Nanomechanical Properties of Artificial Lipid Bilayers Composed of Fluid and Polymerizable Lipids.

Polymerization enhances the stability of a planar supported lipid bilayer (PSLB) but it also changes its chemical and mechanical properties, attenuates lipid diffusion, and may affect the activity of integral membrane proteins. Mixed bilayers composed of fluid lipids and poly(lipids) may provide an appropriate combination of polymeric stability coupled with the fluidity and elasticity needed to maintain the bioactivity of reconstituted receptors. Previously (Langmuir, 2019, 35, 12483-12491) we showed that binary mixtures of the polymerizable lipid bis-SorbPC and the fluid lipid DPhPC form phase-segregated PSLBs composed of nanoscale fluid and poly(lipid) domains. Here we used atomic force microscopy (AFM) to compare the nanoscale mechanical properties of these binary PSLBs with single-component PSLBs. The elastic (Young's) modulus, area compressibility modulus, and bending modulus of bis-SorbPC PSLBs increased upon polymerization. Before polymerization, breakthrough events at forces below 5 nN were observed, but after polymerization, the AFM tip could not penetrate the PSLB up to an applied force of 20 nN. These results are attributed to the polymeric network in poly(bis-SorbPC), which increases the bilayer stiffness and resists compression and bending. In binary DPhPC/poly(bis-SorbPC) PSLBs, the DPhPC domains are less stiff, more compressible, and are less resistant to rupture and bending compared to pure DPhPC bilayers. These differences are attributed to bis-SorbPC monomers and oligomers present in DPhPC domains that disrupt the packing of DPhPC molecules. In contrast, the poly(bis-SorbPC) domains are stiffer and less compressible relative to pure PSLBs; this difference is attributed to DPhPC filling the nm-scale pores in the polymerized domains that are created during bis-SorbPC polymerization. Thus, incomplete phase segregation increases the stability of poly(bis-SorbPC) but has the opposite, detrimental effect for DPhPC. Overall, these results provide guidance for the design of partially polymerized bilayers for technological uses.

Nanodomain Formation in Planar Supported Lipid Bilayers Composed of Fluid and Polymerized Dienoyl Lipids.

Polymerization of synthetic phospholipid monomers has been widely used to enhance the stability of lipid membranes in applications such as membrane-based biosensing, where the inherent instability of fluid-phase lipid bilayers can be problematic. However, lipid polymerization typically decreases membrane fluidity, which may be required to maintain the activity of reconstituted integral proteins and peptides. Prior work has shown that a bilayer composed of binary mixtures of poly(lipid) and fluid lipid exhibits enhanced stability and supports the function of incorporated biomolecules. This work examines the structural basis of these findings using planar supported lipid bilayers (PSLBs) composed of binary mixtures of a polymerizable lipid, 1,2-bis[10-(2',4'-hexadienoloxy)decanoyl]-sn-glycero-3-phosphocholine (bis-SorbPC), and a nonpolymerizable lipid, 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC). Fluorescence recovery after photobleaching (FRAP) measurements showed that long-range lateral diffusion was minimally affected when the poly(lipid) mole ratio was ≤0.7. Atomic force microscopy, used to examine phase segregation in these PSLBs, showed that DPhPC forms a continuous lipid matrix that is 0.2-0.4 nm thicker than the island-like poly(bis-SorbPC) domains, with lateral dimensions of ≤200 nm. The nanoscale phase segregation allows for long-range lateral diffusion of lipid probes in the DPhPC matrix. The combination of fluidity and stability in these materials should make them useful in membrane-based biosensing applications.

Total synthesis of apoptolidin A.

A highly convergent, enantioselective total synthesis of the potent antitumor agent apoptolidin A has been completed. The key transformations include highly selective glycosylations to attach the C27 disaccharide and the C9 6'-deoxy-l-glucose, a cross-metathesis to incorporate the C1-C10 trienoate unit, and a Yamaguchi macrolactonization to complete the macrocycle. Twelve stereocenters in the polypropionate segments and sugar units were established through diastereoselective chlorotitanium enolate aldol reactions.

Enantioselective synthesis of apoptolidinone: exploiting the versatility of thiazolidinethione chiral auxiliaries.

An efficient, enantioselective synthesis of apoptolidinone has been completed, demonstrating the versatility of thiazolidinethione auxiliaries. Three propionate aldol additions and two asymmetric glycolate alkylations function to establish 8 of the 12 stereogenic carbon centers. A cross-metathesis reaction is utilized to assemble the C1-C10 trieneoate fragment and the C11-C28 polypropionate region of the molecule.

Total synthesis of (+/-)-halichlorine, (+/-)-pinnaic acid, and (+/-)-tauropinnaic acid.

The related marine natural products halichlorine, pinnaic acid, and tauropinnaic acid have been synthesized. The described route provided access to all three compounds from a common, late-stage intermediate. The synthesis began with 1-pyrrolidino-1-cyclopentene from which an intermediate possessing the three contiguous stereocenters of the natural products was synthesized in just four steps. Olefin cross metathesis followed by a hydrogenation/hydrogenolysis reaction stereoselectively formed the piperidine ring. Use of a beta-lactam group provided internal protection for the highly congested nitrogen atom during side-chain elaboration. The beta-lactam was subsequently reduced directly to an amino aldehyde, which after the Horner-Wadsworth-Emmons reaction was elaborated to pinnaic acid. The same amino aldehyde was also transformed into halichlorine after a thiol-mediated cyclization sequence to form the dehydroquinolizidine ring system.