Electron-transfer oxidation properties of unsaturated fatty acids and mechanistic insight into lipoxygenases

Rate constants of photoinduced electron-transfer oxidation of unsaturated fatty acids with a series of singlet excited states of oxidants in acetonitrile at 298 K were examined and the resulting electron-transfer rate constants (k(et)) were evaluated in light of the free energy relationship of electron transfer to determine the one-electron oxidation potentials (E(ox)) of unsaturated fatty acids and the intrinsic barrier of electron transfer. The k(et) values of linoleic acid with a series of oxidants are the same as the corresponding k(et) values of methyl linoleate, linolenic acid, and arachidonic acid, leading to the same E(ox) value of linoleic acid, methyl linoleate, linolenic acid, and arachidonic acid (1.76 V vs SCE), which is significantly lower than that of oleic acid (2.03 V vs SCE) as indicated by the smaller k(et) values of oleic acid than those of other unsaturated fatty acids. The radical cation of linoleic acid produced in photoinduced electron transfer from linoleic acid to the singlet excited state of 10-methylacridinium ion as well as that of 9,10-dicyanoanthracene was detected by laser flash photolysis experiments. The apparent rate constant of deprotonation of the radical cation of linoleic acid was determined as 8.1 x 10(3) s(-1). In the presence of oxygen, the addition of oxygen to the deprotonated radical produces the peroxyl radical, which has successfully been detected by ESR. No thermal electron transfer or proton-coupled electron transfer has occurred from linoleic acid to a strong one-electron oxidant, Ru(bpy)3(3+) (bpy = 2,2'-bipyridine) or Fe(bpy)3(3+). The present results on the electron-transfer and proton-transfer properties of unsaturated fatty acids provide valuable mechanistic insight into lipoxygenases to clarify the proton-coupled electron-transfer process in the catalytic function.     

Site-selective electron transfer from purines to electrocatalysts: voltammetric detection of a biologically relevant deletion in hybridized DNA duplexes.

BACKGROUND: The one-electron oxidation of guanine nucleobases is of interest for understanding the mechanisms of mutagenesis, probing electron-transfer reactions in DNA, and developing sensing schemes for nucleic acids. The electron-transfer rates for oxidation of guanine by exogenous redox catalysts depend on the base paired to the guanine. An important goal in developing the mismatch sensitivity is to identify a means for monitoring the current resulting from electron transfer at a single base in the presence of native oligonucleotides that contain all four bases. RESULTS: The nucleobase 8-oxo-guanine (8G) is selectively oxidized by the redox catalyst Os(bpy)(3)(3+/2+) (bpy = 2,2'-bipyridine) in the presence of native guanine. Cyclic voltammograms of Os(bpy)(3)(2+) show current enhancements indicative of nucleobase oxidation upon addition of oligonucleotides that contain 8G, but not in the presence of native guanine. As expected, similar experiments with Ru(bpy)(3)(2+) show enhancement with both guanine and 8G. The current enhancements for the 8G/Os(III) reaction increase in the order 8G-C approximately 8G.T < 8G.G < 8G.A < 8G, the same order as that observed for guanine/Ru(III). This site-selective mismatch sensitivity can be applied to detection of a TTT deletion, which is important in cystic fibrosis. CONCLUSIONS: The base 8G can be effectively used in conjunction with a low-potential redox catalyst as a probe for selective electron transfer at a single site. Because of the high selectivity for 8G, rate constants can be obtained that reflect the oxidation of only one base. The mismatch sensitivity can be used to detect biologically relevant abnormalities in DNA.     

