Quinto G. Mulazzani

Geometrical isomerism of monounsaturated fatty acids: thiyl radical catalysis and influence of antioxidant vitamins

Thiyl radicals generated either from thiols or disulfides act as the catalyst for the cis-trans isomerization of a variety of monounsaturated fatty acid methyl esters in homogeneous solution. Similar results have also been obtained using alpha-lipoic acid and its reduced form. The effectiveness of the isomerization processes in the presence of the most common antioxidants has been addressed. The ability of thiyl radical scavenging was found to increase along the series alpha-tocopherol < ascorbic acid < all-trans retinol. The cis-trans isomerization of fatty acid residues in multilamellar vesicles of dioleoyl phosphatidyl choline by thiyl radical, in the absence and presence of the various antioxidants, has also been studied in detail. The influence of the isomerization process on the phospholipid bilayer has been tested by permeability measurements of vesicles and it is clearly shown that trans fatty acid-containing membranes have intermediate properties between those formed by all-cis and saturated components. This study contributes to the understanding of radical processes that can alter or protect the naturally occurring cis geometry of unsaturated lipids in cell membranes and demonstrates a new role of essential antioxidants.

Radiation treatment of combustion gases: Formulation and test of a reaction model

Abstract A generalized kinetic mechanism for radiation induced oxidation of nitrogen oxides from exhaust gases in the absence of sulfur dioxide is formulated. The responses obtained by the mathematical simulation are in good agreement with reported experimental results.

The thiyl radical-mediated isomerization of cis-monounsaturated fatty acid residues in phospholipids: a novel path of membrane damage?

Thiyl radicals, generated from biologically relevant thiols under biomimetic conditions, reversibly attack the double bonds of unsaturated phospholipids containing cis-fatty acid residues either in lipid solutions or lipid vesicles, thus producing phospholipids containing trans-fatty acid residues in high yield.

Radiolytically-induced one-electron reduction of methyl viologen in aqueous solution: Stability of the radical cation in acidic and highly alkaline media(1)

Abstract Pulse and continuous radiolysis have been used to investigate the stability of the reduced methyl viologen radical cation (MV +. ) in acidic and highly alkaline aqueous solution. The reaction of the methyl viologen dication (MV 2+ ) with (CH 3 ) 2 ĊOH and (CH 3 ) 2 ĊO - radicals generates MV +. rapidly ( k = 2.9±0.2 × 10 9 and 6.7±0.3 × 10 9 M -1 s -1 , respectively). The absorption spectrum of MV +. is the same at pH 1, natural pH, and pH 13 suggesting that MV +. is not involved in acid-base equilibria in that pH range. Between pH 0 and 2, MV +. disappears via second-order kinetics with k obs an inverse function of [MV 2+ ] and pH. The decay of MV +. occurs via H + -assisted disproportionation and yields ultimately a hydrogenated species with λ max 220, 255 nm (∈ max 8.2 × 10 3 , 4.1 × 10 3 M -1 cm -1 , respectively). At pH 13, MV +. is infinitely stable in the absence of O 2 ; further reduction of MV +. yields the moderately stable MV° species with λ max 368 nm(∈ max 3.6 × 10 4 M -1 cm -1 ) and a shoulder at 370 nm(∈3.0 × 10 4 M -1 cm -1 ). Reaction of MV° with O 2 yields MV 2+ quantitatively via two one-electron oxidation steps. Acidification of MV° in the absence of O 2 yields the same air-insensitive hydrogenation product as is obtained from the disproportionation of MV +. in acidic solution. The relationship of these observations to the use of MV 2+ as an electron relay species in photochemical solar energy conversion schemes is examined.

Excess electron transfer in G-quadruplex

The excess electron transfer in a G-quadruplex is successfully probed by using the reaction of hydrated electrons with quadruplex complex of pentamers and the 8-bromoguanine moieties as the detection system.

Reaction of hydrated electrons with guanine derivatives: tautomerism of intermediate species.

Here, we show that two tautomers are produced by the protonation of the guanine-electron adduct. The fate of electron adducts of a variety of substituted guanosines was investigated by radiolytic methods and addressed computationally by means of time-dependent DFT (TD-B3LYP/6-311G**//B1B95/6-31+G**) calculations. The reaction of e(aq-) with guanosine and 1-methylguanosine produces two transient species, whereas the reaction with N2-ethylguanosine and N2,N2-diethylguanosine produces only one. The two short-lived intermediates, which show a substantial difference in their UV-visible spectra, are recognized to be two purine tautomers (i.e., iminic 18 and aminic 19 forms). The tautomerization 18 --> 19 occurs with a rate constant of ca. 1.5 x 106 s(-1) , and theory suggests that it is a water-assisted process.

