Now showing 1 - 10 of 12
  • Publication
    Spectro-electrochemical Studies on [Ru(TAP)2 (dppz)]2+ - Insights into the Mechanism of its Photosensitized Oxidation of Oligonucleotides
    (American Chemical Society, 2019-01-07) ; ; ;
    [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).
      126Scopus© Citations 9
  • Publication
    Photophysical studies of CdTe quantum dots in the presence of a zinc cationic porphyrin
    The photophysical properties of 2.3 nm thioglycolic acid (TGA) coated CdTe quantum dots (QDs) prepared by a reflux method have been studied in the presence of cationic meso-tetrakis(4-N-methylpyridyl) zinc porphyrin (ZnTMPyP4). Addition of the CdTe QDs to the porphyrin in H2O results in a marked red-shift and hypochromism in the porphyrin absorption spectrum, indicative of a non-covalent binding interaction with the QD surface. Only low equivalents of the quantum dot were required for complete quenching of the porphyrin fluorescence revealing that one quantum dot may quench more than one porphyrin. Similarly addition of porphyrin to the quantum dot provided evidence for very efficient quenching of the CdTe photoluminescence, suggesting the formation of CdTe'porphyrin aggregates. Definitive evidence for such aggregates was gathered using small angle X-ray spectroscopy (SAXS). Ultrafast transient absorption data are consistent with very rapid photoinduced electron transfer (1.3 ps) and the resultant formation of a long-lived porphyrin species.
      450Scopus© Citations 25
  • Publication
    Ultrafast IR spectroscopy of polymeric cytosine nucleic acids reveal the long-lived species is due to a localised state
    The decay pathways of UV-excited cytosine polymers are investigated using picosecond time-resolved infrared spectroscopy. Similar yields of a non-emissive (1)nÏ * state are found in the single-stranded dC(30) polymer as in the dCMP monomer, but with a longer lifetime in the polymer (80 ps vs. 39 ps). A longer lifetime is also found in the d(CpC) dinucleotide. No evidence of excimer states is observed, suggesting that localised (1)nÏ * excited states are the most significant intermediates present on the picosecond timescale.
      334Scopus© Citations 11
  • Publication
    Monitoring guanine photo-oxidation by enantiomerically resolved Ru(II) dipyridophenazine complexes using inosine-substituted oligonucleotides
    The intercalating [Ru(TAP)2(dppz)]2+ complex can photo-oxidise guanine in DNA, although in mixed-sequence DNA it can be difficult to understand the precise mechanism due to uncertainties in where and how the complex is bound. Replacement of guanine with the less oxidisable inosine (I) base can be used to understand the mechanism of electron transfer (ET). Here the ET has been compared for both Λ- and Δ-enantiomers of [Ru(TAP)2(dppz)]2+ in a set of sequences where guanines in the readily oxidisable GG step in {TCGGCGCCGA}2 have been replaced with I. The ET has been monitored using picosecond and nanosecond transient absorption and picosecond time-resolved IR spectroscopy. In both cases inosine replacement leads to a diminished yield, but the trends are strikingly different for Λ- and Δ-complexes.
      89Scopus© Citations 13
  • Publication
    A comparative picosecond transient infrared study of 1-methylcytosine and 5'-dCMP that sheds further light on the excited states of cytosine derivatives
    The role of N1-substitution in controlling the deactivation processes in photoexcited cytosine derivatives has been explored using picosecond time-resolved IR spectroscopy. The simplest N1-substituted derivative, 1-methylcytosine, exhibits relaxation dynamics similar to the cytosine nucleobase and distinct from the biologically relevant nucleotide and nucleoside analogues, which have longer-lived excited-state intermediates. It is suggested that this is the case because the sugar group either facilitates access to the long-lived (1)n(O)Ï * state or retards its crossover to the ground state.
      418Scopus© Citations 40
  • Publication
    Long-Lived Excited-State Dynamics of i-Motif Structures Probed by Time-Resolved Infrared Spectroscopy
    UV-generated excited states of cytosine (C) nucleobases are precursors to mutagenic photoproduct formation. The i-motif formed from C-rich sequences is known to exhibit high yields of long-lived excited states following UV absorption. Here the excited states of several i-motif structures have been characterized following 267 nm laser excitation using time-resolved infrared spectroscopy (TRIR). All structures possess a long-lived excited state of ∼300 ps and notably in some cases decays greater than 1 ns are observed. These unusually long-lived lifetimes are attributed to the interdigitated DNA structure which prevents direct base stacking overlap.
