Effect of ortho substitution on the charge localization of dinitrobenzene radical anions

Optical and electron paramagnetic resonance spectroscopies were used to study the radical anions of several m-dinitrobenzenes and p-dinitrobenzenes with substituents on ortho positions relative to the nitro groups. 1,4-Dinitrobenzene, 1,4-dimethyl-2,5-dinitrobenzene, and 2,5-dinitrobenzene-1,4-diamine radical anions are delocalized (class III) mixed valence species, but in the dinitrodurene radical anion the nitro groups are forced out of the ring plane due to the steric hindrance, which results in localization of the charge. The radical anions m-dinitrobenzene, 2,6-dinitrotoluene, and dinitromesitylene are all localized (class II) mixed valence species, as is common for m-dinitrobenzenes, and the rate of intramolecular electron transfer reaction strongly decreases with the number of methyl substituents. The same mechanism of rotation of the nitro groups out of the ring plane due to steric hindrance caused by neighboring methyl groups is also responsible for slowing the reaction. However, 2,6-dinitroaniline radical anion and 2,6-dinitrophenoxide radical dianion are charge-delocalized because the strong electron releasing amino and oxido groups increase the conjugation between the two charge-bearing units.     

PHOTOALLERGIC CONTACT DERMATITIS TO MUSK AMBRETTE: ACTION SPECTRA IN GUINEA PIGS AND MAN*

Abstract— Positive photopatch tests to musk ambrette were elicited using narrow band radiation in two patients with photoallergic contact dermatitis to musk ambrette and in photosensitized guinea pigs. The most effective wavelength range in the two patients was between 334 and 394 nm. Control subjects did not respond to the same light doses either in the presence or absence of musk ambrette. One patient exhibited maximum sensitivity at 346 nm. The action spectrum in guinea pigs extended from 322 to 406 nm with maximum response at 322 nm. Below 322 nm, erythema was observed at the same or higher doses than those required for evoking a minimal erythema response in non-sensitized animals. Morphologically and histologically the response below 322 nm appeared to be normal delayed erythema. No erythema was elicited by radiation between 322 and 430 nm in non-photosensitized animals which had received musk ambrette.     

SPIN-TRAPPING and ESR STUDIES OF THE DIRECT PHOTOLYSIS OF AROMATIC AMINO ACIDS, DIPEPTIDES, TRIPEPTIDES and POLYPEPTIDES IN AQUEOUS SOLUTIONS—III. TRYPTOPHAN and RELATED COMPOUNDS

Abstract— The UV photolysis of tryptophan (Trp) and Trp-containing peptides in aerated aqueous solutions has been studied by ESR and spin-trapping techniques using f-nitrosobutane as the spin-trap. The photolysis of Trp alone at 290 nm gave rise to the addition of the spin-trap to carbon 3 of the indole ring. A large ESR signal from the hydronitroxide spin-adduct was also observed revealing the formation of hydrated electrons. Generally, the photolysis of Trp-containing dipeptides generated the deamination radical of the N-terminal amino acid followed by addition to the spin-trap. In the case of lysyl-Trp, a deamination radical from the side chain of lysine was proposed. A sensitization experiment with Trp as sensitizer and glycine (Gly) as substrate led to the generation of the deamination radical of Gly. Most of the observed free radicals resulting from the photolysis of Trp-containing peptides can be explained in terms of hydrated electrons reacting with the carbonyl group followed by deamination of the N-terminus.     

SPIN-TRAPPING and ESR STUDIES OF THE DIRECT PHOTOLYSIS OF AROMATIC AMINO ACIDS, DIPEPTIDES, TRIPEPTIDES and POLYPEPTIDES IN AQUEOUS SOLUTIONS—II. TYROSINE and RELATED COMPOUNDS

Abstract— The UV-photolysis of peptides containing tyrosine (Tyr) was investigated in aqueous solutions at room temperature at 220 and 265 nm. The short-lived free radicals formed during photolysis were spin-trapped by t-nitrosobutane and identified by electron spin resonance. For N-acetyl- and N-formyl-L-Tyr and for peptides containing L-Tyr as the middle residue, photolysis at 265 nm under neutral conditions produced mainly spin-adducts due to the scission between the alpha carbon and the methylene group attached to the aromatic ring, while at 220 nm decarboxylation radicals were spin-trapped. Photolysis of di- and tripeptides at 275 nm in alkaline solutions predominantly generated deamination radicals. The radicals produced in the photolysis of the oxidized A chain of insulin were tentatively characterized by comparison with the results for di- and tripeptides.     

