High Catalytic Activity of Dendritic C₆₀ Monoadducts in Metal‐Free Superoxide Dismutation

Abstract

Water-soluble fullerenes, in particular the tris-malonyl-C 60 derivative 1 (so-called C 3 ), [1] have been shown to exhibit strong antioxidant activity against reactive oxygen species in vitro and to protect cells and tissue from oxidative injury and cell death in vivo.[2] The ability to destroy the toxic superoxide O 2 C À was suggested to be responsible for fullerene antioxidant activity, [3] although its mechanism is still not clear. Dugan and co-workers offered evidence in support of a catalytic superoxide dismutation mechanism instead of direct radical attack on the C 60 moiety of 1, thus showing that it could act as a metal-free mitochondrial manganese superoxide dismutase (MnSOD) mimetic.[3a] They proposed the formation of a complex between C 3 and O 2 C À . Herein, we present for the first time clear and unambiguous evidence for a catalytic dismutation process, the key steps of which are successive O 2 C À oxidation, within an outersphere electron-transfer process, and fullerene-derivativemediated O 2 C À reduction. At the same time we are able to rationalize structure-property relationships by the systematic investigation of a series of stable, readily accessible, and nontoxic mono-and trisadducts 2-7 of C 60 . [2c, 4] This led to the identification of new lead compounds for neuroprotective applications with significantly improved superoxide dismutation activity.To describe the structure-reactivity relationship with respect to superoxide dismutase (SOD) activity, we first present the redox properties of 2-7. Cyclic voltammetry measurements [5] in DMSO have shown that in the potential range from 0 to À1 V (vs. a saturated calomel electrode, SCE) 2-6 undergo two reversible reductions, whereas 7 exhibits one reversible redox couple (Table 1, Figure 1, Figure S1 in the Supporting Information). The corresponding reduction potentials of the monoadducts 2-4 are significantly higher than those of trisadducts 5-7 and show a prominent charge dependence, especially for the first C 60 /C 60 C À redox couple, with the positively charged derivative 4 being the strongest electron acceptor. The observed redox potentials are considerably higher than expected for fullerene mono-and trisadducts.[6a] It seems that solvent effects (predominantly solvent polarity) [6b] and the amphiphilic nature of the attached addends, which facilitate micellar organization and therefore close C 60 -C 60 interaction of 2-7 in solution, are responsible for the positive shift of their redox potentials.Of special importance is the fact that the first reduction potentials of 2-4 are much higher than the oxidation potential of superoxide (À0.74 V vs. SCE in DMSO). This implies that [a] values obtained by using direct stopped-flow measurements in DMSO (0.06 % water) and an indirect cytochrome c assay (k McCF ) [b] in aqueous solution (pH 7.8).

On the Mechanism of Action of Fullerene Derivatives in Superoxide Dismutation

“…[55] This is the main reason for the relatively small energy difference between int1 (in which the electron has not yet been transferred) and int2 (in which superoxide is converted to an oxygen molecule due to the redox process). This is in concordance with the experimental results by Liu and co-workers, [21] who observed that C 3 derivatives with higher reduction potentials presented higher antioxidant activity. Given that a higher reduction potential corresponds to a greater affinity for accepting electrons, this implies more stabilized HOMO and LUMO.…”

“…The relatively high activation barrier found for the reduction/oxidation process (int1!int2 + O 2 ) for the archetypal C 3 compound studied here is consistent with the low antioxidant activity found experimentally, compared to other C 3 -related derivatives with higher reduction potentials. [21] The third intermediate (int3) is substantially more stable than the initial reactants (À137.8 kcal mol…”

“…[18] Formation of a complex between C 3 and O 2 C À was also proposed. [18,21] Liu et al presented experimental evidence for a catalytic dismutation process in which the key steps of the reaction were oxidation of O 2 C À produced in an outer-sphere electron-transfer process and fullerene reduction.…”

“…[10] Liu et al found that the SOD activity of different carboxyfullerenes was dependent on their reduction potentials: the higher the reduction potential the higher the SOD dismutation activity. [21] They observed that the molecular structure of the fullerene derivative is important, too, as fullerene derivatives with similar reduction potentials but substantially different architectures exhibit completely distinct antioxidant activities. They recently also evaluated the antioxidant activity of several mono-adducts (i.e., only one malonyl unit is attached to the fullerene surface) and found a higher activity compared to the archetypal compound C 3 .…”

“…They recently also evaluated the antioxidant activity of several mono-adducts (i.e., only one malonyl unit is attached to the fullerene surface) and found a higher activity compared to the archetypal compound C 3 . [21] Moreover, mono-adducts are more stable, less toxic, [22] and can be produced in higher yields. Other related compounds, such as fullerene-containing peptide nanoparticles obtained through self-assembly, were found to exhibit antioxidant activity as well.…”