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Spectro-electrochemical Studies on [Ru(TAP)2 (dppz)]2+ - Insights into the Mechanism of its Photosensitized Oxidation of Oligonucleotides
Keane, Páraic M.; Tory, Joanne; Towrie, Michael; Quinn, Susan J.
[Ru(TAP) 2 (dppz)] 2+ (TAP = 1,4,5,8-tetraazaphenanthrene; dppz = dipyrido[3,2-a:2′,3′-c]phenazine) is known to photo-oxidize guanine in DNA. Whether this oxidation proceeds by direct photoelectron transfer or by proton-coupled electron transfer is still unknown. To help distinguish between these mechanisms, spectro-electrochemical experiments have been carried out with [Ru(TAP) 2 (dppz)] 2+ in acetonitrile. The UV-vis and mid-IR spectra obtained for the one-electron reduced product were compared to those obtained by picosecond transient absorption and time-resolved infrared experiments of [Ru(TAP) 2 (dppz)] 2+ bound to guanine-containing DNA. An interesting feature of the singly reduced species is an electronic transition in the near-IR region (with λ max at 1970 and 2820 nm). Density functional and time-dependent density functional theory simulations of the vibrational and electronic spectra of [Ru(TAP) 2 (dppz)] 2+ , the reduced complex [Ru(TAP) 2 (dppz)] + , and four isomers of [Ru(TAP)(TAPH)(dppz)] 2+ (a possible product of proton-coupled electron transfer) were performed. Significantly, these predict absorption bands at λ > 1900 nm (attributed to a ligand-to-metal charge-transfer transition) for [Ru(TAP) 2 (dppz)] + but not for [Ru(TAP)(TAPH)(dppz)] 2+ . Both the UV-vis and mid-IR difference absorption spectra of the electrochemically generated singly reduced species [Ru(TAP) 2 (dppz)] + agree well with the transient absorption and time-resolved infrared spectra previously determined for the transient species formed by photoexcitation of [Ru(TAP) 2 (dppz)] 2+ intercalated in guanine-containing DNA. This suggests that the photochemical process in DNA proceeds by photoelectron transfer and not by a proton-coupled electron transfer process involving formation of [Ru(TAP)(TAPH)(dppz)] 2+ , as is proposed for the reaction with 5′-guanosine monophosphate. Additional infrared spectro-electrochemical measurements and density functional calculations have also been carried out on the free TAP ligand. These show that the TAP radical anion in acetonitrile also exhibits strong broad near-IR electronic absorption (λ max at 1750 and 2360 nm). Spectroelectrochemistry Reading Royal Irish Academy/Royal Society Exchange Programme
Keyword(s): Ruthenium; Phenazines; Phenanthrenes; DNA; Oligonucleotides; Intercalating agents; Ligands; Oxidation-reduction; Light; Models, chemical; Electrochemical techniques; Coordination complexes; Density functional theory
Publication Date:
2020
Type: Journal article
Peer-Reviewed: Unknown
Language(s): English
Institution: University College Dublin
Publisher(s): American Chemical Society
File Format(s): other
First Indexed: 2020-10-01 06:22:18 Last Updated: 2020-10-01 06:22:18