SINGLE- and DOUBLE-STRAND BREAK FORMATION IN DOUBLE-STRANDED DNA UPON NANOSECOND LASER-INDUCED PHOTOIONIZATION

Double-stranded (ds) calf thymus DNA (0.4 mM), excited by 20 ns laser pulses at 248 nm, was studied in deoxygenated aqueous solution at room temperature and pH 6.7 in the presence of a sodium salt (10 mM). The quantum yields for the formation of hydrated electrons (phi c-), single-strand breaks (phi ssb) and double-strand breaks (phi dsb) were determined for various laser pulse intensities (IL). phi c- and phi ssb increase linearly with increasing IL; however, phi ssb has a tendency to reach saturation at high IL (greater than 5 X 10(6) Wcm-2). The ratio phi ssb/phi c-, representing the number of ssb per radical cation, is about 0.08 at IL less than or equal to 5 X 10(6) Wcm-2. For comparison, the number of ssb per OH radical reacting with dsDNA is 0.22. On going from argon to N2O saturation, phi ssb and phi dsb become larger by factors of approximately 5 and 10-15, respectively. This enhancement is produced by attack on DNA bases by OH radicals generated by N2O-scavenging of the photoelectrons. While phi ssb is essentially independent of the dose (Etot), phi dsb depends linearly on Etot in both argon- and N2O-saturated solutions. The linear dependence of phi dsb implies a square dependence of the number of dsb on Etot. This portion of dsb formation is explained by the occurrence of two random ssb, generated within a critical distance (h) in opposite strands. For both argon- and N2O-saturated solutions h was found to be of the order of 40-70 phosphoric acid diester bonds. On addition of electron scavengers such as 2-chloroethanol (or N2O plus t-butanol), phi dsb is similar to that in neat, argon-saturated solutions. Thus, hydrated electrons are not involved in the chemical pathway leading to laser-pulse-induced dsb of DNA.     

Preparation of a new carboranyl lactoside for the treatment of cancer by boron neutron capture therapy: synthesis and toxicity of fluoro carboranyl glycosides for in vivo 19F-NMR spectroscopy

The synthesis of new ortho-carboranyl lactosides 8, 17, 19 and glucosides 22 and 23 for the use in boron neutron capture therapy is reported. Carboranyl lactosides 17 and 19 as well as the glucosides 22 and 23 contain a fluorine atom to allow a noninvasive determination of these compounds in tumor cells by 19F-NMR spectroscopy. In cloning efficiency tests on human bronchial carcinoma cells the carboranyl lactosides 17 and 19 displayed almost no cytotoxicity. Thus, the considerably cytotoxic carboranyl alcohol 11 is detoxified when linked to a sugar moiety such as in carboranyl glucoside 22.     

Octahedral (cis-cyclam)iron(III) complexes with O,N-coordinated o-iminosemiquinonate(1-) pi radicals and o-imidophenolate(2-) anions

Three octahedral complexes containing a (cis-cyclam)iron(III) moiety and an O,N-coordinated o-iminobenzosemiquinonate pi radical anion have been synthesized and characterized by X-ray crystallography at 100 K: [Fe(cis-cyclam)(L(1-3)(ISQ))](PF(6))(2) (1-3), where (L(1-3)(ISQ)) represents the monoanionic pi radicals derived from one-electron oxidations of the respective dianion of o-imidophenolate(2-), L(1), 2-imido-4,6-di-tert-butylphenolate(2-), L(2), and N-phenyl-2-imido-4,6-di-tert-butylphenolate(2-), L(3). Compounds 1-3 possess an S(t) = 0 ground state, which is attained via strong intramolecular antiferromagnetic exchange coupling between a low-spin central ferric ion (S(Fe) = 1/2) and an o-imino-benzosemiquinonate(1-) pi radical (S(rad) = 1/2). Zero-field M?ssbauer spectra of 1-3 at 80 K confirm the low-spin ferric electron configuration: isomer shift delta = 0.26 mm s(-1) and quadrupole splitting DeltaE(Q) = 1.96 mm s(-1) for 1, 0.28 and 1.93 for 2, and 0.33 and 1.88 for 3. All three complexes undergo a reversible, one-electron reduction of the coordinated o-imino-benzosemiquinonate ligand, yielding an [Fe(III)(cis-cyclam)(L(1-3)(IP))](+) monocation. The monocations of 1 and 2 display very similar rhombic signals in the X-band EPR spectra (g = 2.15, 2.12, and 1.97), indicative of low-spin ferric species. In contast, the monocation of 3 contains a high-spin ferric center (S(Fe) = 5/2) as is deduced from its M?ssbauer and EPR spectra.     