Oxidation of 7-deazaguanine by one-electron and oxo-transfer oxidants: mismatch-dependent electrochemistry and selective strand scission

Addition of oligonucleotides containing 7-deazaguanine (Z) to solutions containing Ru(dmb)3(2+) (dmb = 4,4'-dimethyl-2,2'-bipyridine) produces an enhancement in the oxidative current in the cyclic voltammogram of the metal complex that can be used, through digital simulation, to determine the rate of oxidation of 7-deazaguanine by Ru(dmb)3(3+). The measured rate constants are about 10 times higher than those for oxidation of guanine by Ru(bpy)3(3+), even though the redox potential of Ru(dmb)3(3+/2+) is 200 mV lower. A potential of 0.75 V (vs Ag/AgCl) can therefore be estimated for the oxidation of 7-deazaguanine, which can be selectively oxidized over guanine when Ru(dmb)3(3+) is the oxidant. The rate of oxidation was much faster in single-stranded DNA, and the difference between rates of single-stranded and duplex DNA was higher than for guanine. The oxidation rate was also sensitive to the presence of a single-base mismatch at the 7-deazaguanine in the order Z.C < Z.T < Z.G approximately Z.A < single-stranded. The Z.T mismatch was much more readily distinguished than the G.T mismatch, consistent with the overall greater sensitivity to secondary structure for Z. The oxidation reaction was also probed by monitoring piperidine-labile cleavage at the Z nucleotide, which could be generated by treatment with either photogenerated Ru(bpy)3(3+) or the thermal oxidant Ru(tpy)(bpy)O2+ (tpy = 2,2',2' '-terpyridine). These oxidants gave qualitatively similar selectivities to the electron-transfer rates from cyclic voltammetry, although the magnitudes of the selectivities were considerably lower on the sequencing gels.     

Reversed effects of DNA on hydride transfer and electron transfer reactions of acridinium and quinolinium ions

DNA inhibits hydride transfer from 1-benzyl-1,4-dihydronicotinamide to the 10-methylacridinium ion, whereas DNA accelerates photoinduced electron transfer from the excited state of Ru(bpy)3(2+) to the 1-methylquinolinium ion. The reason of such reversed effects of DNA on the hydride transfer and electron transfer reactions is clarified.     

DNA oxidation in anionic reverse micelles: ruthenium-mediated damage at Guanine in single- and double-stranded DNA

One-electron guanine oxidation in DNA has been investigated in anionic reverse micelles (RMs). A photochemical method for generating Ru3+ from the ruthenium polypyridyl complex tris(2-2'-bipyridine)ruthenium(II) chloride ([Ru(bpy)3]Cl2) is combined with high-resolution polyacrylamide gel electrophoresis (PAGE) to quantify piperidine-labile guanine oxidation products. As characterized by emission spectroscopy of Ru(bpy)3(2+), the addition of DNA to RMs containing Ru(bpy)3(2+) does not perturb the environment of Ru(bpy)3(2+). The steady-state quenching efficiency of Ru(bpy)3(2+) with K3[Fe(CN)6] in buffer solution is approximately 2-fold higher than that observed in RMs. Consistent with the difference in quenching efficiency in the two media, a 1.5-fold higher yield of piperidine-labile damage products as monitored by PAGE is observed for duplex oligonucleotide in buffer vs RMs. In contrast, a 13-fold difference in the yield of PAGE-detected G oxidation products is observed when single-stranded DNA is the substrate. Circular dichroism spectra showed that single-stranded DNA undergoes a structural change in anionic RMs. This structural change is potentially due to cation-mediated adsorption of the DNA phosphates on the anionic headgroups of the RMs, leading to protection of the guanine from oxidatively generated damage.     

Automated synthesis of 3'-metalated oligonucleotides

We report the first synthesis of a metallonucleoside bound to a solid support and subsequent oligonucleotide synthesis with this precursor. Large-scale syntheses of metal-containing oligonucleotides are achieved using a solid support modified with [Ru(bpy)(2)(impy')](2+) (bpy is 2,2'-bipyridine; impy' is 2'-iminomethylpyridyl-2'-deoxyuridine). A duplex formed with the metal-containing oligonucleotide exhibits superior thermal stability when compared to the corresponding unmetalated duplex (T(m) = 50 degrees C vs T(m) = 48 degrees C). Electrochemical (E(1/2) = 1.3 V vs NHE), absorption (lambda(max) = 480 nm), and emission (lambda(max) = 720 nm, tau = 44 ns, Phi = 0.11 x 10(-)(3)) data for the ruthenium-modified oligonucleotides indicate that the presence of the oligonucleotide does not perturb the electronic properties of the ruthenium complex. The absence of any change in the emission properties upon duplex formation suggests that the [Ru(bpy)(2)(impy)](2+) chromophore will be a valuable probe for DNA-mediated electron-transfer studies. Despite the relatively high Ru(III/II) reduction potential, oxidative quenching of photoexcited [Ru(bpy)(2)(impy)](2+) does not lead to oxidative damage of guanine or other DNA bases.     

