Development of novel membrane materials and their application to coupled transport phenomenon

Aniruddha Parshuram Patwardhan, Purdue University

Abstract

The synthesis of acyclic and macrocyclic tetraether bolaamphiphiles that are either completely saturated or contain one or more diacetylenic groups were synthesized as archaebacterial membrane mimics for applications in coupled ion transport through a gramicidin channel. A novel route was developed starting from 2-phenyl-5-hydroxy-1,3-dioxane to prepare fully saturated, diacetylenic, and bis(diacetylenic) bolaamphiphiles with acyclic & macrocyclic frameworks. Two different reactions, olefin metathesis reaction catalyzed by [(PCy 3)2Ru=CHPh]Cl2 or copper-mediated Glaser oxidation, were employed in the key synthetic steps. Chemoselective debenzylation of benzyl ethers in presence of an electron rich diacetylenic moiety using trimethylsilyl iodide (TMS-1) reagent has been demonstrated for the first time. Liposomal preparations of 7:3 2,2′-di-O -decyl-3,3′-di-O-(eicosanyl)-bis-( rac-glycero)-1,1′-diphosphocholine Ib :cholesterol formed unilamellar vesicles that retained a concentration gradient for at least 3.5 days. Gradients could not be maintained by vesicles formed from 7:3 2,2′-di-O-octyl-3,3 ′-di-O-(hexadecyl)-bis-(rac-glycero)-1,1 ′-diphosphocholine Ia:chol. Incorporation of gramicidin in these liposomal formulations gave Na+ exchange across the membrane at a rate of 1 × 1010 M−1s −1 as compared to 2 × 107 M−1 s−1 for dimeric gramicidin in bilayer forming lipids. These results demonstrate the bolaamphiphile membrane-gramicidin construct is three orders of magnitude faster than gramicidin channels in a bilayer system. BLM (Black lipid membrane) measurements performed on bolaamphiphile Ib suggested that Ib forms stable membranes with high capacitance (∼1.0–1.5μF/cm2). Incorporation of 1% gramicidin in Ib BLM film, resulted in a rapid change in current as a function of applied potential, suggesting that gramicidin channel is capable of forming a cation-conductive monomeric channel. Bolaamphiphiles were also used in designing a supported membrane biosensor. Two gold electrode-coating methods were developed: (1) top loading and (2) contact method. Time-dependent impedance spectroscopy data showed that the contact method was the best technique for obtaining a stable bolaamphiphile coating on the gold surface. Higher impedance values were observed when the gold surfaces were coated with bolaamphiphiles, however, the impedance of these films decreased significantly upon incorporation of 5 mol % gramicidin within the supported membranes coating on the electrode suggesting that gramicidin is functioning as an ion channel. Thus the bolaamphiphile/ligand-gated gramicidin supported membrane concept may be a viable and sensitive biosensing strategy.

Degree

Ph.D.

Advisors

Thompson, Purdue University.

Subject Area

Organic chemistry|Biochemistry

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