Time-resolved CIDNP Research Articles

Marcus Cross-Relationship Probed by Time-Resolved CIDNP.

The time-resolved CIDNP method can provide information about degenerate exchange reactions (DEEs) involving short-lived radicals. In the temperature range from 8 to 65 °C, the DEE reactions of the guanosine-5'-monophosphate anion GMP(-H)- with the neutral radical GMP(-H)•, of the N-acetyl tyrosine anion N-AcTyrO- with a neutral radical N-AcTyrO•, and of the tyrosine anion TyrO- with a neutral radical TyrO• were studied. In all the studied cases, the radicals were formed in the reaction of quenching triplet 2,2'-dipyridyl. The reorganization energies were obtained from Arrhenius plots. The rate constant of the reductive electron transfer reaction in the pair GMP(-H)•/TyrO- was determined at T = 25 °C. Rate constants of the GMP(-H)• radical reduction reactions with TyrO- and N-AcTyrO- anions calculated by the Marcus cross-relation differ from the experimental ones by two orders of magnitude. The rate constants of several other electron transfer reactions involving GMP(-H)-/GMP(-H)•, N-AcTyrO-/N-AcTyrO•, and TyrO-/TyrO• pairs calculated by cross-relation agree well with the experimental values. The rate of nuclear paramagnetic relaxation was found for the 3,5 and β-protons of TyrO• and N-AcTyrO•, the 8-proton of GMP(-H)•, and the 3,4-protons of DPH• at each temperature. In all cases, the dependences of the rate of nuclear paramagnetic relaxation on temperature are described by the Arrhenius dependence.

Photo- and Radiation-Induced One-Electron Oxidation of Methionine in Various Structural Environments Studied by Time-Resolved Techniques.

Oxidation of methionine (Met) is an important reaction that plays a key role in protein modifications during oxidative stress and aging. The first steps of Met oxidation involve the creation of very reactive and short-lived transients. Application of complementary time-resolved radiation and photochemical techniques (pulse radiolysis and laser flash photolysis together with time-resolved CIDNP and ESR techniques) allowed comparing in detail the one-electron oxidation mechanisms initiated either by ●OH radicals and other one-electron oxidants or the excited triplet state of the sensitizers e.g., 4-,3-carboxybenzophenones. The main purpose of this review is to present various factors that influence the character of the forming intermediates. They are divided into two parts: those inextricably related to the structures of molecules containing Met and those related to external factors. The former include (i) the protection of terminal amine and carboxyl groups, (ii) the location of Met in the peptide molecule, (iii) the character of neighboring amino acid other than Met, (iv) the character of the peptide chain (open vs cyclic), (v) the number of Met residues in peptide and protein, and (vi) the optical isomerism of Met residues. External factors include the type of the oxidant, pH, and concentration of Met-containing compounds in the reaction environment. Particular attention is given to the neighboring group participation, which is an essential parameter controlling one-electron oxidation of Met. Mechanistic aspects of oxidation processes by various one-electron oxidants in various structural and pH environments are summarized and discussed. The importance of these studies for understanding oxidation of Met in real biological systems is also addressed.

Electron transfer vs. proton-coupled electron transfer as the mechanism of reaction between amino acids and triplet-excited benzophenones revealed by time-resolved CIDNP

Hyperfine coupling constants (HFCCs) of the short-lived radicals of 4-carboxy, 4,4'-dicarboxy, and 3,3',4,4'-tetracarboxy benzophenones (4-CBP, DCBP, and TCBP, respectively) formed in their photoreaction with tyrosine were obtained from analysis of geminate CIDNP spectra. These HFCCs were compared to HFCCs calculated using density functional theory. From this comparison, it was established that the CIDNP pattern of TCBP originates from contributions of three types of TCBP radical structures: the non-protonated anion radical and two anion radical structures with a protonated carboxylic group at position 3 or 4 (or 3' or 4'). This allowed us to conclude that the mechanism of the quenching reaction is proton coupled electron transfer (PCET): electron transfer is followed by proton transfer to one of four possible positions with similar probabilities. The same CIDNP pattern and therefore the same reaction mechanism was established for histidine. For 4-CBP and DCBP, triplet quenching proceeds also via PCET, again with formation of the anion radical with a protonated carboxylic group.