HO2 ELIMINATION FROM α-HYDROXYALKYLPEROXYL RADICALS IN AQUEOUS SOLUTION

Abstract— In aqueous solutions α-hydroxyalkylperoxyl radicals undergo a spontaneous and a base catalysed HO₂ elimination. From kinetic deuterium isotope effects, temperature dependence, and the influence of solvent polarity it was concluded that the spontaneous reaction occurs via an HO₂ elimination followed by the dissociation of the latter into H⁺ and O₂^-. The rate constant of the spontaneous HO₂ elimination increases with increasing methyl substitution in α-position ( k (CH₂(OH)O₂) < 10s^-1 k (CH₃CH(OH)O₂) = 52s^-1 k ((CH₃)₂C(OH)O₂) = 665 s^-1). The OH^- catalysed reaction is somewhat below diffusion controlled. The mixture of peroxyl radicals derived from polyhydric alcohols eliminate HO₂ at two different rates. Possible reasons for this behaviour are discussed. The mixture of the six peroxyl radicals derived from d -glucose are observed to eliminate HO₂ with at least three different rates. The fastest rate is attributed to the HO₂ elimination from the peroxyl radical at C-l ( k > 7000s^-1). Because of the HO₂ eliminations the peroxyl radicals derived from d -glucose do not undergo a chain reaction in contrast to peroxyl radicals not containing an α-OH group. In competition with the first order elimination reactions the α-hydroxylalkylperoxyl radicals undergo a bimolecular decay. These reactions are briefly discussed.     

Synthesis and characterization of tris(bipyridyl)ruthenium(II)-modified mono-, di-, and trinuclear manganese complexes as electron-transfer models for photosystem II

With the aim of modeling the arrangement of redox-active and photoactive components along the electron-transfer pathway of photosystem II, tetra- to nonanuclear transition metal complexes have been synthesized, comprising one, two, or three manganese ions, oxidizable phenolates, and tris(2,2'-bipyridyl)ruthenium(II)-type units as photosensitizers. These model complexes are considered to be mononuclear ([LnMn](PF6)m), dinuclear ([L1aMnIV2(mu-O)2](PF6)6), or trinuclear ([LnMnIIMnIIMnIILn](PF6)12) with respect to the number of manganese centers present. Electronic coupling between the manganese ions is strongly antiferromagnetic in the case of the di(mu-oxo)-dimanganese compound [L1aMnIV2(mu-O)2](PF6)6, where the "ligand" [H2L1a]4+ consists of two tris(bipyridyl)ruthenium(II)-type units covalentely bound to a bismacrocyclic Me2dtne backbone to which the manganese ions are coordinated via an additional phenolate oxygen (Me2dtne = 1,2-bis(4-methyl-1,4,7-triazacyclononyl)ethane). Weak antiferromagnetic coupling is observed in compounds [LnMnIIMnIIMnIILn](PF6)12, where the three metals are in a linear arrangement (face-sharing octahedral). They are bridged by three phenolate oxygens of each of the deprotonated "ligands" [H3Ln]6+, respectively. Each ligand [H3Ln]6+ (n = 1, 2) consists of a tacn ring with three pendent arm phenols which are each bound to a tris(bipyridyl)ruthenium(II)-type unit (tacn = 1,4,7-triazacyclononane). In these compounds several electron-transfer steps were detected by electrochemical methods which are assigned to different redox processes located at individual electrochemically active components (Mn, Ru, bipyridyl, phenolate). For example, in the "mononuclear" compounds [LnMn](PF6)m (n = 1 or 2) Mn(II), Mn(III), and Mn(IV) are accessible and three Ru(II) centers are reversibly oxidized to Ru(III), and in addition, the coordinated phenolate can be oxidized to a highly reactive, coordinated phenoxyl radical. In several cases very slow heterogeneous electron-transfer rates were observed for redox processes involving the manganese centers.     

