Radical chemistry of epigallocatechin gallate and its relevance to protein damage.
The radical chemistry of the plant polyphenolics epigallocatechin gallate (EGCG) and epigallocatechin (EGC) were investigated using electron paramagnetic resonance spectroscopy. Radical species formed spontaneously in aqueous solutions at low pH without external oxidant and were spin stabilized with Zn(II). The spectra were assigned to the gallyl radical and the anion gallyl radical, with only 10% of the signal assigned to a radical from the galloyl ester. Spectral simulations were used to establish a pK(a) of 4.8 for the EGCG radical and a pK(a) of 4.4 for the EGC radical. The electrochemical redox potentials of EGCG and EGC varied from 1000 mV at pH 3 to 400 mV at pH 8. The polyphenolics did not produce hydroxyl radicals unless reduced metal ions such as iron(II) were added to the system. Zinc(II)-stabilized EGCG radicals were more effective protein-precipitating agents than unoxidized EGCG and produced irreversibly complexed protein. EGCG and other naturally occurring polyphenolics are effective radical scavengers but their radical products have the potential to damage biological molecules such as proteins.
Department of Chemistry & Biochemistry, Miami University, Oxford, OH 45056, USA. hagermae@muohio.edu
Ann E
AE
Hagerman
Roger T
RT
Dean
Michael J
MJ
Davies
D000975:Antioxidants / Q000737:chemistry; D002392:Catechin / Q000031:analogs & derivatives* / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D005609:Free Radicals; D006863:Hydrogen-Ion Concentration; D010084:Oxidation-Reduction; D012710:Serum Albumin, Bovine / Q000737:chemistry*; D019345:Zinc Acetate / Q000737:chemistry
D000975:Antioxidants; D005609:Free Radicals; D012710:Serum Albumin, Bovine; D002392:Catechin; C045651:epigallocatechin gallate; D019345:Zinc Acetate; C057580:gallocatechol