Photoregulated potassium ion permeation through dihexadecyl phosphate bilayers containing azobenzene and stilbene surfactants

Photoresponsive asymmetrically organized systems based upon small unilamellar dihexadecyl phosphate (DHP) vesicles were constructed by entrapping high concentrations of potassium ion within the vesicular aqueous core and incorporating either an amphiphilic trans-azobenzene-containing phosphate monoester or an amphiphilic trans-stilbene-containing carboxylic acid into its membrane structure. Spectroscopic measurements indicated that the azobenzene derivative was molecularly dispersed in the hydrocarbon phase of the vesicle and that the extent of aggregation of the membrane-localized stilbene derivative was minor. Thermal K+ leak rates from the doped vesicles were very low, with calculated permeability coefficients (P) of similar to 4 x 10(-12) cm/s at 40 degrees C for DHP vesicles containing 5.5 mol % of the trans-azobenzene derivative and similar to 1.5 x 10(-11) cm/s at 38 degrees C for vesicles containing 5.5 mol % of the trans-stilbene derivative; for comparison, P similar or equal to 2 x 10-12 cm/s for undoped vesicles at 40 degrees C. Photoexcitation of the azobenzene-doped vesicles at 360 nm caused >90% trans --> cis photoisomerization over the measured temperature range (25-40 degrees C), with complete reversion to the trans isomer upon photoexcitation at 450 nm. Photoexcitation of deoxygenated suspensions of the stilbene-doped vesicles at 315 nm gave similar to 80% conversion to the cis isomer in the photostationary state, which was not reversible. At 25 degrees C, K+ leak rates for the isomeric azobenzene-doped vesicles were nearly identical; at 40 degrees C, K+ leakage for the DHP vesicles containing the cis-azobenzene isomer corresponded to P similar or equal to 2 x 10(-11) cm/s, 5-fold greater than that of the trans isomer. In trans --> cis --> trans photocycling experiments, K+ leak. rates alternately increased and decreased, indicating that the vesicles remained intact. At 40 degrees C, K+ leakage from the vesicles containing predominantly cis-stilbene was similar to 2-fold greater than that from vesicles with the corresponding trans isomer. In electrochemical experiments, viologen-mediated reduction of the DHP-bound trans-azobenzene occurred at E < -0.44 V (NHE), with hydrazobenzene reoxidation at E similar or equal to -0.16 V; addition of viologen radicals to aqueous suspensions of the trans-azobenzene-doped DHP vesicles caused immediate decolorization of the dye.