有机锡化合物光解活泼自由基的ESR研究

本文用自旋捕捉技术与ESR相结合的方法,研究了六种有机锡化合物的光解反应历程。结果表明:有机锡化合物紫外光照时,碳-锡键发生均裂,产生碳中心和锡中心自由基。其中碳中心自由基易被捕捉剂α-苯基-N-特丁基氮氧化物(PBN)或2,3,5,6-四甲基亚硝基苯(ND)所捕获;而锡中心自由基可与菲醌形成稳定的环状加合物。由于环状加合物中配体的不同,产生的空间效应也不同,从而导致加合物中的未偶电子云密度发生变化,给出相异的ESR信号。     

INTERMEDIATES IN THE ROOM TEMPERATURE FLASH PHOTOLYSIS OF ADENINE AND SOME OF ITS DERIVATIVES

The transient absorption spectra of aqueous solutions of adenine, 2′-deoxyadenosine, 2′-deoxyadenosine-5′-phosphate and 2′-deoxyadenylyl-(3′-5′)-2′-deoxyadenosine have been determinated at different pH values using conventional flash photolysis. Reactives intermediates produced in the flash photolysis of these adenine derivatives present similar absorption regions: two higher intensity bands in the UV and 560–720 nm wavelength region and a third weaker band at 420–560 nm. On the basis of the effects produced by triplet quenchers and/or electron scavengers the bands have been assigned to hydrated electrons, radical cations, radical anions and/or neutral radicals resulting from neutralization reactions of the charged radicals. The results indicate that the bases photoionize via a triplet state under these conditions.     

Aminoxyl radicals as crosslinks for macromolecules of polyvinylpyrrolidone

Using polyvinylpyrrolidone as an example, it has been shown that photolysis of ceric ammonium nitrate at room temperature can result in crosslinking of macromolecules. This process correlates with the formation of stable aminoxyl radicals, which are registered by EPR. The mechanism involves photodissociation of nitrate radicals produced in the primary reaction into nitric oxide or nitrogen dioxide depending on the wavelength of the light, and simultaneous formation of macroradicals. The accumulation of aminoxyl radicals occurs owing to the acceptance of macroradicals by nitroso or nitro groups according to which mechanism of the nitrate radical photodissociation prevails. Similar radical reactions are observed in N-methyl-2-pyrrolidone.     

From molecular chemistry to supramolecular chemistry to superdupermolecular chemistry. Controlling covalent bond formation through non-covalent and magnetic interactions

The reactions of carbon centered radical pairs often involve diffusion controlled combination and/or disproportionation reactions which are non-selective. A triplet geminate pair of radicals is produced by the photolysis of suitable ketones. The reactions of such geminate pairs can be controlled though the application of supramolecular concepts which emphasize non-covalent interaction to "steer" the geminate pair toward a selected pathway. In addition, "superdupermolecular" concepts, which emphasize the control of radical pair reactions through the orientation of electron spins, can be employed to further control the course of geminate pair reactions. Examples of control of a range of the selectivity of geminate radical combinations, which form strong covalent bonds, through supramolecular and superdupermolecular effects will be presented for the photolysis of ketones adsorbed in the supercages of zeolites.     

Electrochemistry and electrogenerated chemiluminescence of 3,6-di(spirobifluorene)-N-phenylcarbazole

We report here the electrochemistry, luminescence, and electrogenerated chemiluminescence (ECL) of 3,6-dispirobifluorene-N-phenylcarbazole (DSBFNPC). DSBFNPC contains two spirobifluorene groups covalently attached to the N-phenylcarbazole core. The optimized geometry as determined from semiempirical MNDO calculations shows that the phenyl group is twisted 89 degrees from the plane of central carbazole, indicating a lack of electron delocalization between these groups. However, the two fluorene rings of each spirobifluorene group are twisted 58 degrees relative to each other and two spirobifluorene groups are twisted 64 degrees from the N-phenylcarbazole ring, suggesting some charge delocalization among these groups. The cyclic voltammetry of this compound shows two reversible oxidation waves (assigned to the formation of the cation and dication) and a two-electron reduction wave that becomes reversible at higher scan rates (assigned to formation of anion). Digital simulations were carried out to obtain details of the electrochemical processes, and electrochemical behavior was compared to that of phenylcarbazole (PC). Upon cycling between the oxidation and reduction waves, ECL is produced by radical ion annihilation. The photophysical properties of DSBFNPC show a strong resemblance to the parent compound, PC, and the ECL spectrum produced via radical ion annihilation shows good agreement with the fluorescence emission spectrum of DSBFNPC.     

