Potential-Dependent-Adsorption and Reorientation Responses Associated With a Phospholipid Monolayer at the Water /Dichloroethane Interface

198 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2001.Total internal reflection fluorescence (TIRF) and electrocapillary measurements were employed to interrogate the potential dependent mechanical and physicochemical properties of adsorbed films at the interface between two immiscible electrolyte solutions (ITIES). Mixed films of a phospholipid, DLPC, and the fluorescent reporter lipid di-N-butylaminonaphthylethenyl-pyridiniumpropylsulfonate, I, were assembled at this interface from the bulk organic and aqueous phases respectively. The mechanical (interfacial tension) properties of the interface were dominated by the phospholipid species over the full range of trans-membrane potentials studied, 0.14 V ≤ Ew-E o ≤ 0.47 V. At the positive end of this range, Ew-Eo > 0.32 V, the affinity of DLPC for the interface is diminished and I becomes a more effective competitor for interfacial sites. The steady state fluorescence signal increased in response to the enhanced population of unprotonated monomeric I at these potentials. Concurrent fluorescence detected linear dichroism (FDLD) measurements found that I reorients towards the interface normal during a positive potential sweep. Because the total interfacial population decreases at these potentials, this behavior cannot be ascribed to a simple compression effect. Rather it reflects the favored geometry at the compositions obtained at positive potentials. Potential-step experiments showed phenomena occurring on three distinct time scales---ion reorganization on the millisecond time scale, an initial excursion of the steady state fluorescence intensity over a few tens of seconds, and then a much longer evolution of the steady state fluorescence and the FDLD signal. The initial fluorescence response can be explained in terms of interfacial reorganization of the fluorophore that occurs in response to the applied potential before significant mass-transport occurs, while the slow time responses of the both the steady state and FDLD signals are attributed to mass-transport-limited partitioning of lipid species into and out of the interfacial region.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD