A theoretical study of the reaction of beta-carotene with the nitrogen dioxide radical in solution

A theoretical study of the reaction of beta-carotene (BC) with the nitrogen dioxide radical (NO2*) in solution is carried out using the density functional theory (DFT) at the B3LYP/6-31G(d) level, to optimize the molecular geometries, and the polarizable continuum model (PCM), to account for solvent effects. The three most important reaction mechanisms--electron transfer from beta-carotene to the radical, hydrogen abstraction by the radical, and radical addition to form an adduct--are studied in detail. Three solvents with different polarities--heptane, methanol, and water--are employed to investigate the effect of the environment on the reaction mechanisms. Our results show that electron transfer is thermodynamically favored only in the polar solvents, the abstraction reactions are spontaneous in the three solvents, although faster in the polar ones, and the addition reactions are all endergonic and, therefore, unlikely to occur in any of the solvents. In both the abstraction and addition mechanisms, the attack of the radical takes place preferentially at the beta-ionone rings, in particular at positions H4 and C5, respectively. The higher stability of the reaction products in these cases is explained in terms of their molecular geometries and electronic structures.