Electron-transfer oxidation of coenzyme B12 model compounds and facile cleavage of the cobalt(IV)-carbon bond via charge-transfer complexes with bases. A negative temperature dependence of the rates

The electron-transfer oxidation and subsequent cobalt-carbon bond cleavage of vitamin B12 model complexes were investigated using cobaloximes, (DH)2Co(III)(R)(L), where DH- = the anion of dimethylglyoxime, R = Me, Et, Ph, PhCH2, and PhCH(CH3), and L = a substituted pyridine, as coenzyme B12 model complexes and [Fe(bpy)3](PF6)3 or [Ru(bpy)3](PF6)3 (bpy = 2,2'-bipyridine) as a one-electron oxidant. The rapid one-electron oxidation of (DH)2Co(III)(Me)(py) (py = pyridine) with the oxidant gives the corresponding Co(IV) complexes, [(DH)2Co(IV)(Me)(py)]+, which were well identified by the ESR spectra. The reorganization energy (lambda) for the electron-transfer oxidation of (DH)2Co(Me)(py) was determined from the ESR line broadening of [(DH)2Co(Me)(py)]+ caused by the electron exchange with (DH)2Co(Me)(py). The lambda value is applied to evaluate the rate constants of photoinduced electron transfer from (DH)2Co(Me)(py) to photosensitizers in light of the Marcus theory of electron transfer. The Co(IV)-C bond cleavage of [(DH)2Co(Me)(py)]+ is accelerated significantly by the reaction with a base. The overall activation energy for the second-order rate constants of Co(IV)-C bond cleavage of [(DH)2Co(IV)(Me)(py)]+ in the presence of a base is decreased by charge-transfer complex formation with a base, which leads to a negative activation energy for the Co(IV)-C cleavage when either 2-methoxypyridine or 2,6-dimethoxypyridine is used as the base.     

Kinetics of metal-mediated one-electron oxidation of guanine in polymeric DNA and in oligonucleotides containing trinucleotide repeat sequences

The oxidation of guanines in DNA by Ru(III) is investigated by catalytic electrochemistry and stopped-flow spectrophotometry. The reactions of calf thymus DNA (20% guanine) and herring testes DNA (25% guanine) with Ru(bpy)3(3+) (bpy = 2,2'-bipyridine) show biexponential decays in stopped-flow spectrophotometric experiments with the fast and slow components in 2:1 ratios and average rate constants in 880 mM NaCl of = 18,700 M-1 s-1 for calf thymus DNA and = 24,600 M-1 s-1 for herring testes DNA. The higher rate constant for the more guanine-rich DNA is possibly due to a higher density of electron-rich guanine multiplets. The observation of a biexponential decay is incorporated into digital simulations of the catalytic voltammograms observed for Ru(bpy)3(2+) in the presence of DNA. For both DNAs, the rates observed by voltammetry are somewhat slower than those observed by stopped-flow spectrophotometry and the dependence of the rate constants on scan rate using the biexponential model is less pronounced than when only one decay is treated, supporting the notion that the scan rate dependence arises from the multiphasic decay. At low salt concentrations, where binding of the metal complex to DNA increases the effective catalytic rate constant, rates can be measured by stopped-flow spectrophotometry only with a less oxidizing complex, Fe(bpy)3(3+/2+), which yields trends in the rate constants similar to those observed for the case of Ru(bpy)3(3+/2+) at high ionic strength. Oligonucleotides based on the trinucleotide repeat sequences (AGT)n and (GAA)n produce significant catalytic currents, which are readily interpreted in terms of the guanine concentration and the secondary structure discerned from gel electrophoresis experiments. These experiments may provide a basis for sensing secondary structures and repeat numbers in biologically relevant DNAs.     