Fluxional processes in diamagnetic and paramagnetic allyl dicarbonyl and 2-methylallyl dicarbonyl molybdenum histidinato complexes as revealed by spectroscopic data and density functional calculations

This work describes a detailed study on the structure and dynamics of pseudooctahedral low-valent complexes of the type [Mo(His-N(epsilon)-R)(eta-2-R'-allyl)(CO)(2)] (His=N(delta),N,O-L-histidinate; R=H, R'=H (1); R=C(2)H(4)CO(2)Me, R'=H (2); R=H, R'=Me (3); R=C(2)H(4)CO(2)Me, R'=Me (4)). These diamagnetic 18-electron complexes were comprehensively characterized spectroscopically and by X-ray crystallography. In the solid state, the (substituted) allyl ligand is in an endo position in all compounds, but it is trans to the His-N(delta) atom in 1 and 2, whereas it is trans to the carboxylate O atom for the 2-Me-allyl compounds 3 and 4. In solution, both isomers are present in a solvent-dependent equilibrium. The third isomer (allyl trans to His-NH(2)) is not spectroscopically observed in solution. This is in agreement with the results from density functional (DFT) computations (BPW 91 functional) for 1 and 3, which predict a considerably higher energy (+6.3 and +5.9 kJ mol(-1), respectively) for this isomer. A likely path for isomerization is calculated, which is consistent with the activation energy determined by variable temperature NMR measurements. At least for 3, the preferred path involves several intermediates and a rotation of the 2-Me-allyl ligand. For the paramagnetic 17-electron congeners, DFT predicts the exo isomer of 3(+) with the 2-Me-allyl ligand trans to the carboxylate O atom to be by far the most stable isomer. For 1(+), an endo-exo equilibrium between the isomers with the allyl ligand trans to the carboxylate O atom is suggested. These suggestions are confirmed by EPR spectroscopy on the electrochemically generated species, which show signals for one- (4) and two- (2) metal-containing compounds. The appearance of the EPR spectra may be rationalized by inspection of the SOMOs from DFT calculations of the species in question. The notion of a metal-centered oxidation is also substantiated by IR spectroelectrochemistry and by UV/Vis spectra of the 17-electron complexes. Upon depleting the metal of electron density, the stretching vibrations of the carbonyl ligands shift more than 100 cm(-1) to higher wavenumbers, and the carbonyl vibration of the metal-coordinated carboxylate shifts by about 50 cm(-1). A color change from yellow to green upon oxidation is observed visually and quantified by the appearance of a new band at 622 nm (2(+)) and 546 nm (4(+)), respectively.     

Tetradentate bis(o-iminobenzosemiquinonate(1-)) pi radical ligands and their o-aminophenolate(1-) derivatives in complexes of nickel(II), palladium(II), and copper(II)