H and13C Hyperfine Coupling Constants of the Tryptophanyl Cation Radical in Aqueous Solution from Microsecond Time-Resolved CIDNP

Relative values of the 1H and 13C isotropic hyperfine couplings in the cationic oxidized tryptophan radical TrpH*+ in aqueous solution are determined. The data are obtained from the photo-CIDNP (chemically induced dynamic nuclear polarization) enhancements observed in the microsecond time-resolved NMR spectra of the diamagnetic products of photochemical reactions in which TrpH*+ is a transient intermediate. The method is validated using the tyrosyl neutral radical Tyr*, whose 1H and 13C hyperfine couplings have previously been determined by electron paramagnetic resonance spectroscopy. Good agreement is found with hyperfine coupling constants for TrpH*+ calculated using density functional theory methods but only if water molecules are explicitly included in the calculation.

Time-resolved CIDNP as a probe of 2,2′-dipyridyl radical anion complexation with β-cyclodextrin

The photochemical reaction of 2,2′-dipyridyl (DP) with 5-sulfosalicylic acid (SSA) was studied using time-resolved CIDNP in homogeneous and β-cyclodextrin (β-CD) aqueous solutions. The degenerate electron exchange rate constant of DP radical anion with diamagnetic precursor was estimated as 1.1 × 10 8 M −1 s −1 from CIDNP kinetic in homogeneous solution. It was found that DP-β-CD inclusion complex formation alter the kinetics of DP polarization. For β-CD solution the drastic decreasing of nuclear relaxation time of DP radical anion was found comparing to the homogeneous solution.

Paramagnetic intermediates in the photoinduced reaction between dodecamethylcyclohexasilane and 9,10-phenanthraquinone: Time-resolved CIDNP study

The reaction between dodecamethylcyclohexasilane (Me 2Si) 6 1 and 9,10-phenanthraquinone 2 has been studied by means of CIDNP method. In the polar solvent, the photodecomposition of 1 is shown to proceed via triplet radical ion pair formed by phenanthraquinone radical anion and cyclohexasilane radical cation. Its transformation leads to the cyclic reaction product – 10-membered cyclic dioxahexasilecine 8 – formally resulting from the addition of linear 1,6-silicon-centered biradical Si(Me) 2–Si 4(Me 2) 4-(Me) 2Si to C O bonds of quinone. Product 8 is unstable, after several hours it converts to dioxasilole 4 via sequential repeated elimination of dimethylsilylenes 3.

Mechanisms of photoinduced electron transfer reactions of lappaconitine with aromatic amino acids. Time-resolved CIDNP study

CIDNP techniques were applied to the investigation of the elementary mechanism of photoinduced interaction between anti-arrhythmic drug lappaconitine and amino acids tyrosine and tryptophan. It has been shown that the reactions involve the formation of lappaconitine radical anion. Lappaconitine radical anion is unstable and rapidly eliminates N-acetyl anthranilic acid via protonation and ether bond cleavage. The rate constant of ether bond cleavage was estimated to be equal to 4 x 10(5) s(-1). The role of single electron transfer is discussed in the light of the model of drug-receptor interactions.

Laser flash photolysis and time-resolved CIDNP study of photochemical reactions between aqueous tryptophan and nucleotides

Triplet tryptophanTTrpH quenching the by nucleotides adenosine-5′-monophosphate (AMP), cytidine-5′-monophosphate (CMP), guanosine-5′-monophosphate (GMP), and uridine-5′-monophosphate (UMP) is reported for the first time. The quenching rate constants for AMP, CMP, and UMP are of the same orderkq=(3–6)·108 M−1s−1, for GMP this value is 5.6·107 M−1s−1. This difference correlates with the reduction potentials of the nucleotides under study, pointing at the involvement of the electron transfer process.TTrpH quenching by AMP, CMP, and GMP does not result in the radical formation, whereas in the reaction ofTTrpH with UMP radicals are formed with the quantum yield Φ≈0.13, the, radical formation is confirmed by both laser flash photolysis and time-resolved chemically induced dynamic nuclear polarization measurements.

