Photophysics of ruthenium(II) complexes with 2-(2′-pyridyl) pyrimidine and 2,2′-bipyridine ligands in fluid solution

The photophysics of three complexes of the form Ru(bpy)₃₋(pypm)²⁺ (where bpy2,2′-bipyridine, pypm 2-(2′-pyridyl)pyrimidine and P=1, 2 or 3) was examined in H₂O, propylene carbonate, CH₃CN and 4:1 (v/v) C₂H₅OH---CH₃OH; comparison was made with the well-known photophysical behavior of Ru(bpy)₃²⁺. The lifetimes of the luminescent metal-to-ligand charge transfer (MLCT) excited states were determined as a function of temperature (between −103 and 90 °C, depending on the solvent), from which were extracted the rate constants for radiative and non-radiative decay and ΔE, the energy gap between the MLCT and metal-centered (MC) excited states. The results indicate that ^*Ru(bpy)₂(pypm)²⁺ decays via a higher lying MLCT state, whereas ^*Ru(pypm)₃²⁺ and ^*Ru(pypm)₂(bpy)²⁺ decay predominantly via the MC state.     

Transients in the oxidative and H-atom-induced degradation of 1,3,5-trithiane. Time-resolved studies in aqueous solution

The (*)OH-induced oxidation of 1,3,5-trithiacyclohexane (1) in aqueous solution was studied by means of pulse radiolysis with optical and conductivity detection. This oxidation leads, via a short-lived (*)OH radical adduct (<1 micros), to the radical cation 1(*+) showing a broad absorption with lambda(max) equal to 610 nm. A defined pathway of the decay of 1(*+) is proton elimination. It occurs with k = (2.2 +/- 0.2) x 10(4) s(-1) and yields the cyclic C-centered radical 1(-H)(*). The latter radical decays via ring opening (beta-scission) with an estimated rate constant of about 10(5) s(-1). A distinct, immediate product (formed with the same rate constant) is characterized by a narrow absorption band with lambda(max) = 310 nm and is attributed to the presence of a dithioester function. The formation of the 310 nm absorption can be suppressed in the presence of oxygen, the rationale for this being a reaction of the C-centered cyclic radical 1(-H)(*) with O(2). The disappearance of the 310 nm band (with a rate constant of 900 s(-1)) is associated with the hydrolysis of the dithioester functionality. A further aspect of this study deals with the reaction of H(*) atoms with 1 which yields a strongly absorbing, three-electron-bonded 2sigma/1sigma* radical cation [1(S therefore S)-H](+) (lambda(max) = 400 nm). Its formation is based on an addition of H(*) to one of the sulfur atoms, followed by beta-scission, intramolecular sulfur-sulfur coupling (constituting a ring contraction), and further stabilization of the S therefore S bond thus formed by protonation. [1(S therefore S)-H](+) decays with a first-order rate constant of about 10(4) s(-1). Its formation can be suppressed by the addition of oxygen which scavenges the H(*) atoms prior to their reaction with 1. Complementary time-resolved conductivity experiments have provided information on the quantification of the 1(*+) radical cation yield, the cationic longer-lived follow-up species, extinction coefficients, and kinetics concerning deprotonation processes as well as further reaction steps after hydrolysis of the transient dithioesters. The results are also discussed in the light of previous photochemical studies.     

A Reevaluation of the Ambident Reactivity of the Guanine Moiety Towards Hydroxyl Radicals

Radically different : Contrary to previous proposals, the main reaction of the HO^. radical with guanosine or 2′‐deoxyguanosine is the hydrogen abstraction from the NH₂ moiety to give a guanyl radical. This radical, characterized by a broad band in the visible region (around 610 nm), undergoes tautomerization to the most stable isomer.

     

Chemical radiation studies of 8-bromo-2'-deoxyinosine and 8-bromoinosine in aqueous solutions

The reactions of hydrated electrons (e(aq) (-)) with 8-bromo-2'-deoxyinosine (8) and 8-bromoinosine (12) have been investigated by radiolytic methods coupled with product studies and have been addressed computationally by means of BB1K-HMDFT calculations. Pulse radiolysis revealed that one-electron reductive cleavage of the C--Br bond gives the C8 radical 9 or 13 followed by a fast radical translocation to the sugar moiety. Selective generation of a C5' radical occurs in the 2'-deoxyribo derivative, whereas in the ribo analogue the reaction is partitioned between the C5' and C2' positions with similar rates. Both C5' radicals undergo cyclizations, 10-->11 and 14-->15, with rate constants of 1.4 x 10(5) and of 1.3 x 10(4) s(-1), respectively. The redox properties of radicals 10 and 11 have also been investigated. A synthetically useful photoreaction has also been developed as a one-pot procedure that allows the conversion of 8 to 5',8-cyclo-2'-deoxyinosine in a high yield and a diastereoisomeric ratio (5'R)/(5'S) of 4:1. The present results are compared with data previously obtained for 8-bromoadenine and 8-bromoguanine nucleosides. Theory suggests that the behavior of 8-bromopurine derivatives with respect to solvated electrons can be attributed to differences in the energy gap between the pi*- and sigma*-radical anions.     

