Antioxidant properties of bee propolis and an important component, galangin, described by X-ray crystal structure, DFT-D and hydrodynamic voltammetry

Abstract

Propolis is produced by honeybees and used to seal their hives for defensive purposes and has been used in ethnopharmacology since ancient times. It is a lipophilic material containing a large collection of naturally produced plant organic molecules, including flavonoids. The flavonoid galangin is consistently found in propolis, independent of the hive geographical location and its X-ray crystal and molecular structure is reported. The antioxidant scavenging of superoxide by galangin and propolis is here presented. Using a cyclic voltammetry technique developed in our lab, we show that galangin is an excellent scavenger of the superoxide radical, perhaps even better than quercetin. Our results show that galangin displays a Superoxide Dismutase (SOD) function. This is described experimentally and theoretically (DFT). Two modes of scavenging superoxide are seen for galangin: (1) superoxide radical extraction of H atom from the hydroxyl moieties located in position 3 and 5 of galangin, which are also associated with proton incorporation defining the SOD action; (2) π-π interaction among several superoxide radicals and the galangin polyphenol ring that evolve towards release of O2 and H2O2. We describe these two actions separately as their relative sequence, and/or combination, cannot be defined; all these processes are thermodynamically spontaneous, or subjected to mild barriers.

Aromatic Polyphenol π-π Interactions with Superoxide Radicals Contribute to Radical Scavenging and Can Make Polyphenols Mimic Superoxide Dismutase Activity

“…A recent study by us has shown the scavenging of superoxide by the flavonoid galangin [ 25 ]. As seen with the coumarin earlier described, a straightforward SOD action is observed when the first attack of superoxide occurs on galangin H3 and is shown in Scheme 1 below.…”

“…An alternative initial π-π approach involving one aromatic ring of galangin and superoxide was also explored and is shown in Scheme 2 The original study [ 25 ] has a typographical error whereby Scheme 1 and Scheme 2 are identical. The correct Scheme 1 is shown in this study.…”

“…The correct Scheme 1 is shown in this study. PlosOne was informed and a corrigendum should appear accordingly [ 25 ]. A molecule of H 2 O 2 results from the superoxide release of its unpaired electron to the galangin ring [steps (d) → (e)], and it is still linked to the ring.…”

“… SOD action for superoxide scavenging by galangin [ 25 ]. Galangin is regenerated after reacting with 2 protons and 2 superoxide radicals.…”

“… Superoxide radical interaction with galangin [ 25 ]. The π-π scavenging of superoxide by galangin: ( a ) Superoxide interacts with galangin ring A (π-π approach); ( b ) the unpaired electron enters the galangin ring; ( c , d ) two protons are incorporated within the O 2 moiety forming H 2 O 2 ; ( e ) a 2nd superoxide is incorporated, ( f ) a proton captured by O 2 forms a HO 2 moiety; ( g ) after an additional proton the original H 2 O 2 (red) detaches from O-1 galangin position.…”