5' modification of duplex DNA with a ruthenium electron donor-acceptor pair using solid-phase DNA synthesis

Incorporation of metalated nucleosides into DNA through covalent modification is crucial to measurement of thermal electron-transfer rates and the dependence of these rates with structure, distance, and position. Here, we report the first synthesis of an electron donor-acceptor pair of 5' metallonucleosides and their subsequent incorporation into oligonucleotides using solid-phase DNA synthesis techniques. Large-scale syntheses of metal-containing oligonucleotides are achieved using 5' modified phosporamidites containing [Ru(acac)(2)(IMPy)](2+) (acac is acetylacetonato; IMPy is 2'-iminomethylpyridyl-2'-deoxyuridine) (3) and [Ru(bpy)(2)(IMPy)](2+) (bpy is 2,2'-bipyridine; IMPy is 2'-iminomethylpyridyl-2'-deoxyuridine) (4). Duplexes formed with the metal-containing oligonucleotides exhibit thermal stability comparable to the corresponding unmetalated duplexes (T(m) of modified duplex = 49 degrees C vs T(m) of unmodified duplex = 47 degrees C). Electrochemical (3, E(1/2) = -0.04 V vs NHE; 4, E(1/2) = 1.12 V vs NHE), absorption (3, lambda(max) = 568, 369 nm; 4, lambda(max) = 480 nm), and emission (4, lambda(max) = 720 nm, tau = 55 ns, Phi = 1.2 x 10(-)(4)) data for the ruthenium-modified nucleosides and oligonucleotides indicate that incorporation into an oligonucleotide does not perturb the electronic properties of the ruthenium complex or the DNA significantly. In addition, the absence of any change in the emission properties upon metalated duplex formation suggests that the [Ru(bpy)(2)(IMPy)](2+)[Ru(acac)(2)(IMPy)](2+) pair will provide a valuable probe for DNA-mediated electron-transfer studies.     

Charge separation in a ruthenium-quencher conjugate bound to DNA

A novel tris heteroleptic dipyridophenazine complex of ruthenium(II), [{Ru(phen)(dppz)(bpy'-his)}{Ru(NH3)5}]5+, containing a covalently tethered ruthenium pentammine quencher coordinated through a bridging histidine has been synthesized and characterized spectroscopically and biochemically in a DNA environment and in organic solvent. Steady-state and time-resolved luminescence measurements indicate that the tethered Ru complex is quenched relative to the parent complexes [Ru(phen)(dppz)(bpy')]2+ and [Ru(phen)(dppz)(bpy'-his)]2+ in DNA and acetonitrile, consistent with intramolecular photoinduced electron transfer. Intercalated into guanine-containing DNA, [{Ru(phen)(dppz)(bpy'-his)}{Ru(NH3)5}]5+, upon excitation and intramolecular quenching, is capable of injecting charge into the duplex based upon the EPR detection of guanine radicals. DNA-mediated charge transport is also indicated using a kinetically fast cyclopropylamine-substituted base as an electron hole trap. Guanine damage is not observed, however, in measurements using the guanine radical as the kinetically slower hole trap, indicating that back electron-transfer reactions are competitive with guanine oxidation. Moreover, transient absorption measurements reveal a novel photophysical reaction pathway for [{Ru(phen)(dppz)(bpy'-his)}{Ru(NH3)5}]5+ in the presence of DNA that is competitive with the intramolecular flash-quench process. These results illustrate the remarkably rich redox chemistry that can occur within a bimolecular ruthenium complex intercalated in duplex DNA.     

