Polymerization in two-dimensional assemblies of sorbyl-containing lipids.

Abstract

Mono- and bis-substituted phosphatidylcholines (PC) containing a polymerizable sorbyl ester group at the acyl chain terminus were synthesized. The thermotropic phase behavior, two-dimensional polymerizability, and polymerized assemblies stability were investigated. The thermotropic phase behavior of mono- and bis-sorbylPCs was examined by differential scanning calorimetry. Each SorbPC exhibited a single endotherm which occurred at temperatures below the main phase transition (T(m)) of the corresponding linear saturated PC. Variations in the chain length of bis-SorbPC resulted in a pronounced odd/even alternation of the T m. The interaction of the sn-2 chain sorbyl ester carbonyl with neighboring methylene chains appears to be predominantly intermolecularly or intramolecularly depending on whether the chain length is even or odd, respectively. Intermolecular orientation of the sorbyl ester carbonyl decreased the T(m) to a greater extent than intramolecular orientation. The magnitude of the odd/even effect diminished as the chain length increased and the van der Waals interchain interactions increased. Lipid bilayers composed of either mono- or bis-SorbPCs were thermally polymerized (60°C) to high conversion with the radical initiator AIBN. Transesterification of poly-(SorbPC), resulting in removal of the lipid headgroup, yielded a soluble polymer which was analyzed by size exclusion chromatography relative to PMMA standards. The relative number-average degree of polymerization (X(n)) varied from 50 to nearly 600, and was proportional to [I]⁻¹. The Xn was identical for both mono- and bis-substituted SorbPCs at constant [I]. These results suggest that at high conversion the chain termination of the growing polymer occurs by primary radical termination, rather than bimolecular chain termination. UV-photopolymerization of mono-Sorb PC yielded oligomers. The stability of poly-(SorbPC) vesicles to detergent dissolution was examined by quasielastic light scattering. Poly-vesicles consisting of linear polymers having a X(n) of 50 did not undergo lysis with detergent, whereas UV-polymerized vesicles with X(n) of 3-5 were lysed. Prolonged UV-irradiation of bis-SorbPC vesicles resulted in stabilization to detergent, a result of extensive crosslinking of the short polymer chains. UV-polymerized vesicles of mono- and bisSorbPC were only stabilized if the mole fraction of bis-SorbPC was ≥ 0.4.