Properties of excited states and one-electron reduced forms of Ruthenium(II)-diimine photosensitizers

In this paper we compare the spectral, acid-base, electrochemical, and kinetic properties of the excited states and one-electron reduced forms of homo- and heteroleptic Ru(II)-diimine photosensitizers that contain 2,2’-bipyridine, 2,2’-bipyrazine, and 2,2’-bipyrimidine ligands. These species, represented as *(RuIIIL.-)2+ and (RuIIL.-)+, respectively, are orbital analogues in that the intra- or intermolecularly transferred electron resides on the same uniquely reduced ligand. The variations in the properties of these species, resulting from changes in the coordinating ligands, permit the fine-tuning of photochemical systems.

Reactivity of Ru(bpy)+3 towards the radicals originating from the scavenging of hydrogen atoms and hydroxyl radicals by methanol, ethanol, propan-2-ol, tert-butyl alcohol and formate ions in aqueous solution: a pulse radiolytic study

Ru(bpy)+3, where Ru(bpy)+3=[RuII(bpy)2(bpy˙–)]+ and bpy = 2,2′-bipyridine, generated in nominally neutral aqueous solution via reduction of Ru(bpy)2+3 by e–aq, reacts [k=(1.9 ± 0.5)× 109 dm3 mol–1 s–1] with the radicals R˙[R˙=˙CH2OH, CH3ĊHOH, (CH3)2ĊOH, and CO˙–2] originating from the scavenging of H˙ and ˙OH by CH3OH, CH3CH2OH, (CH3)2CHOH and HCO–2, respectively, and with the radical ˙CH2C(CH3)2OH originating from the scavenging of ˙OH by (CH3)3COH. The products of these reactions, which are believed to proceed via addition of R˙ to coordinated bpy˙– to give [RuII(bpy)2(bpyR–)]+, show an absorption in the 390 nm region (Iµ= 13 000 ± 1500 dm3 mol–1 cm–1). A subsequent decay of the absorbance at 390 nm follows first-order kinetics with kobs=(a+b[H2PO–4]) s–1; the values of a and b, which are related to the rate constants for proton transfer from H2O and H2PO–4, respectively, to [RuII(bpy)2(bpyR–)]+ to give [RuII(bpy)2(bpyHR)]2+, have been determined for each R˙. In alkaline solution, the formation of [RuII(bpy)2(bpyR–)]+ is observed only when R˙=˙CH2C(CH3)2OH and CO˙–2 owing to the fact that the basic forms of the α-hydroxylalkyl radicals, i.e. CH2O˙–, CH3CHO˙– and (CH3)2CO˙– are rapidly scavenged (k= 1.6 × 109, 3.6 × 109 and 3.1 × 109 dm3 mol–1 s–1, respectively, at pH 13 and µ= 0.1 mol dm–3) by Ru(bpy)2+3 to give Ru(bpy)+3.

Photodecarboxylation of citrate through ion pair photochemistry: the Co(Sep)3+•citraten− (n= 1,2,3) system

Abstract The Co(Sep) 3+ complex (Sep = sepulchrate = 1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane) forms ion pairs with citrate in aqueous solution at pH 4.0, 5.6 and 8.5, where the forms CIT − , CIT 2− and CIT 3− (CIT = citrate) predominate respectively. Irradiation in the ion pair charge transfer band for the three pH values leads to the decarboxylation of citrate with the production of CO 2 and Co(Sep) 2+ . In the presence of colloidal platinum, CO 2 and H 2 are evolved. The quantum yields for formation of CO 2 , Co(Sep) 2+ and H 2 were obtained and are discussed in terms of their dependence on pH.

One-electron reduction of 2,2″-bipyridine in aqueous solution

Pulse radiolytic reduction of 2.2Prime;-bipyridine (bpy) in neutral and alkaline aqueous solution yields an intense band at ⋌max 365 nm (ϵ=3.0 × 104 dm3 mol-1 cm-1); in acidic solution the band maximum is at 375 nm (ϵ=4.5 × 104 dm3 mol-1 cm-1). The acidic form of the one-electron reduced intermediate shows pKa=5.6 and decays via first-order kinetics (k~0.2s -1) at low radiation doses; the basic form decays via a bimolecular mode (k=2.0 × 109 dm3 mol-1 s-1). Comparison is made of the behaviors of the reduced bpy species and Ru(bpy)3+.