Corrigendum to “Extending homeomorphisms from punctured surfaces to handlebodies” [Topology Appl. 155 (6) (2008) 610–621]

We correct the statements of Theorems 9 and 10 of [A. Cattabriga, M. Mulazzani, Extending homeomorphisms from punctured surfaces to handlebodies, Topology Appl. 155 (2008) 610–621], by adding missing generators, and improve the statement of Theorem 10, by removing some redundant generators.     

Selective generation and reactivity of 5'-adenosinyl and 2'-adenosinyl radicals

The reaction of hydrated electrons (e(-)(aq) with 8-bromoadenosine 7 has been investigated by radiolytic methods coupled with product studies. Pulse radiolysis revealed that one-electron reductive cleavage of the C-Br bond gives the C8 radical 8 followed by a fast radical translocation to the sugar moiety. The reaction is partitioned between C5' and C2' positions in a 60:40 ratio leading to 5'-adenosinyl radical 9 and 2'-adenosinyl radical 11. This radical translocation from C8 to different sites of the sugar moiety has also been addressed computationally by means of DFT B3LYP calculations. In addition, ketone 21 was prepared and photolyzed providing an independent generation of C2' radical 11. Both C5' and C2' radicals undergo unimolecular reactions. Radical 9 attacks adenine with a rate constant of 1.0 x 10(4) s(-1) and gives the aromatic aminyl radical 10, whereas C2' radical 11 liberates adenine with a rate constant of 1.1 x 10(5) s(-1).     

The fate of C5' radicals of purine nucleosides under oxidative conditions

The factors that influence the reactivity of C5' radicals in purine moieties under aerobic conditions are unknown not only in DNA, but also in simple nucleosides. 5',8-Cyclopurine lesions are the result of a rapid C5' radical attack to the purine moieties before the reaction with oxygen. These well-known lesions among the DNA modifications were suppressed by the presence of molecular oxygen in solution. Here we elucidate the chemistry of three purine-substituted C5' radicals (i.e., 2'-deoxyadenosin-5'-yl, 2'-deoxyinosin-5'-yl, and 2'-deoxyguanosin-5'-yl) under oxidative conditions using gamma-radiolysis coupled with product studies. 2'-Deoxyadenosin-5'-yl and 2'-deoxyinosin-5'-yl radicals were selectively generated by the reaction of hydrated electrons (e(aq)(-)) with 8-bromo-2'-deoxyadenosine and 8-bromo-2'-deoxyinosine followed by a rapid radical translocation from the C8 to the C5' position. Trapping these two C5' radicals with Fe(CN)6(3-) gave corresponding hydrated 5'-aldehydes in good yields that were isolated and fully characterized. When an oxygen concentration in the range of 13-266 microM (typical oxygenated tissues) is used, the hydrated 5'-aldehyde is accompanied by the 5',8-cyclopurine nucleoside. The formation of 5',8-cyclopurines is relevant in all experiments, and the yields increased with decreasing O2 concentration. The reaction of HO(*) radicals with 2'-deoxyadenosine and 2'-deoxyguanosine under normoxic conditions was also investigated. The minor path of C5' radicals formation was found to be ca. 10% by quantifying the hydrated 5'-aldehyde in both experiments. Rate constants for the reactions of the 2'-deoxyadenosin-5'-yl with cysteine and glutathione in water were determined by pulse radiolysis to be (2.1 +/- 0.5) x 10(7) and (4.9 +/- 0.6) x 10(7) M(-1) s(-1) at 22 degrees C, respectively.     

On the vertex group of n-manifolds

In this paper we obtain the decomposition of the vertex group of n-manifolds, extending the one given by Kauffman and Lins for dimension 3 and solving the related conjecture. The result is obtained in the more general category of gems: the vertex group of a gem , representing an n-manifold M, is the free product of n copies of the fundamental group of M and a free group F of rank N–n, where N is the number of n-residues of . In particular, for crystallizations FZ and consequently the vertex group is an invariant of M.     

Electrochemistry of coordination compounds: Part XIX. Electrochemical study of some cationic hydrido-complexes of iron(II)

The Electrochemical behaviour of a series of cationic hydrido-complexes of Fe(II) of general formula trans-[FeH(L)(DPE)₂] (BPh₄)(L=N₂, C₂H₅N, C₆H₅CN, CH₂CHCN, CH₃CN, P(OCH₃)₃, P(OC₂H₅)₃, CO; DPE=1,2- bis(diphenylphosphino)ethane) has been investigated in 1,2-dimethoxyethane at the platinum electrode. The reduction of these d⁶ complexes has been found to proceed by an ECE mechanism in which a one-electron transfer, generating a transient d⁷ species, is followed by the fast loss of one of the neutral ligands and a further one-electron reduction of the pentacoordinated intermediates to the final d⁸ anionic hydrides. The reduced species are stabilized considerably at low temperature. The ligands, L, are divided into two groups according to their bonding properties, with particular references to their ability to act as π- acceptors. Weak π-bonders (N₂, C₂H₅N, C₆H₅CN, CH₂CHCN, CH₃CN) are lost in the chemical step interposed between the two charge-transfer processes, whereas strong π-accepting ligands (CO and P(OR)₃) favour dissocia tion of one end of a diphosphine.