Tatyana A. Konovalova

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.

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.

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.

Mass Spectrometric and Spectroscopic Studies of the Nutritional Supplement Chromium(III) Nicotinate

Despite chromium nicotinate’s popular use as a chromium nutritional supplement, the structure and composition of chromium nicotinate have only been poorly described. As solid chromium nicotinate is intractable, being insoluble or unstable in common solvents, studies on the solid have been limited, and studies of the solution from which the “compound” precipitates have additionally provided little additional data. The results of mass spectrometric and spectroscopic investigations designed to further elucidate the structure and composition of chromium nicotinate are described. The results demonstrated that the three common methods for producing “chromium nicotinate” all yield different compounds, all of which are polymers of Cr(III), oxygen-bound nicotinate, hydroxide, and water. Implications for interpreting results of nutritional studies of “chromium nicotinate” are discussed.

Persistent spectral hole burning in europium-doped sodium tellurite glass

We have prepared sodium tellurite glasses doped with europium. Emission intensity of the europium-doped glass is at least 500× more than that of the undoped glass. Fluorescence features exhibited dependence on the pump laser wavelength. Persistent spectral hole burning was observed in the F07→D05 transition of Eu3+. Multiple hole burning was possible even if the chemicals were melted in ambient air to make the glass. The hole burning mechanism is also explained. Electron spin resonance measurements confirmed the existence of defect centers.

EPR and ENDOR studies of carotenoid–solid Lewis acid interactions

EPR and ENDOR results provide strong support for the formation of carotenoid radical cations on activated alumina and silica–alumina as a consequence of electron transfer from the adsorbed carotenoid molecules to the Lewis acid sites on these surfaces. This assignment is confirmed by the correlation between the AlIII Lewis acid site content on the activated surfaces and the maximum concentration of the generated carotenoid radical cations. The results are compared with those obtained for a solution of carotenoid and AlCl3, which serves as a model system. The ENDOR line at ca. 6.5 MHz obtained for the carotenoids adsorbed on solid supports and carotenoid–AlCl3 solution, is attributed to the high-frequency feature of a hyperfine doublet, centred about the 27Al Larmor frequency. ENDOR detection of the hyperfine doublet, instead of the single line at the Larmor frequency, indicates the formation of strong complexes between carotenoid molecules and Lewis acid sites on the surface.

CIDNP and EPR Study of Phototransformation of Lappaconitine Derivatives in Solution

Chemically induced dynamic nuclear polarization (CIDNP) and electron paramagnetic resonance (EPR) techniques have been used to study the paramagnetic species formed during the photolysis of the alkaloid lappaconitine and its synthetic analogues in solution. Lappaconitine is a photosensitive antiarrhythmic and hypertension drug, whose major photoproduct (N-acetyl anthranilic acid) is also a potent photosensitizer. Both these compounds are lipophilic and might bind efficiently to cell membranes thereby causing phototoxic damage. Photolysis of natural lappaconitine (I) as well as its N(20) des-ethyl derivatives (N-Bz (II), N-Me (III), N-H (IV), and N(O)-Et (V)) results in cleavage of the ester bond with the formation of N-acetyl anthranilic acid (VIII) and corresponding enamine. The lappaconitine derivative V shows maximum photostability which correlates with reference data about its low toxicity. It was shown that the primary reaction step is electron transfer from the amino group to the anthranilic fragment of lappaconitine resulting in an intermediate biradical. The final products are formed via fragmentation of the neutral lappaconitine radicals.