Reaction pH and protein affect the oxidation products of β-pentagalloyl glucose

To better understand the biochemical consequences when polymeric polyphenols serve as biological antioxidants, we studied how reaction pH (pH 2.1-7.4) and protein affected the oxidation of pentagalloyl glucose (PGG) by NaIO4 in aqueous solution. PGG oxidation produced an o-semiquinone radical intermediate, which tended to form polymeric products at pH values below 5, and o-quinones at higher pH. The model protein bovine serum albumin promoted the formation of quinone even at low pH. Two other polyphenols, procyanidin (epicatechin(16-)(4 -> 8)-catechin) and epigallocatechin gallate, had similar pH-dependent oxidation patterns.