FT EPR Study Research Articles

Oxidation of Cyclic Dipeptides by Photoinduced H-Atom Abstraction. A Laser Flash FT EPR and Optical Spectroscopy Study

Laser flash photolysis with the Fourier transform electron paramagnetic resonance (FT EPR) and optical spectroscopy detection methods on the nanosecond time scale have been employed in order to investigate the oxidation mechanism of cyclic dipeptides glycine, alanine, and sarcosine anhydrides initiated by SO4*- or 9,10-anthraquinone-2,6-disulfonate (2,6-AQDS) triplet in oxygen free aqueous solutions. A direct hydrogen abstraction from the ring C-H position of an anhydride by both oxidants is proposed as the primary reaction, rather then an electron transfer from nitrogen followed by (alpha)C-H deprotonation. The overall second-order rate constants for the reaction with SO4*- were determined to be 7.2 x 10(7) M(-1) s(-1), 1.2 x 10(8) M(-1) s(-1), and 5.2 x 10(8) M(-1) s(-1) for glycine anhydride, alanine anhydride, and sarcosine anhydride, respectively. The rate constants for 2,6-AQDS triplet as oxidizing species are about two times lower. The radical intermediate products derived from cyclic dipeptides observed on the microsecond time scale were assigned to the general structure of piperazine-2,5-dione-3-yl radical. These are spin polarized by the mechanisms of chemically induced dynamic electron polarization (CIDEP). For SO4*- as the oxidant the spectra are exhibiting an E/A* polarization pattern originating partially from F-pairs of two piperazine-2,5-dione-3-yl radicals.

Time-Resolved FT EPR and Optical Spectroscopy Study on Photooxidation of Aliphatic α-Amino Acids in Aqueous Solutions; Electron Transfer from Amino vs Carboxylate Functional Group

Using time-resolved Fourier transform electron paramagnetic resonance, FT EPR, and optical spectroscopy, the photooxidation of glycine, alpha-alanine, alpha-aminoisobutyric acid, and model compounds beta-alanine, methylamine and sodium acetate, by excited triplets of anthraquinone-2,6-disulfonate dianion was studied in aqueous solutions in the pH range 5-13. Anthraquinone radical trianions showing strong emissive spin-polarization (CIDEP) were formed, indicating fast electron transfer from the quenchers to the spin-polarized quinone triplet as the primary reaction. None of the primary radicals formed upon one-electron oxidation of quenchers could be detected at the nanosecond time scale of FT EPR measurements because of their very fast transformation into secondary products. The latter were identified to be decarboxylated alpha-aminoalkyl radicals for alpha-amino acids anions and zwitterions, beta-aminoalkyl radicals for beta-alanine zwitterions, and methyl radicals for acetate anions; corresponding aminyl radicals were the first EPR detectable products from beta-alanine anions and methylamine. Thus, anthraquinone-2,6-disulfonate triplet can take an electron from both NH(2)- and -CO(2)(-) functional groups forming aminium ((+*)NH(2)-) and acyloxyl (-CO(2)(*)) radicals, respectively. Aminium radicals derived from beta-alanine anions and CH(3)-NH(2) stabilize by deprotonation into aminyl radicals, whereas these derived from alpha-amino acids anions are known to suffer ultrafast decarboxylation (tau approximately 10 ps). Analysis of the polarization patterns revealed that decarboxylation from acyloxyl radicals are considerably slower (ns < tau < 0.1 micros). Therefore, in the case of alpha-amino acids, the isoelectronic structures NH(2)-CR(2)-CO(2)(*) and (+*)NH(2)-CR(2)-CO(2)(-) probably do not constitute resonance mesomeric forms of one and the same species and the decarboxylation of aminium radicals is not preceded by the intramolecular carboxylate to amino group electron transfer. Absolute triplet quenching rate constants at zero ionic strength were in the range of 2 x 10(8) to 2 x 10(9) M(-1) s(-1) for R-NH(2) and 2 x 10(7) to 10(8) M(-1) s(-1) for R-CO(2)(-) type of electron donors, reflecting in principle their standard reduction potentials. The strengths of acids: (+)NH(3)-(*)CH(2), (+)NH(3)-(*)C(CH(3))H, and (+)NH(3)-(*)C(CH(3))(2), pK(a) <4, >6, and >7, respectively, were found to be remarkably strongly dependent on alpha-C substitution. The conjugate bases of these alpha-aminoalkyl radicals reduce anthraquinone-2,6-disulfonate dianion ground state with k(sec) = 3 x 10(9) M(-1) s(-1).

Time resolved FT EPR study of the decarboxylation following photo-induced electron transfer between α-amino acids and anthraquinone-2,6-disulfonic acid in aqueous solution

Using laser photolysis of basic anaerobic aqueous solutions of α-glycine, L-α-alanine and α–aminoisobutyric acid containing anthraquinone-2,6-disulfonic acid the two spin-polarized (CIDEP) C•R1R2NH2 and NH2C•R1COO− radicals (R1, R2 = H or CH3) and the radical anion of anthraquinone-2,6-disulfonic acid were detected by time resolved Fourier transform electron paramagnetic resonance (FT EPR) spectroscopy. The FT EPR results reveal that α-glycine, L-α-alanine and α-aminoisobutyric acid were oxidized by the spin-polarized photo-excited triplet of anthraquinone-2,6-disulfonic acid through a one-electron transfer followed by a rapid decarboxylation. In a competing reaction channel the quinone triplet decays by hydrogen transfer from α-glycine and α-alanine yielding the NH2C•R1COO− radical. The rate constants for the electron transfer reaction and of the decarboxylation reaction were determined.

