Craig A Aspinwall

The utilization of photopolymerized phospholipids for the preparation of spatially defined, chemically functionalized, micron-sized domains within enclosed fluidic channels was recently reported (Ross, E. E.; et al. J. Am. Chem. Soc. 2005, 127, 16756-7). Fabrication of the phospholipid patterns is achieved via self-assembly of photoreactive phospholipid membranes that are subsequently cross-linked via UV-irradiation through a photomask. In this work, we have characterized the chemical and physical stability of the self-assembled, chemically functionalized, cross-linked phospholipid patterns and extended this approach to the preparation of cross-linked phospholipid patterns with multiple chemical functionalities. Poly(bis-SorbPC) patterns were found to withstand a number of chemical and physical challenges, including drying/rehydration, solvent or surfactant rinse, and extended storage without compromising the size or morphology of the cross-linked phospholipid patterns. Nonspecific adsorption of proteins was found to be markedly reduced in the presence of UV-photopolymerized poly(bis-SorbPC) compared to bare silica capillaries. The resulting barcode-like patterns were used to prepare protein-functionalized domains via covalent attachment of fluorescent proteins and active enzymes to chemically functionalized lipid headgroups. We also demonstrate multiple component polymer lipid patterns with adjacent chemically functionalized polymer lipid regions. The unique combination of stability, biocompatibility, reduced nonspecific protein adsorption, and the availability of numerous chemically functionalized lipid headgroups suggests the utility of this approach for preparing a widely applicable platform for multicomponent, high-throughput chemical sensing and screening applications.

PMID: 10701254;Abstract:

Confocal microscopy with Zinquin, a fluorogenic Zn²⁺-specific indicator, was used for spatially and temporally resolved measurement of Zn²⁺ efflux from single pancreatic β-cells. When cells were incubated in buffer containing Zinquin, application of insulin secretagogues evoked an increase in fluorescence around the surface of the cell, indicative of detection of Zn²⁺ efflux from the cell. The fluorescence increases corresponded spatially and temporally with measurements of exocytosis obtained simultaneously by amperometry. When images were taken at 266-ms intervals, the detection limit for Zn²⁺ was ~0.5 μM. With this image frequency, it was possible to observe bursts of fluorescence which were interpreted as fluctuations of Zn²⁺ level due to exocytosis. The average intensity of these fluorescence bursts corresponded to a Zn²⁺ concentration of ~7 μM. Since insulin is co-stored with Zn²⁺ in secretory vesicles, it was concluded that the Zn²⁺ efflux corresponded to exocytosis of insulin/Zn²⁺-containing granules from the β-cell. Exocytosis sites identified by this technique were frequently localized to one portion of the cell, indicative of active areas of release.

PMID: 16316200;Abstract:

We report a new molecular approach for in situ generation of micron scale, chemically and biochemically functionalized patterns inside three-dimensional, completely enclosed fluidic channels. The formation of chemical patterns is based upon a combination of lipid bilayer self-assembly and UV photopolymerization of photoreactive, cross-linkable phospholipids. Using this approach, we have functionalized capillaries of varying inner diameters with a range of chemistries useful for protein and peptide immobilization. Here, we demonstrate the ability to produce small molecule and protein-based chemical patterns. Copyright © 2005 American Chemical Society.

Suspended planar lipid membranes (or black lipid membranes (BLMs)) are widely used for studying reconstituted ion channels, although they lack the chemical and mechanical stability needed for incorporation into high-throughput biosensors and biochips. Lipid polymerization enhances BLM stability but is incompatible with ion channel function when membrane fluidity is required. Here, we demonstrate the preparation of a highly stable BLM that retains significant fluidity by using a mixture of polymerizable and nonpolymerizable phospholipids. Alamethicin, a voltage-gated peptide channel for which membrane fluidity is required for activity, was reconstituted into mixed BLMs prepared using bis-dienoyl phosphatidylcholine (bis-DenPC) and diphytanoyl phosphatidylcholine (DPhPC). Polymerization yielded BLMs that retain the fluidity required for alamethicin activity yet are stable for several days as compared to a few hours prior to polymerization. Thus, these polymerized, binary composition BLMs feature both fluidity and long-term stability.

The ability to rapidly screen complex libraries of pharmacological modulators is paramount to modern drug discovery efforts. This task is particularly challenging for agents that interact with lipid bilayers or membrane proteins due to the limited chemical, physical, and temporal stability of conventional lipid-based chromatographic stationary phases. Here, we describe the preparation and characterization of a novel stationary phase material composed of highly stable, polymeric-phospholipid bilayers self-assembled onto silica microparticles. Polymer-lipid membranes were prepared by photochemical or redox initiated polymerization of 1,2-bis[10-(2',4'-hexadieoyloxy)decanoyl]-sn-glycero-2-phosphocholine (bis-SorbPC), a synthetic, polymerizable lipid. The resulting polymerized bis-SorbPC (poly(bis-SorbPC)) stationary phases exhibited enhanced stability compared to particles coated with 1,2-dioleoyl-sn-glycero-phosphocholine (unpolymerized) phospholipid bilayers when exposed to chemical (50mM triton X-100 or 50% acetonitrile) and physical (15min sonication) insults after 30 days of storage. Further, poly(bis-SorbPC)-coated particles survived slurry packing into fused silica capillaries, compared to unpolymerized lipid membranes, where the lipid bilayer was destroyed during packing. Frontal chromatographic analyses of the lipophilic small molecules acetylsalicylic acid, benzoic acid, and salicylic acid showed >44% increase in retention times (P0.0001) for all analytes on poly(bis-SorbPC)-functionalized stationary phase compared to bare silica microspheres, suggesting a lipophilic retention mechanism. Phospholipid membrane-functionalized stationary phases that withstand the chemical and physical rigors of capillary LC conditions can substantially increase the efficacy of lipid membrane affinity chromatography, and represents a key advance toward the development of robust membrane protein-functionalized chromatographic stationary phases.