Determination of synthetic musk compounds in indoor house dust by gas chromatography–ion trap mass spectrometry

A new method for the simultaneous determination of 11 synthetic musks and one fragrance compound in house dust was developed. The nitro musks included musk ketone (MK, 4-tert-butyl-3,5-dinitro-2,6-dimethylacetophenone), musk xylene (MX, 1-tert-butyl-3,5-dimethyl-2,4,6-trinitrobenzene), musk ambrette (1-tert-butyl-2-methoxy-4-methyl-3,5-dinitrobenzene) and musk moskene (1,1,3,3,5-pentamethyl-4,6-dinitroindane). The polycyclic musk compounds were 1,3,4,6,7,8-hexahydro-4,6,6,7,8,8-hexamethylcyclopenta-(γ)-2-benzopyran (HHCB), 7-acetyl-1,1,3,4,4,6-hexamethyl-1,2,3,4-tetrahydronaphthalene (AHTN), 4-acetyl-1,1-dimethyl-6-tert-butylindane, 6-acetyl-1,1,2,3,3,5-hexamethylindane, 5-acetyl-1,1,2,6-tetramethyl-3-isopropylindane, 6,7-dihydro-1,1,2,3,3-pentamethyl-4(5H)-indanon. The one macrocyclic musk was 1,4-dioxacycloheptadecane-5,17-dione. The bicyclic hydrocarbon fragrance compound (1,2,3,4,5,6,7,8-octahydro-2,3,8,8-tetramethylnaphthalen-2yl)ethan-1-one (OTNE) and HHCB-lactone (4,6,6,7,8,8-hexamethyl-1H,3H,4H,6H,7H, 8H-indeno[5,6-c]pyran-1-one), a degradation product of HHCB, were also analysed. NIST SRM 2781 (domestic sludge) and SRM 2585 (organic contaminants in house dust) were analysed for these target compounds. The method was applied for the analysis of 49 paired samples collected using two vacuum sampling methods: a sample of fresh or "active" dust (FD) collected using a Pullman-Holt vacuum sampler, and a household dust (HD) sample taken from the participants' vacuum cleaners. Method detection limits and recoveries ranged from 12 to 48?ng/g and 54 to 117?%, respectively. AHTN, HHCB, OTNE and HHCB-lactone were detected in all samples, with median concentrations of 552, 676, 252 and 453?ng/g for FD samples, respectively; and 405, 992, 212 and 492?ng/g for HD samples, respectively. MX and MK were detected with high frequencies but with much lower concentrations. The two sampling methods produced comparable results for the target analytes. Widely scattered concentration levels were observed for target analytes from this set of 49 house dust samples, suggesting a wide variability in Canadian household exposure to synthetic musks.     

SPIN-TRAPPING AND ESR STUDIES OF THE DIRECT PHOTOLYSIS OF AROMATIC AMINO ACIDS, DIPEPTIDES, TRIPEPTIDES AND POLYPEPTIDES IN AQUEOUS SOLUTIONS—I. PHENYLALANINE AND RELATED COMPOUNDS

Abstract— The direct UV photolysis of l -Phe and peptides containing l -Phe in aqueous solutions has been investigated at room temperature. The short-lived free radicals formed during photolysis were spin-trapped by t -nitrosobutane and identified by electron spin resonance. During the photolysis of l -Phe the decarboxylation and the deamination radicals were spin-trapped. For N-formyl and N-acetyl- l -Phe the decarboxylation radicals were observed. For dipeptides containing Phe the decarboxylation radicals were observed and in some cases the deamination radicals from the N-terminal residue were found. For the tripeptides Gly- l -Phe- l -Ala and Gly-Gly- l -Phe, the C-terminal decarboxylation radical was spin trapped; for l -Phe-Gly-Gly only the deamination radical of the N-terminal residue could be detected. However, for Gly- l -Phe-Gly, five different radicals were identified. The results of the spin-trapping experiments of the 260 nm photolysis of RNase-S-peptide, containing 20 amino acid residues, was interpreted in terms of a chain scission between the alpha carbon of the Phe residue and the adjacent carbonyl group.     