Time-resolved CIDNP and laser flash photolysis study of the photoreaction between triplet 2,2′-dipyridyl and guanosine-5′-monophosphate in water

Laser flash photolysis and time-resolved CIDNP have been applied to the investigation of the kinetics and the mechanism of the photoreaction between triplet 2,2′-dipyridyl (DP) and guanosine-5′-monophosphate (GMP) over a wide pH range in aqueous solution. The pH dependence of the rate constant kq of quenching the triplet dipyridyl by the nucleotide has been measured. Upon pH titration, four pairs of the reacting species contribute to the observed value of kq: pH < 2.4, TDPH+ and GH+; 2.4 < pH < 5.8, TDPH+ and G; 5.8 < pH < 9.4, TDP and G; and pH > 9.4, TDP and G(−H)−, with the corresponding quenching rate constants k1 = 1.3 × 109 M−1 s−1, k2 = 2.7 × 109 M−1 s−1, k3 = 1.6 × 108 M−1 s−1, k4 = 1.1 × 109 M−1 s−1. Based on LFP and CIDNP data, the established mechanism of the quenching reaction is hydrogen atom transfer in neutral solution (5.8 < pH < 9.4), and electron transfer in all other pH regions. Kinetic CIDNP measurements reveal that in acidic and basic solutions the CIDNP kinetics for GMP is determined by the degenerate electron exchange between the GMP radical and its parent molecule with the rate constants 1.3 × 108 M−1 s−1 (acidic conditions) and 4.0 × 107 M−1 s−1 (basic conditions). The nuclear paramagnetic relaxation time for the proton H8 of GMP, T1 = 20 ± 5 μs, obtained from the simulations of the CIDNP kinetics, is found to be independent of the protonation state of the radical.

Sensitized Photolysis of Iodonium Salts Studied by CIDNP. Solvent Dependence and Influence of Lipophilic Substituents

The sensitized photoreactions of diphenyliodonium salt and four of its technically relevant derivatives bearing lipophilic substituents were investigated by pseudo-steady-state and time-resolved CIDNP experiments. Singlet (naphthalene, diphenylanthracene, dimethylanthracene) and triplet (xanthone, thioxanthone, Michler's ketone) sensitizers were used in a variety of solvents (acetonitrile, dimethyl sulfoxide, dimethylformamide, methanol, chloroform, tetrahydrofurane, diethylene glycol dimethyl ether, dioxane, toluene). Under all conditions, the primary step leading to CIDNP was found to be electron transfer from the excited sensitizer *Sens to the onium cation On+. All spin-polarization effects could be explained consistently within the framework of radical pair theory (S−T0-type CIDNP). Pair substitution, i.e., the transformation of the primary radical pairs RP 1 into secondary pairs RP 2 ( where Ph• is the phenyl or an aryl radical) plays a key role for the CIDNP effects and even leads to a field dependence of the polarization phases for the system diphenyliodonium cation/naphthalene in dioxane. Decreasing solvent polarity causes an increase of the rate RP 1 → RP 2. The introduction of a long lipophilic side chain into the onium salt has the same effect, presumably owing to self-solvation.

CIDNP Investigation of Radical Decay Pathways in the Sensitized Photolysis of Triphenylsulfonium Salts

The photodecomposition of triphenylsulfonium hexafluoroantimonate was studied by pseudo-steady-state and time-resolved CIDNP experiments, using naphthalene and 9,10-dimethylanthracene as (singlet) sensitizers. Key intermediates of these reactions are radical pairs consisting of the sensitizer radical cation Sens•+ and the phenyl radical Ph•, which are formed by photoinduced electron transfer followed by cleavage of the resulting neutral onium radical. The chemical fate of Ph• is strongly influenced by the sensitizer. With 9,10-dimethylanthracene, in-cage hydrogen abstraction from the methyl groups of Sens•+ produces benzene and a carbocation that can function as a protic acid; free radicals Ph• attack surplus sensitizer to give addition products. With naphthalene, geminate combination of the two radicals and in-cage oxidation of diphenyl sulfide (the by-product of cleavage of the onium radical) by Sens•+ are observed. In both systems, the solvent scavenges free Ph•, yielding monodeuteriobenzene. The rate ...

