Reactions of e−aq, co2•−, ho, o2•− and o2(1δg) with a dendro[60]fullerene and c60[c(cooh)2]n (n = 2–6)

doi:10.1016/S0891-5849(00)00287-2

Free Radical Biology and Medicine

Volume 29, Issue 1, 1 July 2000, Pages 26-33

https://doi.org/10.1016/S0891-5849(00)00287-2 Get rights and content

Abstract

Using pulse radiolysis and laser flash photolysis, we have investigated the reactions of the deleterious species, e⁻_aq, HO radical dot , O₂^•⁻ and O₂(¹Δ_g) with 10 water-soluble cyclopropyl-fused C₆₀ derivatives including a mono-adduct dendro[60]fullerene (d) and C₆₀ derivatives based on C₆₀[C(COOH)₂]_n=2–6, some of which are known to be neuroprotective in vivo. The rate constants for reactions of e⁻_aq and HO radical dot lie in the range 0.5–3.3 × 10¹⁰ M⁻¹ s⁻¹. The d and bis-adduct monoanion radicals display sharp absorption peaks around 1000 nm (ϵ = 7 000–11 500 M⁻¹ cm⁻¹); the anions of the tris-, tetra-, and penta-adduct derivatives have broader, weaker absorptions. The monohydroxylated radicals have their most intense absorption maxima around 390–440 nm (ϵ = 1000–3000 M⁻¹ cm⁻¹). The anion and hydroxylated radical absorption spectra display a blue-shift as the number of addends increases. The radical anions react with oxygen (k ∼ 10⁷–10⁹ M⁻¹ s⁻¹). The reaction of O₂^•⁻ with the C₆₀ derivatives does not occur via an electron transfer. The rate constants for singlet oxygen reaction with the dendrofullerene and eee-derivative in D₂O at pH 7.4 are k ∼ 7 × 10⁷ and ∼ 2 × 10⁷ M⁻¹ s⁻¹ respectively, in contrast to ∼ 1.2 × 10⁵ M⁻¹ s⁻¹ for the reaction with C₆₀ in C₆D₆. The large acceleration of the rates for electron reduction and singlet oxygen reactions in water is due to a solvophobic process.

Introduction

C₆₀ fullerenes, with their 30 carbon-carbon double bonds, are able to scavenge a large number of radicals per molecule [1], [2]. For the addition of radicals such as the benzyl radical, the rate constants are larger, by three to four orders of magnitude [3], for C₆₀ than those observed for simple alkenes. Such properties have inspired an interest in potential biological applications of fullerenes [4], including the inhibition of radical-initiated lipid peroxidation [5] and the interception of deleterious reactive oxygen species [6]. Dugan et al. have observed the efficiency of water-soluble malonic acid derivatives, C₆₀[C(COOH)₂]₃, as neuroprotective agents in living systems and assessed their ability to scavenge O₂^•⁻ and HO radical dot oxyradicals in solution by EPR analysis [6].

Using pulse radiolysis, we have now measured in aqueous solution the rate constants for the reactions of hydrated electrons (e⁻_aq), CO₂^•⁻, O₂^•⁻, and HO radical dot radicals with a dendritic monoadduct (d) of C₆₀ with 18 carboxylic groups and a series of nine malonic acid C₆₀ derivatives based on C₆₀[C(COOH)₂]_n (n = 2–6). The bis-adducts studied were trans-2-, trans-3-, trans-4-, and e-C₆₀[C(COOH)₂]₂. The tris-adducts were e,e,e- and t333-C₆₀[C(COOH)₂]₃, the tetra-, penta-, and hexa-adducts being the all-e-regioisomers. The structures, symmetries and abbreviated names of the fullerene derivatives under study are shown in Fig. 1.

We have determined in water at pH 7.4 (i) the spectra and the molar absorption coefficients of the monoanion radicals and the mono-OH adduct radicals of the C₆₀ derivatives, (ii) the rate constants of the e⁻_aq, CO₂^•⁻, and HO radical dot reactions, and (iii) the role of the number of addends and of the addition pattern on the spectroscopic properties of the monoanion radicals and the monohydroxylated adduct radicals. The kinetics of the hydrated electron reactions with two different C₆₀ derivatives were also measured at different pH values between 4 and 11. Our pulse radiolysis investigation supplements previous studies performed by Guldi et al. [7] on the radical anions of trans-2-, trans-3-, e-C₆₀[C(COOH)₂]₂, and e,e,e-C₆₀[C(COOH)₂]₃ in homogeneous (H₂O, 10 vol% 2-Propanol, at pH 9.7) and heterogeneous (H₂O, 5% Triton X-100, 10 vol% 2- Propanol, at pH 9.7) solution.

In addition, using laser flash photolysis, we have compared the rate constants for the reaction of O₂(¹Δ_g) with the dendro[60]fullerene and the eee-tris adduct in D₂O with that of C₆₀ in C₆D₆.