Mechanisms of electron-transfer oxidation of NADH analogues and chemiluminescence. Detection of the keto and enol radical cations

The radical cation of an NADH analogue (BNAH: 1-benzyl-1,4-dihydronicotinamide) has been successfully detected as the transient absorption and ESR spectra in the thermal electron transfer from BNAH to Fe(bpy)(3)(3+) (bpy = 2,2'-bipyridine) and Ru(bpy)(3)(3+). The ESR spectra of the radical cations of BNAH and the dideuterated compound (BNAH-4,4'-d(2)) indicate that the observed radical cation is the keto form rather than the enol form in the tautomerization. The deprotonation rate and the kinetic isotope effects of the keto form of BNAH(*)(+) were determined from the kinetic analysis of the electron-transfer reactions. In the case of electron transfer from BNAH to Ru(bpy)(3)(3+), the chemiluminescence due to Ru(bpy)(3)(2+) was observed in the second electron-transfer step from BNA(*), produced by the deprotonation of the keto form of BNAH(*)(+), to Ru(bpy)(3)(3+). The observation of chemiluminescence due to Ru(bpy)(3)(2+) provides compelling evidence that the Marcus inverted region is observed even for such an intermolecular electron-transfer reaction. When BNAH is replaced by 4-tert-butylated BNAH (4-t-BuBNAH), no chemiluminescence due to Ru(bpy)(3)(2+) has been observed in the electron transfer from 4-t-BuBNAH to Ru(bpy)(3)(3+). This is ascribed to the facile C-C bond cleavage in 4-t-BuBNAH(*)(+). In the laser flash photolysis of a deaerated MeCN solution of BNAH and CHBr(3), the transient absorption spectrum of the enol form of BNAH(*)(+) was detected instead of the keto form of BNAH(*)(+), and the enol form was tautomerized to the keto form. The rate of intramolecular proton transfer in the enol form to produce the keto form of BNAH(*)(+) was determined from the decay of the absorption band due to the enol form and the rise in the absorption band due to the keto form. The kinetic isotope effects were observed for the intramolecular proton-transfer process in the keto form to produce the enol form.     

Photocatalytic generation of a non-heme oxoiron(IV) complex with water as an oxygen source

The photocatalytic formation of a non-heme oxoiron(IV) complex, [(N4Py)Fe(IV)(O)](2+) [N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine], efficiently proceeds via electron transfer from the excited state of a ruthenium complex, [Ru(II)(bpy)(3)](2+)* (bpy = 2,2'-bipyridine) to [Co(III)(NH(3))(5)Cl](2+) and stepwise electron-transfer oxidation of [(N4Py)Fe(II)](2+) with 2 equiv of [Ru(III)(bpy)(3)](3+) and H(2)O as an oxygen source. The oxoiron(IV) complex was independently generated by both chemical oxidation of [(N4Py)Fe(II)](2+) with [Ru(III)(bpy)(3)](3+) and electrochemical oxidation of [(N4Py)Fe(II)](2+).     

Probing the solvent accessibility and electron density of adenine: oxidation of 7-deazaadenine in bent DNA and purine doublets

The effect of DNA bending on nucleobase electron transfer was investigated by studying the oxidation of double-stranded sequences containing seven repeats of the known bent sequence d(GGCA(1)A(2)A(3)A(4)A(5)A(6)C) where 7-deazaadenine (zA) was substituted at the A(3) position. Native gel electrophoresis was used to show that the sequence remained bent upon substitution of zA, which provides for oxidation of the sequence by Ru(bpy)(3)(3+) (bpy = 2,2'-bipyridine). The Ru(III) oxidant was generated by photolysis of Ru(bpy)(3)(2+) in the presence of ferricyanide, and the oxidation was visualized by high-resolution gel electrophoresis of the radiolabeled DNA sequence following base treatment. Cleavage of the DNA strand at the guanine residues and at the zA residues was observed. Comparison of the oxidation of zA in bent DNA versus the normal B form showed that hybridization of the B form sequence to its Watson-Crick complement produced a reduction in cleavage by a factor of 5.19 +/- 0.46 while hybridization of the bent sequence only reduced cleavage by a factor of 1.58 +/- 0.23. This result implies that the zA in the double-stranded, bent sequence is much more solvent-exposed than in normal B-form DNA. When the zA occurred in a B-form 5'-zA-G doublet, the reactivity was 6.63 +/- 0.10 times higher for the zA compared to the G. This implies an even greater effect of a 3'-guanine on the oxidation potential of zA than in the well-known 5'-GG doublet.     