      102Scopus© Citations 11
  • Publication
    Enantiomeric conformation controls rate and yield of photoinduced electron transfer in DNA sensitized by Ru(II) dipyridophenazine complexes
    Photosensitized oxidation of guanine is an important route to DNA damage. Ruthenium polypyridyls are very useful photosensitizers, as their reactivity and DNA-binding properties are readily tunable. Here we show a strong difference in the reactivity of the two enantiomers of [Ru(TAP)2(dppz)]2+, by using time-resolved visible and IR spectroscopy. This reveals that the photosensitized one-electron oxidation of guanine in three oligonucleotide sequences proceeds with similar rates and yields for bound Δ-[Ru(TAP)2(dppz)]2+, whereas those for the λ enantiomer are very sensitive to base sequence. It is proposed that these differences are due to preferences of each enantiomer for different binding sites in the duplex.
      128Scopus© Citations 26
  • Publication
    Long-lived excited states in i-motif DNA studied by picosecond time-resolved IR spectroscopy
    The transient IR absorption spectrum for UV-excited i-motif DNA is reported for the first time and found to possess complex dynamics pointing to multiple decay processes, including possible charge transfer between packed hemi-protonated C bases.
      184Scopus© Citations 22
  • Publication
    Inosine Can Increase DNA′s Susceptibility to Photo‐oxidation by a RuII Complex due to Structural Change in the Minor Groove
    Weinheim Key to the development of DNA-targeting phototherapeutic drugs is determining the interplay between the photoactivity of the drug and its binding preference for a target sequence. For the photo-oxidising lambda-[Ru(TAP)2(dppz)]2+ (Λ-1) (dppz=dipyridophenazine) complex bound to either d{T1C2G3G4C5G6C7C8G9A10}2 (G9) or d{TCGGCGCCIA}2 (I9), the X-ray crystal structures show the dppz intercalated at the terminal T1C2;G9A10 step or T1C2;I9A10 step. Thus substitution of the G9 nucleobase by inosine does not affect intercalation in the solid state although with I9 the dppz is more deeply inserted. In solution it is found that the extent of guanine photo-oxidation, and the rate of back electron-transfer, as determined by pico- and nanosecond time-resolved infrared and transient visible absorption spectroscopy, is enhanced in I9, despite it containing the less oxidisable inosine. This is attributed to the nature of the binding in the minor groove due to the absence of an NH2 group. Similar behaviour and the same binding site in the crystal are found for d{TTGGCGCCAA}2 (A9). In solution, we propose that intercalation occurs at the C2G3;C8I9 or T2G3;C8A9 steps, respectively, with G3 the likely target for photo-oxidation. This demonstrates how changes in the minor groove (in this case removal of an NH2 group) can facilitate binding of RuIIdppz complexes and hence influence any sensitised reactions occurring at these sites. No similar enhancement of photooxidation on binding to I9 is found for the delta enantiomer.
      106Scopus© Citations 16
  • Publication
    Direct observation by time-resolved infrared spectroscopy of the bright and the dark excited states of the [Ru(phen)2(dppz)]2+ light-switch compound in solution and when bound to DNA
    The [Ru(phen)2(dppz)]2+ complex (1) is non-emissive in water but is highly luminescent in organic solvents or when bound to DNA, making it a useful probe for DNA binding. To date, a complete mechanistic explanation for this “light-switch” effect is still lacking. With this in mind we have undertaken an ultrafast time resolved infrared (TRIR) study of 1 and directly observe marker bands between 1280–1450 cm−1, which characterise both the emissive “bright” and the non-emissive “dark” excited states of the complex, in CD3CN and D2O respectively. These characteristic spectral features are present in the [Ru(dppz)3]2+ solvent light-switch complex but absent in [Ru(phen)3]2+, which is luminescent in both solvents. DFT calculations show that the vibrational modes responsible for these characteristic bands are predominantly localised on the dppz ligand. Moreover, they reveal that certain vibrational modes of the “dark” excited state couple with vibrational modes of two coordinating water molecules, and through these to the bulk solvent, thus providing a new insight into the mechanism of the light-switch effect. We also demonstrate that the marker bands for the “bright” state are observed for both Λ- and Δ-enantiomers of 1 when bound to DNA and that photo-excitation of the complex induces perturbation of the guanine and cytosine carbonyl bands. This perturbation is shown to be stronger for the Λ-enantiomer, demonstrating the different binding site properties of the two enantiomers and the ability of this technique to determine the identity and nature of the binding site of such intercalators.
      154Scopus© Citations 43