Section snippets

Materials and methods

The malonic acid derivatives C₆₀[C(COOH)₂]_n (n = 2–6) were synthesized as described by Hirsch et al. [8], [11]. The synthesis of the dendritic monoadduct of C₆₀ with 18 carboxylic groups was reported by Brettreich and Hirsch [12].

The aqueous solutions were buffered at pH 7. 4 using 5.0 × 10⁻³ M phosphate. For solutions buffered from pH 4 to pH 11, we used the Teorell and Stenhagen buffer with 1 × 10⁻³ M citric acid, 1 × 10⁻³ M phosphoric acid, and 5 × 10⁻⁴ M orthoboric acid, the pH being

Results and discussion

Within 10⁻⁹ s of high-energy radiation deposition, the radiolysis products of water present at neutral pH are the reducing radicals: hydrated electron (e⁻_aq) and hydrogen atom H radical dot ; the oxidizing radical HO; and the molecular products, hydrogen peroxide and hydrogen. The radiolysis yields (G) expressed in radical or molecules formed per 100 eV absorbed have been recently reviewed and discussed by Ferradini and Jay-Gerin [19]: $e^{−}_{aq} (G=2.65)+H^{•} (G=0.60)+HO^{•} (G=2.88)+H_{2} (G=0.45)+H_{2} O_{2} (G=0.68)$ In

Acknowledgements

This work was performed under European contract FMRX-CT98-0192. The experiments were performed at the Paterson Institute for Cancer Research Free Radical Research Facility, Manchester, UK, with the support of the European Commission through the Access to Large-Scale Facilities activity of the TMR Programme. We are grateful to Drs. I. Hamblett and S. Navaratnam for expert assistance and to Dr. D. Allan, Mr. B. W. Hodgson, and Mr. P. Lucas for technological support. We are also grateful to

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Cited by (0)

¹: Senior Research Fellow in Chemistry, Keele University.

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Original contributionReactions of e−aq, co2•−, ho, o2•− and o2(1δg) with a dendro[60]fullerene and c60[c(cooh)2]n (n = 2–6)1

Abstract

Introduction

Section snippets

Materials and methods

Results and discussion

Acknowledgements

Tetrahedron Lett.

Radiat. Phys. Chem.

J. Electroanal. Chem.

Chem. Phys. Lett.

Radical reactions of C60

Science

Addition of free radicals to C60

Acc. Chem. Res.

Rate constants for the addition of the benzyl radical to C60 in solution

J. Phys. Chem.

Medicinal chemistry with fullerenes and fullerene derivatives

J. Chem. Soc. Chem. Commun.

C60 and water-soluble fullerene derivatives as antioxidants against radical-initiated lipid peroxidation

J. Med. Chem.

Carboxyfullerenes as neuroprotective agents

Proc. Natl. Acad. Sci. USA

Inhibition of cluster phenomena in truly water soluble fullerene derivativesBimolecular electron and energy transfer processes

J. Phys. Chem. B

Water-soluble malonic acid derivatives of C60 with a defined three-dimensional structure

J. Chem. Soc. Chem. Commun.

Fullerene chemistry in three dimensionsisolation of seven regioisomeric bisadducts and chiral trisadducts of C60 and di(ethoxy-carbonyl)methylene

Angew. Chem. Int. Ed. Engl.

Regiochemistry of multiple additions to the fullerene coresynthesis of a Th-symmetric hexakis adduct of C60 with Bis(ethoxycarbonyl)methylene

J. Am. Chem. Soc.

Reversible template-directed activation of equatorial double bonds of the fullerene frameworkregioselective direct synthesis, crystal structure, and aromatic properties of Th-C66(COOEt)12

Angew. Chem. Int. Ed. Engl.

Pulse radiolysis apparatus

J. Sci. Instr.

Original contribution
Reactions of e⁻_aq, co₂^•⁻, ho, o₂^•⁻ and o₂(¹δ_g) with a dendro[60]fullerene and c₆₀[c(cooh)₂]_n (n = 2–6)¹

Radical reactions of C₆₀

Addition of free radicals to C₆₀

Rate constants for the addition of the benzyl radical to C₆₀ in solution

C₆₀ and water-soluble fullerene derivatives as antioxidants against radical-initiated lipid peroxidation

Water-soluble malonic acid derivatives of C₆₀ with a defined three-dimensional structure

Fullerene chemistry in three dimensionsisolation of seven regioisomeric bisadducts and chiral trisadducts of C₆₀ and di(ethoxy-carbonyl)methylene

Regiochemistry of multiple additions to the fullerene coresynthesis of a T_h-symmetric hexakis adduct of C₆₀ with Bis(ethoxycarbonyl)methylene

Reversible template-directed activation of equatorial double bonds of the fullerene frameworkregioselective direct synthesis, crystal structure, and aromatic properties of T_h-C₆₆(COOEt)₁₂