Photocurrent amplification by an energy/electron transfer cascade in polymer Langmuir-Blodgett films

Effective photocurrent generation by visible light irradiation on hetero-deposited polymer Langmuir-Blodgett (LB) films containing tris(bipyridine) ruthenium(II) (Ru(bpy)3(2+)) and anthracene derivatives was observed. The photocurrent amplification was found to be assisted by the photoinduced energy/electron transfer cascade, which consists of the interlayer triplet-triplet energy transfer process from photoexcited Ru(bpy)3(2+) to anthracene, and then electron transfer processes from the triplet anthracene to a viologen acceptor, from Ru(bpy)3(2+) to the oxidized anthracene and from the electrode to Ru(bpy)3(3+).     

Redox-switchable direction of photoinduced electron transfer in an Ru(bpy)3(2+)-viologen dyad

Quenching of the 3MLCT excited state of [Ru(bpy)3]2+ (bpy=bipyridine) by the reduction products (MV*+ and MV0) of methyl viologen (MV2+) was studied by a combination of electrochemistry with laser flash photolysis or femtosecond pump-probe spectroscopy. Both for the bimolecular reactions and for the reactions in an Ru(bpy)3(2+)-MVn+ dyad, quenching by MV*+ and MV0 is reductive and gives the reduced ruthenium complex [Ru(bpy)3]+, in contrast to the oxidative quenching by MV2+. Rate constants of quenching (kq), and thermal charge recombination (krec) and cage escape yields (phi(ce)) were determined for the bimolecular reactions, and rates of forward (kf) and backward (kb) electron transfer in the dyad were measured for quenching by MV2+, MV*+, and MV0. The reactions in the dyad are very rapid, with values up to kf = 1.3 x 10(12) s(-1) for *Ru(bpy)3(2+)-MV*+. In addition, a long-lived (tau = 15 ps) vibrationally excited state of MV*+ with a characteristically structured absorption spectrum was detected; this was generated by direct excitation of the MV*+ moiety both at 460 and 600 nm. The results show that the direction of photoinduced electron transfer in a Ru(bpy)3-MV molecule can be switched by an externally applied bias.     

Synthesis and characterization of dinuclear ruthenium complexes covalently linked to Ru(II) tris-bipyridine: an approach to mimics of the donor side of photosystem II

To mimic the electron-donor side of photosystem II (PSII), three trinuclear ruthenium complexes (2, 2a, 2b) were synthesized. In these complexes, a mixed-valent dinuclear Ru2(II,III) moiety with one phenoxy and two acetato bridges is covalently linked to a Ru(II) tris-bipyridine photosensitizer. The properties and photoinduced electron/energy transfer of these complexes were studied. The results show that the Ru2(II,III) moieties in the complexes readily undergo reversible one-electron reduction and one-electron oxidation to give the Ru2(II,III) and Ru2(III,III) states, respectively. This could allow for photooxidation of the sensitizer part with an external acceptor and subsequent electron transfer from the dinuclear ruthenium moiety to regenerate the sensitizer. However, all trinuclear ruthenium complexes have a very short excited-state lifetime, in the range of a few nanoseconds to less than 100 ps. Studies by femtosecond time-resolved techniques suggest that a mixture of intramolecular energy and electron transfer between the dinuclear ruthenium moiety and the excited [Ru(bpy)3]2+ photosensitizer is responsible for the short lifetimes. This problem is overcome by anchoring the complexes with ester- or carboxyl-substituted bipyridine ligands (2a, 2b) to nanocrystalline TiO2, and the desired electron transfer from the excited state of the [Ru(bpy)3]2+ moiety to the conduction band of TiO2 followed by intramolecular electron transfer from the dinuclear Ru2(II,III) moiety to photogenerated Ru(III) was observed. The resulting long-lived Ru2(III,III) state decays on the millisecond timescale.     