Time-resolved FT-EPR study of photoreduction of duroquinone by triethylamine in methanol

Using time resolved Fourier transform EPR spectroscopy the photoreduction of duroquinone by triethylamine in methanol solution was investigated. It is found that the spin-polarized (CIDEP) duroquinone triplet deactivates by electron transfer from triethylamine generating duroquinone radical anion and amine radical cation, and by hydrogen transfer from the solvent generating durosemiquinone radical and hydroxymethyl radical, respectively. All radicals are observed at different conditions and are spin-polarized by triplet mechanism and partially by ST0 radical pair mechanism. The time dependence of FT-EPR intensities of radical cation and radical anion on the amine concentration is investigated in the range of 1 to 100 mM triethylamine. The contribution of the triplet mechanism to the spin polarization of radicals changes with different triethylamine concentrations. The durosemiquinone radical is found to be transformed into duroquinone radical anion in the presence of triethylamine in the solution. CIDNP experiments indicate that the hydrogen back transfer between the durosemiquinone radical and hydroxymethyl radical pair has a significant influence on the time behaviour of duroquinone radical anion. The intensity of triethylamine radical cation is found to be decreased with the increase of triethylamine concentration, which is interpreted that the triethylamine radical cation is deprotonated by the amine. Based on the FT-EPR results, a new complete mechanism is proposed.

Effects of Lewis acid on the sign of exchange interaction of photogenerated radical ion pairs

FTEPR study shows that the sign of the exchange interaction of the radical ion pair including the 4,4′-dimethoxybenzophenone radical anion and 1,4-diazabicyclo[2,2,2]octane (DABCO) radical cation changes from positive to negative by addition of BF3.

FT EPR Study of the Hydrated Electron Generated by Laser Excitation of Phenothiazine in Quinone-SDS Micellar Systems

We have studied nanosecond kinetics of the hydrated electron (e{sup -}{sub aq}) generated by laser excitation of phenothiazine (PTH) in the presence of quinone in SDS solution using FT EPR. Using two different quinones, idebenone (IDB) and CoQ{sub 0}, we have observed the unique photochemical processes due to the micellar effect in the system. The analyses of FT EPR results suggested that IDB with the long alkyl alcohol chain at the 6-position became the corresponding anion radical by quenching of e{sup -}{sub aq}. On the other hand, CoQ{sub 0}, without the long chain, was reduced by two processes, quenching of e{sup -}{sub aq} and direct electron transfer from the triplet precursor. Moreover, the distinctive dispersive pattern, which implied the radical pair of [PTH{sup .+}...e{sup -}{sub aq}], was observed at 30 ns after the excitation of PTH in IDB/SDS micellar system but was not observed for CoQ{sub 0}. 20 refs., 7 figs.

Photoionization of Benzophenone Carboxylic Acids in Aqueous Solution. A FT EPR and Optical Spectroscopy Study of Radical Cation Decay

Benzophenone and some of its carboxylic acid derivatives were photoionized by a biphotonic process in aqueous solution. For all substances studied, the EPR spectra of the hydrated electrons eaq- and of radicals produced by the decay of the benzophenone radical cation could be detected on the nanosecond and microsecond time scale. Time-resolved optical measurements revealed additional reaction channels for the benzophenone radical cation decay. In general three different reaction channels were found, namely, the reaction of the radical cation with nucleophilic OH- under cyclohexadienyl radical formation, the generation of aryl radicals by β-scission, and the generation of aryl radicals by CO2 elimination.

FT-EPR study of the CIDEP of 2-propanolyl radical produced by the reaction of acetone with triethylamine

The results of an FT-EPR study of the CIDEP of 2-propanolyl radical produced by the photochemical reaction of acetone with triethylamine are reported. The time profiles of the signals are explained satisfactorily on the basis of a hydrogen abstraction reaction involving the acetone singlet and/or triplet excited states depending upon the amine concentration and CIDEP mechanisms involving singlet and/or triplet geminate pairs and triplet F pairs together with spin-lattice relaxation.

A Comparative CW EPR and FT EPR Study of Spin Polarization Created by Doublet-Triplet Encounters

A Comparative CW EPR and FT EPR Study of Spin Polarization Created by Doublet-Triplet Encounters

FT-EPR study of triplet state C60. Spin dynamics and electron transfer quenching

A FT-EPR study was made of paramagnetic species formed by pulsed-laser excitation of C60 in fluid solution. Earlier findings that photoexcitation of C60 in fluid solution gives rise to an EPR signal with narrow linewidth were confirmed. The lifetime of the signal corresponds to that of the C60 triplet as measured by flash photolysis. In the presence of donors, the rate of signal decay is increased and matches the growing in of EPR signals from oxidized donors. The time dependence of the FT-EPR spectra gives values for electron transfer rate constants which agree with those derived from flash photolysis measurements on the same systems. Based on these findings, we assign the narrow line signal to the triplet of C60. The time evolution of the FT-EPR signal establishes that the triplets are born with less spin polarization that the thermal equilibrium value. As a result, signal growth is controlled by spin-lattice relaxation.