Singlet Oxygen Reactions with Flavonoids. A Theoretical - Experimental Study

Detection of singlet oxygen emission, lambda(max) = 1270 nm, following laser excitation and steady-state methods were employed to measure the total reaction rate constant, k(T), and the reactive reaction rate constant, k(r), for the reaction between singlet oxygen and several flavonoids. Values of k(T) determined in deuterated water, ranging from 2.4x10(7) M(-1)s(-1) to 13.4x10(7) M(-1)s(-1), for rutin and morin, respectively, and the values measured for k(r), ranging from 2.8x10(5) M(-1)s(-1) to 65.7x10(5) M(-1)s(-1) for kaempferol and morin, respectively, being epicatechin and catechin chemically unreactive. These results indicate that all the studied flavonoids are good quenchers of singlet oxygen and could be valuable antioxidants in systems under oxidative stress, in particular if a flavonoid-rich diet was previously consumed. Analysis of the dependence of rate constant values with molecular structure in terms of global descriptors and condensed Fukui functions, resulting from electronic structure calculations, supports the formation of a charge transfer exciplex in all studied reactions. The fraction of exciplex giving reaction products evolves through a hydroperoxide and/or an endoperoxide intermediate produced by singlet oxygen attack on the double bond of the ring C of the flavonoid.