Tetranuclear polybipyridyl complexes of Ru(II) and Mn(II), their electro- and photo-induced transformation into di-mu-oxo Mn(III)Mn(IV) hexanuclear complexes

Three heterotetranuclear complexes, [{Ru(II)(bpy)(2)(L(n))}(3)Mn(II)](8+) (bpy = 2,2'-bipyridine, n = 2, 4, 6), in which a Mn(II)-tris-bipyridine-like centre is covalently linked to three Ru(II)-tris-bipyridine-like moieties using bridging bis-bipyridine L(n) ligands, have been synthesised and characterised. The electrochemical, photophysical and photochemical properties of these complexes have been investigated in CH(3)CN. The cyclic voltammograms of the three complexes exhibit two successive very close one-electron metal-centred oxidation processes in the positive potential region. The first, which is irreversible, corresponds to the Mn(II)/Mn(III) redox system (E(pa) approximately 0.82 V vs Ag/Ag(+) 0.01 M in CH(3)CN-0.1 M Bu(4)NClO(4)), whereas the second which is, reversible, is associated with the Ru(II)/Ru(III) redox couple (E(1/2) approximately 0.91 V). In the negative potential region, three successive reversible four electron systems are observed, corresponding to ligand-based reduction processes. The three stable dimeric oxidized forms of the complexes, [Mn(2)(III,IV)O(2){Ru(II)(bpy)(2)(L(n))}(4)](11+), [Mn(2)(IV,IV)O(2){Ru(II)(bpy)(2)(L(n))}(4)](12+) and [Mn(2)(IV,IV)O(2){Ru(III)(bpy)(2)(L(n))}(4)](16+) are obtained in fairly good yields by sequential electrolyses after consumption of respectively 1.5, 0.5 and 3 electrons per molecule of initial tetranuclear complexes. The formation of the di-micro-oxo binuclear complexes are the result of the instability of the {[Ru(II)(bpy)(2)(L(n))](3)Mn(III)}(9+) species, which react with residual water, via a disproportionation reaction and the release of one ligand, [Ru(II)(bpy)(2)(L(n))](2+). A quantitative yield can be obtained for these reactions if the electrochemical oxidations are performed in the presence of an added external base like 2,6-dimethylpyridine. Photophysical properties of these compounds have been investigated showing that the luminescence of the Ru(II)-tris-bipyridine-like moieties is little affected by the presence of manganese within the tetranuclear complexes. A slight quenching of the excited states of the ruthenium moieties, which occurs by an intramolecular process, has been observed. Measurements made at low concentration (<1 x 10(-5) M) indicate that some decoordination of Mn(2+) arises in 1a-c. These measurements allow the calculation of the association constants for these complexes. Finally, photoinduced oxidation of the tetranuclear complexes has been performed by continuous photolysis experiments in the presence of a large excess of a diazonium salt, acting as a sacrificial oxidant. The three successive oxidation processes, Mn(II)--> Mn(III)Mn(IV), Mn(III)Mn(IV)--> Mn(IV)Mn(IV) and Ru(II)--> Ru(III) are thus obtained, the addition of 2,6-dimethylpyridine in the medium giving an essentially quantitative yield for the two first photo-induced oxidation steps as found for electrochemical oxidation.     

Electrochemical determination of triple helices: electrocatalytic oxidation of guanine in an intramolecular triplex

Electrocatalytic oxidation of the oligonucleotide 5'- GAA GAG GTT TTT CCT CTT CTT TTT CTT CTC C (TS) by Ru(bpy)(3)(2+) was studied by cyclic voltammetry. This oligonucleotide forms either an intramolecular triplex, hairpin, or single strand, depending on the pH (Plum, G. E.; Breslauer, K. J. J. Mol. Biol. 1995, 248, 679-695). In the triplex form, the guanine doublet in TS is buried inside the folded structure, and as such is less susceptible to oxidation by electrogenerated Ru(bpy)(3)(3+). Digital simulations of the catalytic voltammograms gave a rate constant of 3.5 +/- 0.2 x 10(2) M(-1) s(-1) for oxidation of the triplex form, while oxidation of the duplex and single-stranded forms occurred with much faster rate constants of (3.5-9.1) x 10(4) M(-1) s(-1). Experiments using a truncated form of TS that lacked the third strand of the triplex were consistent with these measurements. The Ru(bpy)(3)(3+) complex was also generated by photolyzing Ru(bpy)(3)(2+) in the presence of Fe(CN)(6)(3-). This reaction produced strand scission following piperidine treatment, which was visualized using high-resolution gel electrophoresis. These experiments showed decreased reactivity for the triplex form, and also gave an unusual reversal of a common selectivity for the 5'-G of GG doublets generally seen in B-form DNA. This reversal was ascribed to strain caused by the location of the GG doublet adjacent to the hairpin loop.     

