V. P. Gubskaya

Synthesis, structure and biological activity of nitroxide malonate methanofullerenes

Two nitroxide methanofullerenes was synthesized for the first time, and their structures and biological activities studied. It was shown by X-ray single crystal analysis that the methanofullerene with two nitroxide groups forms a 1 : 2 inclusion complex with chloroform and has a nearly tetrahedral (diamond-like) arrangement of fullerene-fullerene interactions in the crystal. For the first time, it has been found that malonate nitroxide methanofullerene in combination with the known anticancer drug cyclophosphamide (CPA) shows high antitumor activity against leukemia P-388.

TR-EPR of single and double spin labelled C60 derivatives: observation of quartet and quintet excited states in solution

Time-resolved EPR spectra recorded after pulsed laser photoexcitation of two fullerene nitroxide radicals 1a and 1b and two fullerene nitroxide biradicals 2a and 2b in solution have been investigated at room temperature. The TR-EPR spectra of all the radicals are interpreted as arising from spin-polarized ground and excited states, the latter being obtained by the spin coupling of the nitroxide unpaired spins with the fullerene excited triplet state. The TR-EPR spectra of the monoradicals are consistent with strong (1a) or weak (1b) spin coupling, giving rise to pure quartet/doublet excited states (1a) or mixed states (1b). In the latter, the spectral simulation allowed the determination of the magnitude of the exchange coupling constant J TR between the triplet and the radical. The spin polarization of the excited and ground states is attributed to CIDEP effect induced by the radical triplet pair mechanism (RTPM). The sign of J TR is inferred from the sign of the spin polarization. In the case of biradicals 2a and 2b, the TR-EPR spectra are assigned, on the basis of hyperfine pattern and g-values, to quintet states obtained by spin coupling of the fullerene triplet photoexcited state with the two radicals. The spin polarization of the ground state is attributed to the triplet–triplet annihilation process and the sign of the polarization allows the assignment of the radical–radical exchange constant J RR.

Redox-switchable binding of the Mg2+ ions by 21,31-diphenyl-12,42-dioxo-7,10,13-trioxa-1,4(3,1)-diquinoxaline-2(2,3),3(3,2)-diindolysine-cyclopentadecaphane

The binding of the Li+, Na+, K+, Mg2+, and Co2+ ions by 21,31-diphenyl-12,42-dioxo-7,10,13-trioxa-1,4(3,1)-diquinoxaline-2(2,3),3(3,2)-diindolysine-cyclopentadecaphane containing two indolysine fragments, two quinoxaline fragments, and 3,6,9-trioxyundecane spacer in the acetonitrile/0.1 M Bu4NBF4 environment is studied by the method of cyclic voltammetry. It is demonstrated that the Li+, Na+, K+, and Co2+ ions are not bound by this macrocycle, whereas selective redox-switchable binding is observed for the Mg2+ ions. The macrocycle binds the Mg2+ ions way more efficiently as compared with its radical cation and dication. The indolysinequinoxaline fragments play the determining role in the binding.

Electrochemical reduction and oxidation of fullerenopyrrolidines* and the ESR spectra of paramagnetic intermediates

Electroreduction and electrooxidation of monosubstituted N-methyl[60]fullerenopyrrolidines were studied by cyclic voltammetry and potentiostatic microelectrolysis in the cavity of an ESR spectrometer. Stepwise reversible transfer of three electrons to the fullerenopyrrolidine molecule results in the formation of stable radical anions (according to ESR, g = 2.0000, ΔH = 0.8 G), dianions, and radical trianions (according to ESR, g = 2.0015, ΔH = 1.5 G). The reduction potentials vary over narrow limits depending on the nature of the substituents in the pyrrolidine fragment of the compounds. Electrooxidation is irreversible and occurs in either one or two steps. For compounds containing the aniline, indole, or phenol fragment, the first step is associated with oxidation of these fragments and only after that, is the fullerenopyrrolidine core oxidized. Oxidation of the pyrrolidine fragment is substantially more difficult than that of tertiary amines.

Fulleropyrrolidine-containing sterically hindered phenol. Synthesis, structure, and properties

Fulleropyrrolidine containing a sterically hindered phenolic fragment was synthesized by the reaction of fullerene C60 with N-methylglycine and 3,5-di-tert-butyl-4-hydroxybenzaldehyde. Electrochemical reduction of fulleropyrrolidine-containing phenol 1 and the corresponding phenoxide ion proceeded stepwise to form stable radical anions, dianions, and trianions. The radical anion (g = 2.0000) and the phenoxyl radical (g = 2.0045) obtained by chemical oxidation with lead dioxide were identified by ESR spectroscopy. The electron affinity of fulleropyrrolidine was estimated at 2.58 eV. For the phenoxide ion, the electrochemical gap was determined (ΔE = EIox – EIred = 0.47 V). The heats of formation and the energies of the frontier orbitals of fulleropyrrolidine and its transformation products were evaluated by the PM3 method.