Free radicals in the electrochemical reduction of certain mononitro and dinitro derivatives of pyridine

In the primary process of electrochemical reduction of substituted 3-nitropyridines in dimethylformamide, their anion radicals are formed. This also takes place in the reduction of 3,5-dinitropyridines and 3-nitropyridines with a nitrophenyl substituent at position 2 or 4. For these dinitro derivatives, however, secondary free radicals are formed as well; in a basic medium, these are the products of reduction of the corresponding Meisenheimer complexes. Serving as the reaction center for electroreduction is the nitro group on the pyridine ring, not the group on the phenyl ring. For the mononitropyridines and dinitropyridines that were studied, free radicals of the nitropyridine type are formed as the primary species. The structure of the primary and secondary free radicals was established by analysis of the hyperfine structure of their ESR spectra.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1079–1087, August, 1993.     

Alpha- and beta-thujone radical rearrangements and isomerizations. A new radical clock

Radical clocks have been extensively used in chemical and biochemical mechanistic studies. The C4 radicals of alpha- and beta-thujone can undergo two distinct rearrangement reactions that could, in principle, serve as simultaneous but independent radical clocks. We have therefore generated these C4 radicals by photolysis of the corresponding N-hydroxypyridine-2-thione ester precursors and have investigated their fates and lifetimes. Photolysis of either alpha- or beta-thujone generates the same 6:100 mixture of alpha- and beta-thujone when the radicals are quenched by thiophenol. Hydrogen atom transfer from thiophenol to the radical thus occurs preferentially from the less sterically hindered alpha-face to give beta-thujone. The third product formed in the photolysis via opening of the cyclopropyl ring is 2-methyl-5-isopropylcyclopent-2-enone. The ratio of ring opened to unopened products gives very similar values of kralpha = 4.4 x 10(7) s(-1) and krbeta = 1.0 x 10(8) s(-1) for ring opening of the radicals generated from alpha- and beta-thujone, respectively. If the C4 cation rather than radical is generated, it is converted to carvacrol, a phenol that is not obtained in the radical reactions. Thujone therefore differentiates between radical and cation pathways and provides a measure of the radical lifetime.     

Photoreactivity of biologically active compounds. XVI. Formation and reactivity of free radicals in mefloquine

The formation and reactivity of the triplet state and free radicals of mefloquine hydrochloride (MQ) have been investigated by pulse radiolysis and flash photolysis. The excited triplet, cation radical and anion radical have been produced and their absorption characteristics determined. The triplet-triplet absorption spectrum of MQ showed a maximum at 430 nm, with a molar absorption coefficient of 3600 M(-1) cm(-1) and the quantum yield for intersystem crossing was determined to be close to unity. Deactivation of the triplet, in the absence of oxygen, led to the formation of MQ cation and/or anion radicals. The molar absorption coefficient of the cation radical at 330 nm was determined to be 2300 M(-1) cm(-1), whilst that for the anion radical was 2400 M(-1) cm(-1) at 620 nm and 3600 M(-1) cm(-1) at 350 nm. The molar absorption coefficients of the proposed neutral radical at 320 nm and 520 nm were 4000 M(-1) cm(-1) and 1300 M(-1) cm(-1) respectively. The quantum yield for the formation of singlet oxygen, sensitized by MQ triplet, was determined to be close to unity. Aqueous solutions of MQ were found to photoionize to yield hydrated electron and cation radical of MQ in a biphotonic process. The influences of pH, buffer concentration, oxygen concentration and addition of sodium azide on the formation and reactivity of the transients were evaluated. The reactions between MQ and solvated electrons and superoxide anion were also studied.     

PHOTOLYSIS OF PHEOMELANIN PRECURSORS: AN ESR-SPIN TRAPPING STUDY

The photolysis of 5-S-cysteinyldopa, 2,5-S,S-dicysteinyldopa, 4-hydroxybenzothiazole and cysteinyldopa melanins has been studied by ESR-spin trapping methods using 5.5-dimethyl-1-pyrroline-1-oxide and 2-methyl-2-nitrosopropane as spin traps for hydrated electrons, hydrogen atoms, and carbon-centered alanyl radicals. The photochemistry of these materials is shown to resemble dopa in that both photoionization and photohomolysis occur. However, unlike dopa, the cysteinyldopa also forms the carbon-centered alanyl radicals via photodecomposition of the cysteinyl group. Action spectra and quantum yields are reported. Mechanism(s) of radical formation are discussed.     