H and 13C Nuclear Polarization in Consecutive Biradicals during the Photolysis of 2,2,12,12-Tetramethylcyclododecanone

CIDNP effects arising in the geminate evolution of consecutive biradicals formed during the photolysis of 2,2,12,12-tetramethylcyclododecanone were investigated by the low- and high-field 1H and 13C CIDNP and time-resolved 1H CIDNP methods. The 1H field dependence of nuclear polarization induced in the recombination and disproportionation products of the consecutive biradicals as well as the 13C CIDNP spectra at low and high magnetic fields was analyzed. It was confirmed that two types of biradicals (acyl−alkyl and bisalkyl) of different chain lengths contribute to the spin polarization. Model calculations of the geminate recombination kinetics were based on the numerical solution of the stochastic Liouville equation for the Fourier transform of the biradical spin density matrix. A detailed kinetic analysis of the CIDNP effects allowed us to reveal all chemical transformations of the alkyl−acyl and bisalkyl biradicals and to estimate the probability of the “hidden” recombination of the biradicals to the starting ketone, as well as to determine the product distribution.

Singlet and Triplet State Back Electron Transfer from Photogenerated Radical Ion Pairs Studied by Time-Resolved CIDNP

Radical ion pairs are generated in acetonitrile solution by photoinduced electron transfer from naphthalene derivatives to cyanobenzenes and undergo reverse electron transfer to the singlet ground and to excited triplet states of the parent compounds. Both pathways lead to chemically induced nuclear polarization of the ground state products. They can be separated because the triplet contribution appears delayed by the triplet lifetime. Variation of the radical pair energy leads to a dominance of either the singlet or triplet channel. The analysis of the absolute net CIDNP effects and their time dependencies yields reaction probabilities for the reverse electron transfers which depend on the appropriate energy gaps as described by the Marcus theory, as well as rate constants for degenerate electron and spin exchange processes. For the donor/acceptor system naphthalene/(E)-1,2-dicyanoethene, the triplet pathway is also found dominant.

Laser Flash Photolysis and CIDNP Studies of 1-Naphthyl Acetate Photo-Fries Rearrangement

The steady-state and time-resolved CIDNP and flash photolysis methods were used in a detailed study of the photo-Fries rearrangement of 1-naphthyl acetate (I) in acetonitrile and methanol. The main reaction channel is the decay of I through the excited singlet state with the quantum yields 0.17 ± 0.02 in acetonitrile and 0.42 ± 0.04 in methanol at room temperature. The absorption spectra of the naphthoxyl radical and triplet state of 1-naphthyl acetate were detected. The quantum yield of triplet was estimated as 0.4 ± 0.2 and 0.35 ± 0.17 in acetonitrile and methanol, respectively. It has been established that the triplet-born radical pairs make a main contribution to the CIDNP of the photo-Fries rearrangement products. The involvement in the process of two different triplet states of I was supposed. The main decay channel of the lowest triplet state is the triplet−triplet annihilation, while the CIDNP of photo-Fries rearrangement products results from the decay of the upper triplet state of I with a lifetime of a few nanoseconds. The kinetics of CIDNP formation in reaction products has been analyzed, and the rate constants of the rearrangement of the preceding intermediates at room temperature have been estimated.

Radical Addition Rates to Alkenes by Time-Resolved CIDNP: 2-Hydroxy-2-propyl Radicals

A novel time-resolved CIDNP technique is used to measure absolute rate constants for the addition of 2-hydroxy-2-propyl radicals to 10 1,1- and two 1,2- disubstituted alkenes in propan-2-ol-d8 and toluene-d8 solvents and in part in acetonitrile-d3 and cyclohexane-d12, at room temperature. 2-Hydroxy-2-propyl radicals are produced by photolysis of 2,4-dihydroxy-2,4-dimethylpentan-3-one and lead to a multiplet effect CIDNP of the self-termination product, propan-2-ol, which follows a second-order termination kinetics. Addition of alkene partially quenches this polarization through the addition reaction. Quantitative measurement of the time-dependence and its simulation by kinetic equations yields the addition rate constant ka. The technique is applicable to systems with rate constants between 105 and 109 M-1 s-1, a range covered by the alkenes used. The data correlate with the alkene electron affinity and confirm the 2-hydroxy-2-propyl radical as very nucleophilic. A solvent dependence of ka up to 1 order magnitude is observed and is related to the hydrogen-bonding ability of the solvent and radical and to the polarity of the solvent. A β-steric effect of the alkene substituents is also noted.

