic phospholipids containing 32-44 ring atoms were synthesized by a route involving a high-temperature Glaser oxidation as the key step. These lipids are analogous to mammalian phospholipids except a single extra carbon-carbon bond joins the chain termini. The new lipids offered, therefore, an opportunity to examine thermotropic properties of their membranes when the chains within a given molecule are unable to move independently of one another. It was concluded that chain "tethering" (a) raises the transition temperatures substantially for all but the shortest lipids, (b) lowers enthalpies of transition by, in part, reducing the number of gauche C-C linkages created during the melting process, and (c) lowers entropies of transition by impeding motional freedom within the liquid-crystalline phase. Molecular mechanics calculations on the macrocyclic lipids are described briefly.