Photocontrol of Ion Permeation in Lipid Vesicles Incorporating Amphiphilic Dithienylethene and Spirooxazine Derivatives
The photocontrol of ion permeation across a lipid bilayer membrane incorporating amphiphilic photochromic compounds, such as dithienylethenes (DTEs) and spirooxazines (SpOxs), has been investigated. In general, photochromic compounds change their molecular structure during photoisomerization. The effect of this photoisomerization on the membrane permeability of lipid vesicles has been examined. Specifically, three DTE derivatives 21–23 and three SpOx derivatives 24–26 were incorporated into a variety of lipid vesicles to study their potential as membrane disruptors. Initially, a regioselective approach was used to synthesize 21–23 that contain a dodecyl or hexyl chain terminated with a quaternary ammonium substituent, and methyl or phenylethynyl substituents at the reactive carbons. Similar to DTEs, SpOxs 24 and 25 contain charged tethers that differ only in the alkyl chain length, whereas 26 is the first bolaamphiphilic SpOx dimer with two large photochromic units. Two different assays were used to assess the photocontrol of proton and potassium ion permeation in lipid vesicles. These assays examine the effect of the inclusion and photoisomerization of DTEs and SpOxs on membrane permeability. The results from the proton permeation assay showed that the open-ring isomers of DTEs 21– 23 were more disruptive than the closed-ring isomers in the four lipid vesicle systems studied, regardless of their lamellar phase at room temperature. Also, a steric effect was clearly observed as DTEs incorporating the comparatively smaller methyl group exhibited lower rates of ion permeation than the bulkier phenylethynyl group. The inclusion of SpOxs 24–26 in lipid vesicles also significantly disrupts the bilayer membrane and enhances proton permeation. The effect of chain length on membrane permeability was more pronounced for these SpOxs than the DTEs because DTEs 21 and 22 does not show any significant difference in either the rate constant of proton permeation or in the extent of proton permeation. Overall, 1,2-dioleoyl-sn-glycero-3- phosphatidylcholine (DOPC) vesicles incorporating 26 showed the highest permeability to protons. Potassium ion permeation was examined in 1,2-dipalmitoyl-sn-glycero-3- phosphatidylcholine (DPPC) vesicles and DOPC vesicles. The rate constant for ion permeation and net change in the percentage of release were highly dependent on the lipid bilayer phase state and the alkyl chain length of the photochromic compound. In general, lipid vesicles including DTEs were less permeable to potassium ions than vesicles including SpOxs. Also, the open-ring isomers of all SpOxs were more disruptive than their closed-ring isomers. In addition, the difference in potassium ion permeability under UV and visible irradiation was more pronounced than previously reported photoresponsive membrane disruptors with reversible photocontrols. The membrane permeability of both DPPC and DOPC vesicles incorporating 24–26 increased in the following order of 25 < 24 < 26. The structure-activity relationship exists for SpOxs in both lipids for the potassium ion permeation but only in DOPC vesicles for the proton permeation. A comparison of the activity of SpOxs in DPPC and DOPC vesicles revealed that 26 in DOPC vesicles exhibits good photocontrol of potassium ion permeation. All together, the proton and potassium ion permeation studies suggest that 26 in DOPC vesicles is the most photoresponsive system for the delivery of small molecules.