Total internal reflection fluorescence and electrocapillary investigationsof adsorption at the water-dichloroethane electrochemical interface. 2. Fluorescence-detected linear dichroism investigation of adsorption-driven reorientation of di-N-butylaminonaphthylethenylpyridiniumpropylsulfonate

The potential-dependent adsorption and orientation of the zwitterionic amph iphile, di-N-butylaminonaphthylethenylpyridiniumpropylsulfonate, I, in the presence of dilauroylphosphatidylcholine (DLPC) at the H2O-1,2-dichloroetha ne (DDCE) interface was investigated using a combination of steady-state fl uorescence; fluorescence-detected linear dichroism, and electrocapillary me asurements. From electrocapillary measurements, DLPC was found to dominate the interfacial composition at all potentials, when DLPC and I were present in bulk DCE and H2O at ca. 2 and 1 muM concentrations, respectively. At po tentials E-w - E-0 > 0.32 V, the affinity of DLPC:for the interface is dimi nished, and I becomes a more effective competitor for interfacial sites. Ov er the potential range 0.32 V less than or equal to E-w - E-0 less than or equal to 0.47 V, the total interfacial excess of species Gamma ((o,w))(DLPC +1) is reduced, but the ratio Gamma ((o,w))(I))/Gamma ((o,w))(DLPC) is enha nced. The DC fluorescence signal increased in response to the enhanced inte rfacial population of the unprotonated monomeric I at positive potentials. Concurrent fluorescence-detected linear dichroism (FDLD) measurements found that I reorients toward the interface normal on the positive scan. Because the total interfacial excess 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. Poten tial step experiments showed phenomena occurring on three distinct time sca les: ion reorganization on the millisecond time scale, an initial excursion of the De fluorescence intensity over a few tens of seconds, and then a mu ch longer evolution of the DC fluorescence (opposing the initial change) an d the FDLD signal. The initial DC response can be explained by an interfaci al reorganization of the chromophore that occurs in response to the applied potential before significant mass-transport occurs,while the slow time res ponses of both the I-DC and the FDLD signal are attributed to a mass-transp ort-limited partitioning of the lipid species, into and out of the interfac ial region.