TR-EPR of single and double spin-labeled C60 derivatives in frozen matrices

Time-resolved electron paramagnetic resonance spectra of two single spin-labeled and two double spin-labeled C60 derivatives in frozen solution are recorded with pulsed laser excitation and 100 ns time resolution. Quartet and quintet excited species are detected which arise from the electron spin coupling of the triplet excited fullerene moiety with the unpaired spin(s) of the nitroxide label(s). Despite the similar molecular structure, in both series of single and double labeled derivatives a different behavior was found, which is due to substantial difference of the energy of exchange coupling.

Retro-Bingel reaction in the electrochemical reduction of bis(dialkoxyphosphoryl)methanofullerenes

Four steps of reduction were detected for bis(diethoxyphosphoryl)- and bis(diisopropoxyphosphoryl)methano[60]fullerenes (1, 2) and bis(diethoxyphosphoryl)methano[70]fullerene (3) by cyclic voltammetry in the o-dichlorobenzene—DMF (3 : 1, v/v)/Bu4NBF4 (0.1 mol L–1) system on a glass-carbon electrode. At the first step the reversible transfer of one electron affords stable radical anions 1 and 2 (g = 1.9999, ΔH = 1.9 G). When two electrons per molecule are transferred, the methano fragment is rapidly eliminated (retro-Bingel reaction). This process involves the step-by-step cleavage of two C—C bonds of exo-carbon with the fullerene shell in combination with the stepwise transfer of other two electrons and a proton to form finally the carbanion of the methano fragment and fullerene dianion. For all studied compounds, the elimination rate is much higher than that for bis(alkoxycarbonyl)- and dialkoxyphosphoryl(alkoxycarbonyl)methano[60]fullerenes, which makes it possible to propose bisphosphorylmethane groups as protective in synthesis of new fullerene derivatives.

Competitive Conversions of Carbonyl-Containing Methanofullerenes Induced by Electron Transfer

The electrochemical reduction of carbonyl-containing methanofullerenes is studied by methods of cyclic voltammetry and ESR with in situ electrolysis in the environment of 0.1 M Bu4NBF4 in o-dichlorobenzene-dimethylformamide (3:1 by volume). It is shown that the one-electron transfer onto the fullerene sphere of these compounds induces opening of the three-membered cycle, which leads to processes of retrocyclopropanation, transfer of the adduct onto the fullerene sphere of another molecule with the formation of bismethanofullerenes, and transformation into fullerodihydrofuran. The totality of competitive processes and the balance between these is defined by the nature of substituents at the exocarbon atom and the experimental conditions. The opening process is interpreted as the intramolecular electron transfer from the fullerene sphere onto the three-membered cycle.

Study of the mechanism of action of p-chloromercuribenzoate on endonuclease from the bacterium Serratia marcescens.

The mechanism of action of p-chloromercuribenzoate (PCMB) on Serratia marcescens nuclease was investigated. The analysis showed that PCMB forms complexes with DNA. Binding of C7H5O2Hg+ to DNA changes the secondary structure of the DNA. These changes alter the enzymatic activity of S. marcescens nuclease, which was previously found to be sensitive to the secondary structure of the substrates. The nuclease activity was either suppressed or stimulated in the presence of PCMB depending on the C7H5O2Hg+ to nucleotide equivalent ratio. Binding of C7H5O2Hg+ to DNA did not form an abortive enzyme–substrate complex. Binding of Mg2+ to the C7H5O2Hg–DNA complex caused appropriate changes in secondary structure of the substrate. Since Mg2+ and C7H5O2Hg+, though differing in the type of metal cation, are similar in their mechanisms of influence on enzymatic activity of S. marcescens nuclease, the identity of other metal-containing effectors in their mechanism of action on Serratia marcescens nuclease is assumed.

Electrochemical reduction of some methanofullerenes. On the mechanism of the retro-Bingel reaction

The reactions of electrochemical reduction of methanofullerenes bearing phosphonate and alkoxycarbonyl groups at theexo-carbon atom were studied. The mechanism of the retro-Bingel reaction as the cleavage of two C−C bonds between the C(61) atom and the fullerene shell accompanied by electrochemical electron transfer was proposed.