Tatyana V. Leshina

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.

Water Soluble Complexes of Carotenoids with Arabinogalactan

We present the first example of water soluble complexes of carotenoids. The stability and reactivity of carotenoids in the complexes with natural polysaccharide arabinogalactan were investigated by different physicochemical techniques: optical absorption, HPLC, and pulsed EPR spectroscopy. Compared to pure carotenoids, polysaccharide complexes of carotenoids showed enhanced photostability by a factor of 10 in water solutions. A significant decrease by a factor of 20 in the reactivity toward metal ions (Fe(3+)) and reactive oxygen species in solution was detected. On the other hand, the yield and stability of carotenoid radical cations photoproduced on titanium dioxide (TiO(2)) were greatly increased. EPR measurements demonstrated efficient charge separation on complex-modified TiO(2) nanoparticles (7 nm). Canthaxanthin radical cations are stable for approximately 10 days at room temperature in this system. The results are important for a variety of carotenoid applications, in the design of artificial light-harvesting, photoredox, and catalytic devices.

Complex of Calcium Receptor Blocker Nifedipine with Glycyrrhizic Acid

Physicochemical methods were used to explore the regularities of complexing between the calcium channel blocker nifedipine (NF) and pharmaceutically acceptable complex-forming glycyrrhizic acid (GA) in view of the discovered influence of GA on the therapeutic activity of NF. 1H NMR (including relaxation measurements) and UV-vis spectra have produced illustrative evidence that NF forms stable complexes with GA within a wide concentration range, from 0.05 to 5 mM. At low GA concentrations, below 0.5 mM, NF forms an inclusion complex where each NF molecule is bound by two molecules of GA. Computer simulations of the NMR experimental data have shown that, in aqueous solution, the stability constant of this complex, K, is about 10(5) M(-1). At higher concentrations, GA forms large micelle-like aggregates which increase the water solubility of NF. Quenching of chemically induced dynamic nuclear polarization effects in the photoinduced interaction of the NF-GA complex with tyrosine suggests that complex formation with GA completely blocks the single electron-transfer step between NF and the amino acid. This, arguably, could explain the increased therapeutic activity of GA complexes, since GA might protect the drug molecule from the reaction with amino acid residues of the receptor binding site.

Enhancement of the Photocatalytic Activity of TiO2 Nanoparticles by Water-Soluble Complexes of Carotenoids †

Photoirradiation of TiO(2) nanoparticles by visible light in the presence of the water-soluble natural polysaccharide arabinogalactan complexes of the hydrocarbon carotenoid β-carotene leads to enhanced yield of the reactive hydroxyl (OH) radicals. The electron paramagnetic resonance (EPR) spin-trapping technique using α-phenyl-N-tert-butyl nitrone (PBN) as the spin-trap has been applied to detect this intermediate by trapping the methyl and methoxy radicals generated upon reaction of the hydroxyl radical with dimethylsulfoxide (DMSO). The free radicals formed in this system proceed via oxygen reduction and not via the reaction of holes on the TiO(2) surface. As compared with pure carotenoids, carotenoid-arabinogalactan complexes exhibit an enhanced quantum yield of free radicals and stability toward photodegradation. The observed enhancement of the photocatalytic efficiency for carotenoid complexes, as measured by the quantum yield of the desired spin adducts, arises specifically from the decrease in the rate constant for the back electron transfer to the carotenoid radical cation. These results are important for a variety of TiO(2) applications, namely, in photodynamic therapy, and in the design of artificial light-harvesting, photoredox, and catalytic devices.

Photo-CIDNP Study of the Interaction of Tyrosine with Nifedipine. An Attempt to Model the Binding Between Calcium Receptor and Calcium Antagonist Nifedipine¶

Abstract This article proposes a new approach to the modeling of the molecular-level mechanism of ligand–receptor interaction for Ca2+ receptor binding site. Chemically induced dynamic nuclear polarization (CIDNP) technique has been used to unravel fine details of the reaction in the model system composed of one of the known Ca2+ antagonist drugs, nifedipine (NF), and isolated amino acid residuals (e.g. tyrosine [Tyr]) of Ca2+ receptor binding site. It has been conclusively demonstrated that the reaction between NF and Tyr resulting in the oxidation product—nitroso form of NF—obeys the radical mechanism. CIDNP data in combination with the results of mathematical modeling of the structures of ligand–receptor complexes have allowed to propose the mechanism of the interaction of NF with Ca2+ receptor binding site.

EPR spin trapping detection of carbon-centered carotenoid and β-ionone radicals

Free radical intermediates were detected by the electron paramagnetic resonance spin trapping technique upon protonation/deprotonation reactions of carotenoid and beta-ionone radical ions. The hyperfine coupling constants of their spin adducts obtained by spectral simulation indicate that carbon-centered radicals were trapped. The formation of these species was shown to be a result of chemical oxidation of neutral compounds by Fe(3+) or I(2) followed by deprotonation of the corresponding radical cations or addition of nucleophilic agents to them. Bulk electrolysis reduction of beta-ionone and carotenoids also leads to the formation of free radicals via protonation of the radical anions. Two different spin adducts were detected in the reaction of carotenoid polyenes with piperidine in the presence of 2-methyl-2-nitroso-propane (MNP). One is attributable to piperidine radicals (C(5)H(10)N*) trapped by MNP and the other was identified as trapped neutral carotenoid (beta-ionone) radical produced via protonation of the radical anion. Formation of these radical anions was confirmed by ultraviolet-visible spectroscopy. It was found that the ability of carotenoid radical anions/cations to produce neutral radicals via protonation/deprotonation is more pronounced for unsymmetrical carotenoids with terminal electron-withdrawing groups. This effect was confirmed by the radical cation deprotonation energy (H(D)) estimated by semiempirical calculations. The results indicate that the ability of carotenoid radical cations to deprotonate decreases in the sequence: beta-ionone > unsymmetrical carotenoids > symmetrical carotenoids. The minimum H(D) values were obtained for proton abstraction from the C(4) atom and the C(5)-methyl group of the cyclohexene ring. It was assumed that deprotonation reaction occurs preferentially at these positions.

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.

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.

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 .