PHOTO-INDUCED ELECTRON TRANSFER FROM NUCLEOTIDES TO RUTHENIUM-TRIS-1,4,5,8TETRAAZAPHENANTHRENE: MODEL FOR PHOTOSENSITIZED DNA OXIDATION

The luminescence quenching of ruthenium-tris-1,4,5,8-tetraazaphenanthrene [Ru(tap)3(2+)] by nucleotides approaches the diffusion rate only with guanosine-5'-monophosphate (GMP), the most reducing nucleotide, and leads to an electron transfer with the production of the monoreduced complex and the oxidized base. The resulting deprotonated GMP(-H).radical recombines with the monoreduced complex according to a bimolecular equimolar process. The pH dependence of the decay of the transient reduced complex, in the presence of an oxidant (oxygen or benzoquinone) indicates the formation of Ru(tap)2(tapH)2+, i.e. the reduced protonated species, subsequent to the electron transfer, with a pKa of 7.6 as confirmed from pulse radiolysis experiments. As the non-protonated reduced complex, Ru(tap)2(tap-.)+, has a higher reducing power than the protonated one, oxygen is able to reoxidize only the non-protonated species, whereas benzoquinone reoxidizes both species but with different rate constants. The flash photolysis of Ru(tap)3(2+) in the presence of DNA and the effect of Mg2+ ions and GMP as supplementary additives also show the existence of a photo-induced electron transfer with the nucleic acid, which can be correlated to the photosensitized cleavage of DNA by this complex.     

Photodriven electron and energy transfer from a light-harvesting metallodendrimer

Intermolecular electron and energy transfer from a light-harvesting metallodendrimer [Ru[bpy(C-450)(4)](3)](2+), where bpy(C-450)(4) is a 2,2'-bipyridine derivative containing 4 coumarin-450 units connected together through aryl ether linkages, is observed in acetonitrile solutions at room temperature. The model complex [Ru(dmb)(3)](2+), where dmb is 4,4'-dimethyl-2,2'-bipyridine, is included for quantitative comparison. The excited states of both compounds are metal-to-ligand charge transfer in nature and participate in excited-state electron and triplet energy transfer processes. Quenching constants were determined from luminescence and time-resolved absorption experiments at constant ionic strength. [Ru[bpy(C-450)(4)](3)](2+) displays significantly slower quenching rates to molecular oxygen and methyl viologen relative to the other processes investigated. Triplet energy transfer from [Ru[bpy(C-450)(4)](3)](2+) to 9-methylanthracene is quantitatively indistinguishable from [Ru(dmb)(3)](2+) while reductive electron transfer from phenothiazine was slightly faster in the former. With the exception of dioxygen quenching, our results indicate that the current dendritic structure is ineffective in shielding the core from bimolecular electron and triplet energy transfer reactions. Electrochemical measurements of [Ru[bpy(C-450)(4)](3)](2+) reveal irreversible oxidative processes at potentials slightly negative to the Ru(III/II) potential that are assigned to oxidations in the dendritic structure. Excited-state oxidative electron-transfer reactions facilitate this process resulting in the reduction of ground-state Ru(III) to Ru(II) and the trapping of the methyl viologen radical cation (MV(*+)) when methyl viologen serves as the quencher. This process generates a minimum of 9 MV(*+)'s for every [Ru[bpy(C-450)(4)](3)](2+) molecule and disassembles the metallodendrimer, resulting in the production of a [Ru(dmb)(3)](2+)-like species and "free" C-450-like dyes.