Electron spin resonance study of free radicals formed from a procyanidin-rich pine (Pinus maritima) bark extract,...

The free radical generated from the oxidation of a French maritima pine bark extract Pycnogenol (PYC), by the horseradish peroxidase (HRP)-hydrogen peroxide (H2O2) system at pH 7.4-10.0 was studied using electron spin resonance (ESR) spectrometer. The formation rate of the PYC radical (aH = 0.92 G; g = 2.0055) was dependent on the PYC and HRP concentrations and pH; the lifetime of the radical was up to 90 min. Furthermore, it was found that the PYC radical was mainly composed of the secondary radical formed from procyanidin B3, one of major procyanidins in PYC. The primary radical signal of procyanidin B3 with hyperfine splitting constants aH = 3.67 G (1H), aH = 0.92 G (3H), and g = 2.0055 was transient and disappeared quickly, whereas its secondary radical signal appeared and increased with time. The secondary radical from dimer procyanidin B3 showed quite high stability, differing from the radical from monomer (+)-catechin that could not be observed possibly because of its instability. These results provide evidence to support the idea that the intramolecular hydrogen bond between the O* at the 4' position in one B ring and an OH group in the other B ring of procyanidin B3 is formed during its oxidation in the presence of HRP and H2O2.