The coordination chemistries of the potential tetradentate ligands N,N'-bis(3,5-di-tert-butyl-2-hydroxyphenyl)ethylenediamine, H4[L1], the unsaturated analogue glyoxal-bis(2-hydroxy-3,5-di-tert-butylanil), H2[L2], and N,N'-bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2-dimethylpropylenediamine, H4[L3], have been investigated with nickel(II), palladium(II), and copper(II). The complexes prepared and characterized are [Ni(II)(H3L1)2] (1), [Ni(II)(HL2)2].5/8CH2Cl2 (2), [Ni(II)(L3**)] (3), [Pd(II)(L3**)][Pd(II)(H2L3) (4), and [Cu(II)(H2O)(L4)] (5), where (L4)2- is the oxidized diimine form of (L3)4- and (L3**)2- is the bis(o-iminosemiquinonate) diradical form of (L3)4-. The structures of compounds 1-5 have been determined by single crystal X-ray crystallography. In complexes 1 and 2, the ligands (H3L1)- and (HL2)- are tridentate and the nickel ions are in an octahedral ligand environment. The oxidation level of the ligands is that of an aromatic o-aminophenol. 1 and 2 are paramagnetic (mu(eff) approximately 3.2 mu(B) at 300 K), indicating an S = 1 ground state. The diamagnetic, square planar, four-coordinate complexes 3 and [Pd(II)(L3**)] in 4 each contain two antiferromagnetically coupled o-iminobenzosemiquinonate(1-) pi radicals. Diamagnetic [Pd(II)(H2L3)] in 4 forms an eclipsed dimer via four N-H.O hydrogen bonding contacts which yields a nonbonding Pd.Pd contact of 3.0846(4) A. Complex 5 contains a five-coordinate Cu(II) ion and two o-aminophenolate(1-) halves in (L4)2-. The electrochemistries of complexes 3 and 4a ([Pd(II)(L3**)] of 4) have been investigated, and the EPR spectra of the monocations and -anions are reported.     

Molecular and electronic structure of four- and five-coordinate cobalt complexes containing two o-phenylenediamine- or two o-aminophenol-type ligands at various oxidation levels: an experimental, density functional, and correlated ab initio study

The bidentate ligands N-phenyl-o-phenylenediamine, H(2)((2)L(N)IP), or its analogue 2-(2-trifluoromethyl)anilino-4,6-di-tert-butylphenol, ((4)L(N)IP), react with [Co(II)(CH(3)CO(2))(2)]4H(2)O and triethylamine in acetonitrile in the presence of air yielding the square-planar, four-coordinate species [Co((2)L(N))(2)] (1) and [Co((4)L(O))(2)] (4) with an S=1/2 ground state. The corresponding nickel complexes [Ni((4)L(O))(2)] (8) and its cobaltocene reduced form [Co(III)(Cp)(2)][Ni((4)L(O))(2)] (9) have also been synthesized. The five-coordinate species [Co((2)L(N))(2)(tBu-py)] (2) (S=1/2) and its one-electron oxidized forms [Co((2)L(N))(2)(tBu-py)](O(2)CCH(3)) (2 a) or [Co((2)L(N))(2)I] (3) with diamagnetic ground states (S=0) have been prepared, as has the species [Co((4)L(O))(2)(CH(2)CN)] (7). The one-electron reduced form of 4, namely [Co(Cp)(2)][Co((4)L(O))(2)] (5) has been generated through the reduction of 4 with [Co(Cp)(2)]. Complexes 1, 2, 2 a, 3, 4, 5, 7, 8, and 9 have been characterized by X-ray crystallography (100 K). The ligands are non-innocent and may exist as catecholate-like dianions ((2)L(N)IP)(2-), ((4)L(N)IP)(2-) or pi-radical semiquinonate monoanions ((2)L(N)ISQ)(*) (-), ((4)L(N)ISQ)(*) (-) or as neutral benzoquinones ((2) L(N)IBQ)(0), ((4) L(N)IBQ)(0); the spectroscopic oxidation states of the central metal ions vary accordingly. Electronic absorption, magnetic circular dichroism, and EPR spectroscopy, as well as variable temperature magnetic susceptibility measurements have been used to experimentally determine the electronic structures of these complexes. Density functional theoretical (DFT) and correlated ab initio calculation have been performed on the neutral and monoanionic species [Co((1)L(N))(2)](0,-) in order to understand the structural and spectroscopic properties of complexes. It is shown that the corresponding nickel complexes 8 and 9 contain a low-spin nickel(II) ion regardless of the oxidation level of the ligand, whereas for the corresponding cobalt complexes the situation is more complicated. Spectroscopic oxidation states describing a d(6) (Co(III)) or d(7) (Co(II)) electron configuration cannot be unambiguously assigned.