Cross-relaxation mechanism for the formation of nuclear polarization: a quantitative time-resolved CIDNP study

The kinetics of the nuclear polarization formed during the photolysis of acetone in isopropanol- d 8 were analyzed quantitatively. Model calculations show that the spin-selective recombination of radicals gives rise to the electron polarization and, with regard to the electron-nuclear cross-relaxation, are adequate to describe the formation of the net nuclear polarization of the reaction products even if the solution contains only one type of radical. For the 2-hydroxy-2-propyl radicals at a magnetic field of 7 T, fitting the theoretical results to the experimental data gives the electron relaxation time T 1 e = 1.0 ± 0.2 μs and the cross-relaxation time T x = 92 ± 18 μs.

Direct analysis of hydrogen transfer reaction from p-benzosemiquinone radical to p-benzoquinone by time-resolved CIDNP method

Abstract The rate constant of the hydrogen transfer reaction from p -benzosemiquinone radical to p -benzoquinone was measured by the time-resolved CIDNP technique. The time dependence of the CIDNP intensity of the products, p -benzoquinone and p -hydrobenzoquinone, can be simulated by kinetic treatment. From the simulation, the rate constant of the hydrogen transfer is obtained.

Electron transfer to triplet fullerene C60 and subsequent radical ion reactions studied by time-resolved CIDNP

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTElectron transfer to triplet fullerene C60 and subsequent radical ion reactions studied by time-resolved CIDNPErik Schaffner and Hanns FischerCite this: J. Phys. Chem. 1993, 97, 50, 13149–13151Publication Date (Print):December 1, 1993Publication History Published online1 May 2002Published inissue 1 December 1993https://doi.org/10.1021/j100152a019RIGHTS & PERMISSIONSArticle Views84Altmetric-Citations18LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (325 KB) Get e-Alerts

Sensitization of α-aminoketone photoinitiators: a time-resolved CIDNP and laser spectroscopy investigation

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSensitization of α-aminoketone photoinitiators: a time-resolved CIDNP and laser spectroscopy investigationG. Rist, A. Borer, K. Dietliker, V. Desobry, J. P. Fouassier, and D. RuhlmannCite this: Macromolecules 1992, 25, 16, 4182–4193Publication Date (Print):August 1, 1992Publication History Published online1 May 2002Published inissue 1 August 1992https://doi.org/10.1021/ma00042a020RIGHTS & PERMISSIONSArticle Views388Altmetric-Citations53LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts Get e-Alerts

Evidence for singlet state β cleavage in the photoreaction of α-(2,6-dimethoxyphenoxy)-acetophenone inferred from time-resolved CIDNP spectroscopy

The photochemistry of α-(2,6-dimethoxyphenoxy)-acetophenone ( 1) was studied using CIDNP spectroscopy; an XeCl excimer laser (308 nm) and a 250 MHz 1H nuclear magnetic resonance (NMR) spectrometer were employed. The results are compared with the photoreactions of α-hydroxyacetophenone ( 2) and α-methoxyacetophenone ( 3). The CIDNP spectra of 1 can be interpreted as a result of a β cleavage from the singlet state. With time-resolved CIDNP spectroscopy it is possible to detect the enol of acetophenone in a second reaction path and to observe the tautomerization to acetophenone. Evidence is presented that this reaction path starts with the photoreduction of 1 from the triplet state. Compound 2 shows α cleavage from the triplet state and no β cleavage. As expected no CIDNP spectra are observed from 3 which is known to react via a Norrish type II mechanism.