Alexander I. Kruppa

Carotenoids as antioxidants: spin trapping EPR andoptical study

The role of several natural and synthetic carotenoids as scavengers of free radicals was studied in homogeneous solutions. A set of free radicals: *OH, *OOH, and *CH(3) were generated by using the Fenton reaction in dimethyl sulfoxide. It was shown that the spin trapping technique is more informative than optical methods for the experimental conditions under study. 5,5-Dimethyl-pyrroline-N-oxide (DMPO) and N-tert-butyl-alpha-phenylnitrone (PBN) were used as spin traps for the EPR studies. The results show that the scavenging ability of the carotenoids towards radical *OOH correlates with their redox properties.

The mechanisms of the oxidation of NADH analogues 1. Photochemical oxidation of N-unsubstituted 1,4-dihydropyridines by various acceptors

Abstract The reactivity of different active centers has been studied during the photo-oxidation of N -unsubstituted 1,4- dihydropyridines (1,4-DHPs) by various electron acceptors. The participation of that or another reactive center (NH, (C-4)H, (C-3)CO group) in the reaction has been observed, depending on the type of process, 1,4- DHP structure, medium polarity and related properties. In the case of photo-initiated interaction with electron acceptors, the elementary mechanism of the process includes the sequential stages of electron, proton and hydrogen atom transfer (e − , H + , H . ) if the electron acceptors are quinones which are able to form charge transfer complexes with the initial 1,4-DHP. When acceptors of another type are used, the primary act of the photo-oxidation process is the abstraction of hydrogen atoms from the C-4 position of the 1,4-DHP, and the oxidation takes place as the two-stage (H . , H . ) process.

The mechanisms of the oxidation of NADH analogues 2. N-Methyl-substituted 1,4-dihydropyridines

Abstract The mechanism of the photoinitiated oxidation of N -methyl-substituted 1,4-dihydropyridines (1,4-DHPs) by quinones was studied in acetonitrile. Electron transfer with the formation of a radical ion pair (radical cation of 1,4-DHP and radical anion of acceptor) forms the initial stage of the process. Abstraction of a hydrogen atom from the 1,4-DHP radical cation in the bulk leads to the formation of the main product of photo-oxidation — pyridinium cation. Traces of water in the reaction mixture and the pH exert a significant influence on the photo-oxidation of N -substituted 1,4-DHPs.

CIDNP study of the third spin effect on the singlet–triplet evolution in radical pairs

Abstract The time-resolved 1 H photo-CIDNP formed upon photocleavage of diisopropylketone in the presence and absence of stable radical (TEMPO) and dodecanetiol-1 as radical traps has been studied. It was found that TEMPO affects CIDNP formation in a primary radical pair not only as a radical acceptor but also through spin exchange. The rate constant of the recombination of isopropyl radicals and TEMPO in acetonitrile at ambient temperature was determined as (2.4±0.5)×10 8 M −1 s −1 .

The mechanisms of oxidation of NADH analogues 3. Stimulated nuclear polarization (SNP) and chemically induced dynamic nuclear polarization (CIDNP) in low magnetic fields in photo-oxidation reactions of 1,4-dihydropyridines with quinones

Abstract Radical particles formed during the photo-oxidation of 1,4-dihydropyridines (1,4-DHPs) by quinones in benzene and acetone have been characterized using stimulated nuclear polarization (SNP) and low-magnetic-field chemically induced dynamic nuclear polarization (CIDNP) techniques. The experimentally obtained hyperfine coupling constants of the 1,4-DHP radical ion and neutral radical qualitatively agree with those calculated using the INDO method. It has been found that the disproportionation reaction of two neutral nitrogen-centred 1,4-DHP radicals provides the main contribution to CIDNP in low magnetic fields.

CIDNP 1H study of the photolysis of 7-sila- and 7-germa-norbornadienes

Abstract The photochemical decomposition of 7-sila- and 7-germa-norbornadienes (Ia,b) was studied by the CIDNP 1H technique. The reactions proceeds by a two-step mechanism via the reversible formation of singlet biradicals, II. The triplet biradical (II), formed as a result of S-T conversion of (II)(S), irreversibly decomposes giving Me2E (E = Si, Ge). The insertion of Me2E into the CBr bond of PhCH2Br and the SnCl bond of Me3SnCl occurs via a radical mechanism, as deduced from the CIDNP effects observed in these reactions.

Paramagnetic intermediates in the photolysis of 7-silanorbornadiene studied by means of spin chemistry method

Abstract The influence of an external magnetic field on the yield of photo-decomposition products of 7,7′-dimethyl-7-silanorbornadiene derivative has been detected in laser pulse photolysis experiments. The observations of magnetic field effects alterations in the presence of scavengers, O 2 and PPh 3 , in combination with 1 H-CIDNP data form the basis for the identification of the structure of paramagnetic intermediates involved in the process. It has been shown that magnetic field effects originate in biradical intermediates. These species result from both endocyclic SiC bond cleavage in the initial compound and the reaction of dimethylsilylene (in a singlet or a triplet excited state) with starting 7-silanorbornadiene. To explain the influence of O 2 upon the magnetic field effects, the reversible formation of oxygen complex with biradical species has been suggested.

The mechanism of oxidation of NADH analogues 4. Photooxidation of N-acetyl-substituted 1,4-dihydropyridine in the presence of quinones

Abstract Mechanism of photoinitiated oxidation of N -acetyl-3,5-dicarbomethoxy-4-phenyl-1,4-dihydropyridine ( I ) in the presence of quinones has been studied by means of CIDNP method in polar solvents. 3,5-Dicarbomethoxy-4-phenyl-pyridine ( II ) has been identified to be the main reaction product. It has been found that the process involves the radical ions as well as neutral radicals of dihydropyridine and quinone. Sigmatropic rearrangement (1–3 shift) has been suggested to occur in N-centered dihydropyridine radicals.

Low-field CIDNP study of photoinduced electron transfer reactions

Abstract Chemically induced dynamic nuclear polarization in low magnetic field (low-field CIDNP) has been detected and studied in photoinduced electron transfer reactions in the polar solvent acetonitrile. For the radical-ion reactions two different approaches to interpret the low-field CIDNP are demonstrated: interpretation of the low-field CIDNP sign on the basis of quality relationships, and numerical calculations of the CIDNP field dependence. Analysis shows that low-field CIDNP in these reactions is sensitive to the value of the electron exchange interactions in radical-ion pairs.

Modeling magnetic field effects in multispin systems

A model is proposed for calculating magnetic field effects formed in a radical triad composed of a biradical and a paramagnetic particle. To describe the influence of the “third” spin on the spin evolution in a biradical, the electron spin exchange interaction of the added spin with one of the paramagnetic centers of the biradical has been considered. Calculating the field dependence of the recombination probability of the biradical-oxygen complex revealed both an increase in recombination probability earlier attributed to spin catalysis and the influence of the values and signs of the exchange interaction in the complex on the shape of the magnetic-field effect dependence. Calculation results are in agreement with the experimental data on the photolysis of 7,7’-dimethyl-silanorbornadiene in aerated and deaerated solution.