Addition of sulfur radicals to fullerenes

摘要

The addition of oxygen-centered radicals to fullerenes has been intensively studied due to their role in cell protection against against hydrogen peroxide induced oxidative damage. However, the analogous reaction of sulfur-centered radicals has been largely overlooked. Herein, we investigate the addition of S-centered radicals to C_(50), C_(60), C_(70), and C_(100) fullerenes by means of DFT calculations. The radicals assayed were: S, SH, SCH_3, SCH_2CH_3, SC _6H_5, SCH_2C_6H_5, and the open-disulfide SCH_2CH_2CH_2CH_2S. Sulfur, the most reactive species, prefers to be attached to a 66-bond of C _(60) with a binding energy (E_(bind)) of 2.4 eV. For the SR radicals the electronic binding energies to C_(60) are 0.77, 0.74, 0.58, 0.67, and 0.35 eV for SH, SCH_3, SCH_2CH_3, SCH_2C_6H_5, and SC_6H_5, respectively. The reactivity of C_(60) toward SR radicals can be increased by lithium doping. For Li@C_(60), the Ebind is increased by 0.65 eV with respect to C_(60), but only by 0.33 eV for the exohedral doping. Fullerenes act like free radical sponges. Indeed, the C _(60)-SR E_(bind) can be duplicated if two radicals are added in ortho or para positions. The enhanced reactivity because of multiple additions is mostly a local effect, although the addition of one radical makes the whole cage more reactive. Therefore, as observed for hydroxylated fullerenes, they should protect cells from oxidative damage. However, the thiolated fullerenes have one advantage, they can be easily attached to gold nanoparticles. For the addition on pentagon junctions smaller fullerenes like C_(50) are more reactive than C_(60). Interestingly, C_(70) is as reactive as C_(60), even for the addition on the equatorial belt. For larger fullerenes like C_(100), reactivity decreases for the carbon atoms belonging to hexagon junctions.