Synthesis, transport activity and selectivity of an artificial photogated ion channel
Date
1995
Authors
Shan, Chengjin
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Abstract
An 18-crown-6 derived ion transporter 1 was synthesized as a potential photogated ion channel. This compound is composed of a macrocyclic core bearing four wall units to which polar head groups attach. The macrocyclic core is a tartaric acid derived 18-crown-6. The wall units are azobenzene derivatives capable of photoisomerization. The design assumed that the open and closed states of the ion channel would be controlled by configuration conversion of the azobenzene.
The azobenzene moiety was constructed by diazonium coupling reaction between phenol and 4'-(5,6-dihydroxyhexyloxy)diazonium chloride. The wall unit was completed by a Williamson ether coupling of the phenol with 6-phthalimidohexyl bromide followed by deprotection to give an aminodiolazobenzene derivative. Protection of the diol as the isopropylidene ketal followed by coupling with R,R,R,R-18-crown-6 tetraacid chloride and deprotection gave the target 1. The target compound and intermediates were all characterized by 1H and 13C NMR spectroscopy and mass spectroscopy (CI or electrospray), and elemental analysis.
The photoisomerization of compound 1 was explored in homogeneous solution by irradiation of the compound in methanol-chloroform (v/v 1:1). The UV-visible spectra indicated that the initial form was trans-azobenzene (λmax = 360 nm, E = 76300 M-1 cm-1) which shifted to a photostationary state that was predominantly cis-azobenzene (λmax = 320 nm). It was found that the extinction coefficient of trans-1 is only 3.4 times that of a compound with one azobenzene moiety. A thermal back reaction of cis-to trans- was non-first order with an approximate half life of 15 minutes. Compound 1 can also photoisomerize between trans- and cis- isomers reversibly.
The target compound was poorly soluble in polar solvents, thus it could not be added as a solution to pre-formed vesicles. The compound was directly
incorporated into pre-formed vesicles from the solid state, leading to a modified pH-stat titration experiment. The transport activity of this compound is
approximately 10 times lower than some previously reported ion transporters with similar structures. The transport selectivity of the compound is in the
order of K+ >Rb+> Cs+> Na+> Li+.
The reversible photoisomerization of compound 1 in vesicle solution was detected by irradiation followed by extraction with chloroform. Conversion to
a predominantly cis- photostationary state was achieved after 15 minutes irradiation at 360 nm. The lifetime of the cis- isomer was found to be approximately 15 minutes in vesicle solution. This limits the exploration of the photogating using pH-stat titration because the lifetime is too short for the pH-stat experiment. In general, the poor solubility, which is caused by the contradiction of the strong polarity of the head groups and strong nonpolarity of the wall units, is a major problem in the detection of the photogate effect and should be avoided in future photogated ion channel designs.