Photoregulated transmembrane charge separation by linked spiropyran-anthraquinone molecules.
Amide-linked spiropyran-anthraquinone (SP-AQ) conjugates were shown to mediate ZnTPPS(4-)-photosensitized transmembrane reduction of occluded Co(bpy)3(3+) within unilamellar phosphatidylcholine vesicles by external EDTA. Overall quantum yields for these reactions were dependent upon the isomeric state of the dye; specifically, 30-35% photoconversion of the closed-ring spiropyran (SP) moiety to the open-ring merocyanine (MC) form caused the quantum yield to decrease by 6-fold in the simple conjugate and 3-fold for an analogue containing a lipophilic 4-dodecylphenoxy substituent on the anthraquinone moiety. Transient spectroscopic and fluorescence quenching measurements revealed that two factors contributed to these photoisomerization-induced changes in quantum yields: increased efficiencies of fluorescence quenching of 1ZnTPPS4- by the merocyanine group and lowered transmembrane diffusion rates of the merocyanine-containing redox carriers. Transient spectrophotometry also revealed the sequential formation and decay of two reaction intermediates, identified as 3ZnTPPS4- and a species with the optical properties of a semiquinone radical. Kinetic profiles for Co(bpy)3(3+) reduction under continuous photolysis in the presence and absence of added ionophores indicated that transmembrane redox mediated by SP-AQ was electroneutral, but reaction by the other quinone-containing mediators was electrogenic. The minimal reaction mechanism suggested from the combined studies is oxidative quenching of vesicle-bound 3ZnTPPS4- by the anthraquinone unit, followed by either H+/e- cotransport by transmembrane diffusion of SP-AQH* or, for the other redox mediators, semiquinone anion-quinone electron exchange leading to net transmembrane electron transfer, with subsequent one-electron reduction of the internal Co(bpy)3(3+). Thermal one-electron reduction of Co(bpy)3(3+) by EDTA is energetically unfavorable; the photosensitized reaction therefore occurs with partial conversion of photonic energy to chemical and transmembrane electrochemical potentials.
Zhu
Linyong
L
Department of Chemistry, Washington State University, Pullman, Washington 99164-4630, USA.
Khairutdinov
Rafail F
RF
Cape
Jonathan L
JL
Hurst
James K
JK
D015082:2,2'-Dipyridyl / Q000031:analogs & derivatives / Q000737:chemistry; D000880:Anthraquinones / Q000737:chemistry*; D001578:Benzopyrans / Q000737:chemistry*; D004492:Edetic Acid / Q000737:chemistry; D007211:Indoles / Q000737:chemistry*; D007700:Kinetics; D008567:Membranes, Artificial; D008665:Metalloporphyrins / Q000737:chemistry; D009574:Nitro Compounds / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry; D010084:Oxidation-Reduction; D010713:Phosphatidylcholines / Q000737:chemistry*; D010777:Photochemistry; D011789:Quantum Theory; D013050:Spectrometry, Fluorescence; D013056:Spectrophotometry, Ultraviolet
D000880:Anthraquinones; D001578:Benzopyrans; D007211:Indoles; D008567:Membranes, Artificial; D008665:Metalloporphyrins; D009574:Nitro Compounds; D009942:Organometallic Compounds; D010713:Phosphatidylcholines; C088184:spiropyran; C474512:zinc-tetrakis(p-sulfonatophenyl)porphyrin; C119791:tris(2,2'-bipyridyl)cobalt(III); D015082:2,2'-Dipyridyl; D004492:Edetic Acid