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Cyclic Transmembrane Charge Transport Mediated by Pyrylium and Thiopyrylium Ions
journal contribution
posted on 2001-07-10, 00:00 authored by Rafail F. Khairutdinov, James K. HurstTransient spectroscopy revealed that 2,4,6-trimethylpyrylium, 2,4,6-triphenylpyrylium, and 2,4,6-triphenylthiopyrylium ions oxidatively quench excited triplet [5,10,15,20-tetrakis(4-sulfonatophenyl)porphinato]zinc(II) to form the corresponding neutral radicals and the zinc porphyrin π-cation. The measured quenching
rate constants were proportional to the pyrylium one-electron reduction potentials, that is, the reaction driving
force. In the presence of anionic dihexadecyl phosphate vesicles, only the fraction of pyrylium not bound to
the vesicle was capable of reacting with the photoexcited zinc porphyrin. Nonetheless, the pyrylium radicals
mediated highly efficient transmembrane reduction of tris(2,2‘-bipyridine)cobalt(III) contained within the inner
aqueous core of the vesicles with apparent quantum yields that approached unity. Permeability coefficients
(P) determined for the pyrylium radicals, pyrylium cations, and the proton were 10-4−2 × 10-5 cm/s, 10-10
cm/s, and <5 × 10-7 cm/s, respectively, so that only the neutral radicals are membrane-permeable on the time
scale of the transmembrane redox reactions. However, each electron carrier was demonstrated to transport up
to 200 electrons, at which point the internal pool of electron acceptors was exhausted. Since the cations are
membrane-impermeable, a reaction cycle is proposed that includes hydrolysis of the pyrylium cations formed
within the aqueous core to the corresponding 1,5-diketones which, as neutral molecules, can diffuse across the
bilayer. According to this mechanism, while undergoing redox cycling the pyrylium ions function as cyclical
antiporters of OH- and the electron, thereby maintaining electroneutrality in the reaction compartments.