Antiradical activity of dioxolane derivatives

1,3-Benzodioxoles synthesized by condensation of 3,6-di-tert-butylbenzene-1,2-diol with carbonyl compounds showed antiradical activity due to their ability to undergo one-electron oxidation with formation of stable radical cations. On this basis, the antiknock effect of their structural analogs, 1,3-dioxolanes derived from vicinal diols, was interpreted in terms of oxidation of these compounds with active radicals generated from fuel hydrocarbons to produce more stable radical or radical ion species, depending on the fuel composition. The formation of radical species was detected in model oxidation reactions of 2,2-dimethyl-1,3-dioxolane and 2,2-dimethyl-1,3-dioxolan-4-ylmethanol with radicals generated by photolysis of iron(III) chloride and benzoyl peroxide.     

Cyclopropylcarbinyl --> homoallyl-type ring opening of ketyl radical anions. Structure/reactivity relationships and the contribution of solvent/counterion reorganization to the intrinsic barrier

[reaction: see text] Following a protocol developed by Mathivanan, Johnston, and Wayner (J. Phys. Chem. 1995, 99, 8190-8195), the radical anions of several cyclopropyl- and oxiranyl-containing carbonyl compounds were generated in an effort to measure the rate constants for their ring opening (k(o)) by laser flash photolysis. The results of these experiments are compared to those obtained from earlier electrochemical studies, and the combined data set is used to rationalize the kinetics of radical anion ring opening in a general context by using Saveant's theory pertaining to stepwise dissociative electron transfer (Acc. Chem. Res. 1993, 26, 455-461). Compared to cyclopropylcarbinyl --> homoallyl rearrangements of neutral free radicals, at comparable driving force, the radical anion ring openings are slightly slower. The small difference in rate is attributed to the contribution of an additional, approximately 2 kcal/mol, solvent reorganization component for the radical anion rearrangements. The solvent reorganization energy for ring opening of these radical anions is believed to be small because the negative charge does not move appreciably in the progression reactant --> transition state --> product.     

THE PHOTOREDUCTION OF FLAVINS BY AMINO ACIDS AND EDTA. A CONTINUOUS AND FLASH PHOTOLYSIS STUDY

Abstract— Primary and secondary photochemical processes in oxygen-free aqueous solution have been characterised for FMN alone and in the presence of EDTA and four amino acids using nanosecond and microsecond flash photolysis and continuous photolysis techniques. The relative contributions of oneelectron and two-electron (group or hydride transfer) reactions to the deactivation of the triplet has been determined by comparing the radical concentration (560 nm) with the bleaching of the ground state (446 nm). It was concluded that one-electron reactions (hydrogen atom or electron abstraction) are the major mode of reactivity of the flavin triplet state with all the suhstrates studied.
The nature of the reactions of the flavin semiquinone radical have been studied quantitatively by microsecond flash photolysis. These secondary reactions consist of either a 'back reaction' between the flavin and substrate radicals (tryptophan or glycyl-tyrosine) or the transfer of a second electron (or hydrogen atom) from the substrate radical to the flavin radical (EDTA, methionine and possibly cysteine) to form reduced flavin and oxidised substrate. From a comparison of the quantum yields of formation of reduced flavin using 'flash' and continuous irradiation, an additional pathway for the decay of the flavin radical is suggested to occur at low light intensities in the presence of glycyl-tyrosine or histidine.     

Efficient production of enol ether radical cations by heterolytic cleavage of beta-mesylate radicals

[reaction: see text] alpha-Methoxy-beta-mesyloxy radicals were produced in laser flash photolysis reactions, and yields of enol ether radical cations formed by heterolytic fragmentation of the mesylate group were determined. The mesylate heterolysis reaction is faster than heterolyses of phosphate and bromide groups in analogous radicals and highly efficient in medium-polarity solvents.     

MECHANISTIC STUDIES OF THE PHOTOREACTION OF FLAVIN WITH N, N-DIMETHYLPROPARGYLAMINE

Abstract Both photoexcited free flavin and protein-bound flavin react with N, N-dimethylpropargyla-mine (DMPA) to yield dihydroflavin-adducts which are not reoxidizable by oxygen. The mechanism of the photoreaction was reinvestigated by flash photolysis and continuous illumination experiments. We suggest that a mechanism occurs in which a flavosemiquinone and a DMPA radical result in the first photoinduced reaction. The flavin-DMPA adducts are then formed by combinations of the two radicals.